- Last Update On : 2016-05-02
Urinalysis begins with a macroscopic examination of the urine which describes the color and clarity of the urine. In healthy individuals urine color ranges from pale yellow to amber, depending on their state of hydration. Many factors affect urine color including fluid balance, diet, medications and disease. The following table includes a list of the most common causes of abnormal urine coloration.
|Color||Pathologic Causes||Food & Drug Causes|
|Cloudy white||Phosphorus, pyuria, chyluria, lipiduria, hyperoxaluria, uricosuria||Diet high in purine-rich foods causing uricosuria, propofol, hypercalciuria, phosphaturia|
|Brown||Bile pigments, myoglobin, hemolytic anemia, porphyria||Fava beans, rhubarb, Levodopa, metronidazole (Flagyl), nitrofurantoin, anti-malarial drugs, acetaminophen overdose|
|Brownish-Black||Bile pigments, melanin, methemoglobin, alcaptonuria, porphyria||levodopa, methyldopa, Senna, Cascara, iron, methocarbamol, metronidazole, nitrofurantoin, sorbitol|
|Green or Blue||Pseudomonas UTI, biliverdin, Hartnup disease, herbicide poisoning||Amitriptyline, indigo, carmine, IV cimetidine (Tagamet), IV promethazine (Phenergan), methylene blue, triamterene (Dyrenium), indomethacin, methocarbamol, metoclopramide, propofol, Zaleplon|
|Orange||Bile pigments, urinary tract infection||Phenothiazines, phenazopyridine (Pyridium), isoniazid, sulfasalazine, riboflavin|
|Red||Hematuria, hemoglobinuria, myoglobinuria, porphyria||Beets, blackberries, rhubarb, Phenolphthalein, phenoazopyridine, rifampin, chloroquine, deferoxamine, hydroxycobalamin, warfarin|
|Yellow||Concentrated urine||Carrots, Cascara|
|Purple||Urine bag syndrome due to gram negative bacteruria|
Aycock RD and Kass DA, Abnormal Urine Color. Southern Med J. 2012;105:43-37.
Urine samples are initially screened with dipsticks. Performing microscopic analysis on only dipstick positive urine samples is cost effective when the patient population being tested has a low incidence of potential disease. Numerous studies have determined that 6 to 20% of patients with urine sediment abnormalities are missed by this testing strategy. However, most of the missed cases are clinically insignificant and are often due to contaminating bacteria multiplying after urine collection. Urine dipsticks are plastic strips with attached reagent pads for pH, protein, glucose, ketone, bilirubin, urobilinogen, blood, nitrite, and leukocyte esterase. The principle and performance of each dipstick test is summarized below.
The test is based on a double indicator method (methyl red and bromthymol blue) that covers the entire range of urine pH. Colors range from orange through yellow and green to blue. pH should be measured in fresh urine and read quickly.
The pH of urine is an indication of the kidney’s ability to maintain a normal plasma pH. Metabolism produces acids that are excreted by the lungs and kidneys. The average adult urine pH varies between 5 and 8. A diet high in protein produces a more acid urine, while a vegetarian diet often produces a pH greater than 6. Heavy bacterial growth may cause an alkaline shift in urine pH by converting urea to ammonia. Pigmented urine can interfere with pH readings.
Bacterial contaminants, blood in the urine and contamination by genital secretions can alter urine pH.
The protein test is based on a change in color of a pH indicator (e.g. tetrabromophenol blue) in the presence of varying concentrations of protein when the pH is held constant. The reagent pad contains the indicator and a buffer that holds the pH of the pad at approximately 3. Yellow indicates undetectable protein. The color of positive reactions ranges from yellow-green to green to green-blue. The accuracy of this test depends on having urine that is slightly acidic. Dipsticks can detect protein concentrations as low as 5 to 30 mg/dL. Urine protein concentrations are reported as 30, 100, 300, or 2000 mg/dL.
This test is optimized to detect albumin and is less sensitive in detecting globulins. Dipsticks do not detect beta-2- microglobulin or immunoglobulin light chains. Standard urine dipsticks are much less sensitive at detecting urine albumin than other assays. Dipsticks do not detect microalbuminuria.
|Method||Typical Detection Limit(mg/dL)||
Sensitivity (Relative to Urine Dipstick)
|Spectrophotometric Urine Protein||6||3X more sensitive|
|Immunoassay for Urine Albumin||0.3||60X more sensitive|
Dipstick testing is useful only when urinary protein exceeds 300 to 500 mg/day or albumin exceeds 10 to 20 mg/day.
The major cause of a false positive urine protein is a highly alkaline sample. False positive reactions can also be caused by contamination with quaternary ammonium compounds (zepharin, chlorhexidine) used to clean the skin for a clean catch urine. Excessive contact with urine may wash out the buffering system and lead to a false positive result. Confirmatory tests only need to be performed on those urine samples with positive protein and a pH of 7.5 or greater.
Proteinuria can have many causes. Postural proteinuria occurs in 3 to 5% of healthy adults and is characterized by the presence of protein in the urine during the day but not the night. Strenuous exercise, fever, and exposure to extreme heat or cold, pregnancy, eclampsia, shock, and CHF cause functional proteinuria. Hematologic malignancies, such as multiple myeloma, may produce excess immunoglobulin that is excreted in the urine. Renal diseases are a common source of proteinuria.
Approximately 25% of urine specimens containing bacteria will have a positive protein reaction as the only positive dipstick reaction. The esterase reagent is sensitive to 15 leukocytes per hpf, but the protein reagent is sensitive to 6 leukocytes per hpf.
The dipstick test is based on a double enzyme method employing glucose oxidase and peroxidase. Color change ranges from green to brown. Small amounts of glucose (<15 mg/dL) are normally excreted by the kidney, which is below the 75 mg/dL lower limit of detection of dipsticks, Glucose oxidase is specific for glucose and does not react with lactose, galactose, fructose, or reducing metabolites of drugs. Glucose is reported as 100, 250, 500, 1000, or >1000 mg/dL.
Urine specific gravity and temperature may affect test reactivity. High urine specific gravity can reduce color development. Urine should be at room temperature before the test is performed to obtain optimum sensitivity. False positive reactions rarely occur, but may be produced by strong oxidizing cleaning agents. Beta lactam antibiotics such as the penicillins, cephalosporins, carbapenems, and monobactams can cause false positive reactions. Massive amounts of ascorbic acid (vitamin C), salicylates or levodopa can decrease the sensitivity of the test.
Negative urine samples from pediatric patients under the age of one should be confirmed with a copper reduction method, such as Clinitest, to detect galactose or lactose. Confirmation only needs to be performed once on a patient.
Glucosuria usually occurs when the blood glucose level exceeds 180 mg/dL. Glucosuria most commonly occurs in patients with diabetes, infections, myocardial infarction, liver disease, and obesity. Thiazides, corticosteroids, and birth control pills may precipitate glucosuria.
Dipsticks use the nitroprusside reaction to test for acetoacetic acid. They are less sensitive to acetone and do not detect beta-hyroxybutyrate. The typical diabetic patient with ketoacidosis usually excretes 78% beta-hyroxybutyrate, 20% acetoacetate, and 2% acetone. The reaction of acetoacetic acid with nitroprusside results in the development of color ranging from buff pink to shades of purple. Color reactions are categorized as trace, small, moderate and large that correspond to ketone concentrations of 5, 15, 40 to 80 and 80 to 160 mg/dL of urine, respectively. Dipsticks reliably detect ketone concentrations of 40 mg/dL or more, so moderate and large readings do not need to be confirmed. Trace and small readings should be confirmed by using Acetest. The detection level for Acetest tablets is 20 mg/dL. The presence of ketonuria does not signal the need to do further microscopic evaluation.
Normally, urine contains <2 mg/dL of acetoacetic acid, which is not detectable. A healthy individual may have detectable ketones if he/she has been fasting, strenuously exercising, or is pregnant. Ketones are also detected in children consuming high fat diets. Ketonuria is commonly seen in hospitalized patients due to fasting. Ketones are clinically significant only in the presence of urine glucose. Drugs with free sulfhydryl groups such as penicillamine, N-acetylcysteine, BAL and ACE inhibitors (captopril and enalapril) cause false positive reactions.
Ketones are volatile and evaporate from the specimen with time. False negative results can occur with old urine samples. The reagent pads are extremely sensitive to moisture and may become non-reactive after exposure to humid room air for a few hours.
The dipstick test for blood is based on the peroxidase-like activity of hemoglobin. Red cells are lysed on contact with the strip, allowing free hemoglobin to catalyze the liberation of oxygen from organic peroxide. Tetramethylbenzidine is oxidized, producing a color change from orange to green-blue. If intact red cells do not lyse, they may produce speckles on the pad. The sensitivity of dipsticks for hemoglobin is 0.015 to 0.062 mg/dL. This concentration corresponds to 5 to 21 RBCs/uL or 1 to 4 RBCs/hpf of concentrated urine sediment.
The reference range for RBCs in normal urine is 0-3 RBC/hpf in males and 0-12 RBCs/hpf in females when concentrated urine sediment is examined. This range corresponds to a concentration of 3 to 20 RBCs/uL of urine. Dipstick sensitivity extends into the reference range. Therefore, trace to 1+ reading may be obtained on urine from as many as 3% of healthy individuals.
In healthy individuals, fewer than 1000 red cells are excreted in the urine per minute. When 3000 to 4000 red cells are excreted per minute, 2 to 3 red cells will be seen per high power field, indicating microscopic hematuria. Gross hematuria occurs when more than 1 million red cells are excreted per minute. Hematuria can be due to lesions within the GU tract involving the kidneys, ureters, bladder, prostate, or urethra. The most common disorders include cancer, kidney stones, renal disease, urinary tract infection, and benign prostatic hyperplasia. Transient hematuria can result from menstruation, viral illnesses, strenuous exercise, and mild trauma. Anticoagulant therapy and chemotherapy may also cause hematuria. No etiology can be determined in approximately 45% of cases of microscopic hematuria.
A positive dipstick test for blood does not tell whether the reaction is due to red cells, red cell casts, hemoglobin casts, or myoglobin. Many conditions can lead to discrepant dipstick and microscopic findings. Any situation that causes red cell hemolysis will give a positive dipstick and negative microscopic result. Urine should be tested shortly after collection because red cell lysis may occur as the sample ages, if the pH is alkaline, or if the specific gravity is 1.010 or less. Bacterially contaminated urine specimens may contain sufficient peroxidase activity to produce a false positive reaction. False positive reactions can also be caused by vegetable peroxidase.
|False Positive Dipstick||False Negative Dipstick|
|Myoglobin||Dipsticks exposed to air|
|Oxidizing agents - bleach, detergent, iodine||RBCs settle out & urine not mixed|
|Bacterial peroxidase||Ascorbic acid (high concentration)|
|Vegetable peroxidase||Formaldelhyde (preservative tablets)|
|Betadine||High specific gravity|
|Very high protein|
|Urine pH <5.1|
|High nitrite from UTI|
The bilirubin dipstick test detects conjugated bilirubin and has a sensitivity of 0.5 to 1.0 mg/dL. This test is based on the binding of conjugated bilirubin to diazotized salts fixed in the test pad in a strong acidic environment to produce a colored compound that is various shades of tan or magenta. Positive dipstick tests are confirmed with the Ictotest. Normal adult urine contains about 0.02 mg/dL of bilirubin, which is not detectable by even the most sensitive methods. Confirmation of positive dipstick bilirubin results is most valuable when the urine specimen is pale yellow.
Ictotest is a tablet test that uses a similar chemical reaction but a different test environment. Urine is placed on an absorbent test mat that captures substances within the urine. The reagent tablet is then placed on top of the absorbed urine and water is added to the tablet. The water dissolves the solid diazonium salt and acid in the tablet so that they run onto the mat. The reaction of conjugated bilirubin with the diazonium salt in the acid environment results in the formation of a blue ring around the dissolving tablet. The sensitvity of the tablet test is 0.05 to 0.1 mg/dL, which is about 10 times more sensitive than the dipstick test. The tablet test is also more specific than the dipstick test for bilirubin and its primary use is the detection of false positive dipstick reactions. Since the urine is placed on the mat first in the tablet test, abnormal pigments due to medications or blood metabolites can be detected before the chemical reaction ensues. Other interfering substances are washed through the mat and do not come into contact with the diazonium salt. Also, because the reaction product is blue rather than tan or magenta, fewer interpretation problems are encountered. Examples of medications that produce false positive dipstick and negative Ictotest results include rifampin, phenazopyridium (Pyridium), and nonsteroidal antiinflammatory agents (etodolac, mefenamic acid and flufenamic acid).
Bilirubin and urobilinogen tests are valuable in detecting hemolysis, hepatic dysfunction, and biliary obstruction. The results of these two tests should be interpreted together. Bilirubin is unstable and rapidly decomposes during exposure to light. False negative reactions are common if urine is not tested shortly after collection. Chlorpromazine (Thorazine) and selenium can produce false negative results.
Most dipsticks use para-dimethylaminobenzaldehyde in a strongly acid medium to test for urobilinogen. A positive reaction produces a pink-red color. Urobilinogen is normally present in urine at concentrations up to 1.0 mg/dL. A result of 2.0 mg/dL represents the transition from normal to abnormal. False positive results can be caused by medications such as para-aminosalicylic acid, antipyrine, chlorpromazine, phenazopyridine, phenothiazine, sulfadiazine, and sulfonamide. High nitrite concentrations can cause false negative reactions. Pigmented urine can interfere with detection of urobilinogen.
Conjugated bilirubin is normally excreted into the bowel where bacteria metabolize it to urobilinogen. Urobilinogen is partially reabsorbed from the gut and excreted in the urine. A positive test indicates increased bilirubin delivery to the gut. Hepatitis produces positive urine bilirubin and urobilinogen. Biliary tract obstruction results in positive urine bilirubin but negative urobilinogen. Hemolytic anemia causes negative urine bilirubin and positive urobilinogen.
|Icteric liver disease||Increased||Positive|
Pyuria (the presence of leukocytes in the urine) can be detected using the leukocyte esterase reagent strip test. The assay is based on the chemical detection of esterases, which are enzymes contained within the azurophilic granules of polymorphonuclear leukocytes. Esterase level is directly proportional to the number of leukocytes present in a urine sample. The basis of the chemical reaction is the hydrolysis of an ester to form an aromatic alcohol and acid. The aromatic compound combines with a diazonium salt to form an azo-dye that changes to purple. Color intensity read at two minutes is proportional to the number of granulocytes in a sample. Positive results are reported semiquantitatively as trace, 1+, 2+, or 3+. The sensitivity for Multistix reagent strips is 5 cells per high power field (hpf) to 15 cells/hpf while Chemstrip reagent strips have a sensitivity of 20 leukocytes per uL of urine. Because of this relative insensitivity, the absence of leukocyte esterase does not rule out urinary tract infection (UTI). A positive esterase reaction indicates inflammation secondary to UTI or renal disease.
Esterase activity from either intact or lysed granulocytes can give a positive result. Lysed granulocytes may produce apparent discrepancies between positive dipstick results and negative microscopic examinations. Lymphocytes do not produce a positive reaction. Other sources of esterase such as eosinophils, Trichomonas, or epithelial cells in vaginal fluid may give false positive results. Oxidizing agents such as bleach or colored substances can produce false positives.
False negative results can be caused by high concentrations of ascorbic acid (vitamin C), albumin or other proteins (>500mg/dL), glucose (>3000 mg/dL), or ketones. Urine with high specific gravity can cause a false negative reaction because enzyme is not as readily released from crenated white blood cells. These samples should be examined microscopically so as not to miss clinically significant pyuria. WBC clumping may prevent dispersion of leukocyte esterase and cause a false negative result. Outdated or deteriorated dipsticks are another cause of false-negative results.
Doxycycline, gentamicin and some cephalosporins reduce the reactivity of leukocyte esterase and produce false negative results. Conversely, imipenem, meropenem, and clavulanic acid can cause false positive leukocyte esterase reactions.
Most studies comparing the sensitivity of nitrite and leukocyte esterase tests compared to urine culture have demonstrated that leukocyte esterase is a more sensitive indicator of UTI than nitrite.
The nitrite test is a rapid, indirect method for detecting bacteriuria. The reaction principle is based on bacterial reduction of dietary nitrate, which is normally present in urine, to nitrite, which is not normally present. Nitrite reacts with para-arsanilic acid on the dipstick to form a diazonium compound that reacts with a benoquinoline to form a pink color. Many of the bacteria that cause UTIs have the ability to reduce nitrate, including Escherichia coli, Klebsiella, Pseudomonas, Enterobacter, and Citrobacter. The optimal specimen is a freshly voided, first morning urine that has been retained in the bladder a minimum of 4 hours, permitting adequate time for conversion of nitrate into nitrite by the bacterial enzymes. A positive nitrite test result indicates UTI with significant bacteriuria. Test sensitivity has been standardized to correspond to a urine bacterial count of 100,000 colony forming units/mL (CFU/mL). Color intensity is not proportional to the degree of bacteriuria; results are simply reported as positive or negative.
False positive results can be caused by colored substances in the urine (e.g. phenazopyridine) and prolonged specimen storage at room temperature that allows proliferation of contaminating bacteria. If urinalysis cannot be done within two hours after collection, specimens should be refrigerated to prevent bacterial growth.
False-negative nitrite results can occur even in the presence of significant bacteriuria due to a number of possible factors. The causative organisms may lack the reductase enzyme needed to convert nitrate to nitrite. For example, both yeast and gram positive bacteria are reductase negative. Malnourished patients and patients receiving intravenous feeding may have insufficient dietary nitrate to promote the chemical reaction. The duration of urine retention in the bladder may be too short (<4 hours) to facilitate nitrate reduction. Previous antimicrobial therapy may inhibit bacterial metabolism. In the presence of high numbers of bacteria, nitrite may be further reduced to nitrogen, which is not detected. High concentrations of ascorbic acid or urobilinogen can inhibit the chemical reaction. Of course, outdated or deteriorated dipsticks can also yield false-negatives. Microscopic examination of urine sediment or urine culture should be performed, even with negative nitrite, when clinical symptoms suggest UTI.
Vitamin C is a strong reducing agent and interferes with a number of dipstick tests. An evaluation of 4379 urinalysis specimens from outpatients in a single laboratory revealed that 23% contained measurable vitamin C. An oral dose of 100 mg of vitamin C caused falsely negative dipstick tests for blood, glucose and leukocyte esterase in urine samples tested within 4 hours of ingestion. Vitamin C consumption is a likely cause of discrepancies between urine dipstick and microscopic analysis.
The specific gravity of a solution is the ratio of the mass per unit volume of the solution to the mass per unit volume of distilled water. It is a relative measure by weight of the amount of dissolved urinary solutes. All urine contains some solutes and will always have a specific gravity higher than pure water (1.000). Normally, an adult should be able to concentrate the urine to a specific gravity of 1.016 to 1.022. A first morning urine with a specific gravity of 1.023 or higher after overnight fluid deprivation indicates normal renal concentrating capacity.
The dipstick specific gravity test is based on the apparent pKa change of polyelectrolytes in relation to ionic concentration. In the presence of an indicator, colors range from deep blue-green in urine of low ionic concentration through green and yellow-green in urines of increasing ionic concentration.
Diabetes mellitus is associated with increased urinary volume and elevated specific gravity due to urinary glucose, which increases the solute content.
Diabetes insipidus results in a large urinary volume with low specific gravity. Hyposthenuria means a persistently low urine specific gravity of <1.007.
Renal tubular disease is often manifested early by a loss of concentrating capacity of the kidneys; specific gravity is <1.018. Later in the disease process, the capacity to dilute the urine is lost and the patient can only produce isothenuric urine with a fixed specific gravity of 1.010.
The kidneys cannot concentrate urine to a specific gravity of >1.035. Specific gravity readings greater than 1.035 by refractometer, accompanied by normal specific gravity by reagent strips, usually contain higher molecular weight solutes such as glucose, protein, radiopaque contrast media or drugs. Organic iodides in contrast media such as meglumine diatrizoate (Renograffin, Hypaque) may be seen in the urine sediment for a brief time after injection of the dye. The crystals resemble cholesterol crystals.
Dipstick Reference Range
|Analysis||Reference Value||If positive, reported as:|
|Specific gravity||1.003 - 1.030||Number|
|Blood||Negative||Small, moderate, large|
|Ketones||Negative||Small, moderate, large|
|Glucose||Negative||100, 250, 500, 1000, >1000 mg/dL|
|Protein||Negative||30, 100, 300, >2000 mg/dL|
|pH||4.5 - 8.0||Number|
A microscopic exam is performed if blood, protein, or leukocyte esterase results are abnormal or if a microscopic exam is specifically requested. The urine is centrifuged and examined microscopically for WBC, RBC, crystals, casts, bacteria and yeast. Both dipstick and microscopic exam should be performed for patient populations with a high incidence of genitourinary tract disease.
Microscopic urinalysis cannot be completely eliminated because multiple clinically significant findings can only be detected by examining urine sediment directly. For example, a positive dipstick reaction for blood does not distinguish between red cells, hemoglobin, or casts. Likewise, a positive leukocyte esterase reaction does not distinguish between free WBCs that occur in cystitis, from WBC casts that are characteristic of pyelonephritis. Microscopy can detect several other clinically significant abnormalities that are not detected by dipsticks including renal tubular epithelial cells and casts, fatty casts, oval fat bodies, crystalline casts, and crystals.
The urinary tract is lined by epithelial cells, including squamous, transitional and renal tubular cells. Squamous epithelial cells line the bladder trigone, urethra and vagina. They are large cells, measuring 30 to 50 um in diameter, with a single small round nucleus. They may appear round, polygonal or rectangular in shape with curled edges. Their presence in urine has no clinical significance.
Transitional cells line the urinary tract from the renal pelvis to the trigone of the bladder in females and the first part of the urethra in males. They are 4 to 6 times larger than a red blood cell, appear spherical or polyhedral in shape and have a larger round or oval nucleus. Small numbers of transitional cells have no clinical significance. Large clusters are seen after bladder catheterization or washing. Large number transitional cells with dysplastic nuclei might be indicative of malignancy.
Renal tubular epithelial cells line the proximal and distal convoluted tubules and collecting ducts of the kidney. They are 3 to 5 times larger than a red blood cell and appear elongated and polyhedral. One side often appears flattened and microvilli may be seen. Their cytoplasm is granulated and they have a single eccentric nucleus. A small number of renal tubular cells is not clinically significant. The presence of more than 15 cells in 10 high power fields suggests active renal disease or injury. Examples include acute tubular necrosis, drug or metal toxicity, infections and renal transplant rejection.
Renal tubular epithelial cells filled with fat droplets are called oval fat bodies and are detected in the nephrotic syndrome. Renal tubular cells may also accumulate hemosiderin, which appears as coarse yellow-brown cytoplasmic granules. The granules stain blue with Prussian blue. They are detectable 2 to 3 days after intravascular hemolysis.
Under high power, unstained RBCs appear as pale, homogeneous, biconcave discs with no nucleus. They vary somewhat in size, but are usually about 7 microns in diameter. Red cells tumble when the fluid on the slide is set in motion. If the specimen is not fresh when it is examined, the cells will appear as faint, colorless circles (shadow or ghost cells) because the hemoglobin has leached out of the cell. In dilute urine, red cells absorb water, swell and lyse, releasing their hemoglobin and leaving only ghost cells. In urine with a very high specific gravity, the red cells become crenated, which may appear as spikes in the cell. In urine with a low specific gravity or with extreme pH, the cells may be lysed.
Red blood cells must be distinguished from yeast and fat droplets. Yeast often exhibit budding and fat droplets are highly refractile. Positive identification of red blood cells can be accomplished by adding 2% acetic acid to the urine sediment, which lyses red blood cells.
Zero to 6 RBCs/hpf are considered to be within normal limits. Physiological increases in the number of RBCs in urine may occur following vigorous exercise or fever without indicating significant urinary tract disease. Presence of large numbers of red cells (smoky urine) indicates bleeding into the urinary tract, anywhere from the kidney to the urethra. This may be due to several causes including urinary tract infection, tumor in the kidney or urinary tract, calculi or urinary tract stones, derangements of blood clotting, trauma and renal disease such as glomerulonephritis. The presence of red cells without the presence of protein or casts suggests that the bleeding occurred distal to the kidney. Dysmorphic (distorted) red cells may indicate that the bleeding has occurred in the kidney glomeruli.
WBCs (leukocytes) appear as round granular spheres about 14 μm in diameter (about twice the size of a red cell) and have a nucleus. Some WBCs in hypotonic urine appear larger because they have absorbed water and the cytoplasmic granules exhibit Brownian movement. These cells are called glitter cells.
A few WBCs can be found in normal centrifuged urine. Pyuria generally indicates an infectious or inflammatory process somewhere in the kidney (pyelonephritis), bladder (cystitis) or urethra (urethritis). Clumps of WBCs should be noted because they can affect urine white cell count.
The presence of increased numbers of eosinophils may be indicative of drug-induced interstitial nephritis. However, special staining procedures using Hansel's stain must be performed before the eosinophils can be reliably identified.
Lymphocytes in the urine cannot be clearly identified without the use of a Wright's or Papanicolaou stain. The presence of lymphocytes may be seen in renal transplant rejection, inflammatory processes, and other disorders.
Urinary casts are formed in the distal convoluted tubule or the collecting duct of the kidney. Cast formation increases with stasis, increased protein excretion and lower pH. Three categories of casts may be seen: acellular, cellular, and mixed. The type and number of casts seen per low power field is reported.
Hyaline casts are composed primarily of a mucoprotein, which is called Tamm-Horsfall protein, and is secreted by cells lining the distal convoluted tubules. The factors that favor protein cast formation are low flow rate, high salt concentration, and low pH because these conditions favor protein precipitation. A few hyaline casts are normal, but all other casts need to be evaluated. Cast width is described as narrow (one to two RBCs in width), medium broad (three to four RBCs in width), or broad (five RBCs in width). Casts that form in the collecting tubules tend to be very broad and usually indicate end stage renal disease.
Pigment casts are hyaline casts with absorbed pigments such as bile or hemoglobin, are yellow to brown in color, and may have a smooth or granular surface. These casts may be seen in patients with viral hepatitis as well as multiple myeloma and those patients with renal amyloidosis.
Fatty casts form when fat from lipid-laden tubular cells is incorporated into the cast. They contain yellowish-tan fat globules on their surface. They usually indicate the nephrotic syndrome.
RBC casts contain numerous orange-red erythrocytes or unpigmented ghost RBC embedded within a hyaline matrix. The unpigmented form is more frequent and can often be recognized by its hemoglobin pigmentation. RBC casts are usually accompanied by the presence of free RBCs in the sediment. RBC casts are frequently found in glomerulonephritis. Occasionally, they may be seen in individuals playing contact sports. In the latter situation, the urine usually returns to normal within 24 to 48 hours.
The typical renal tubular cell casts are often described as hyaline matrix casts showing two rows of well-delimited tubular cells. However, the two row criterion is not reliable because WBC casts can also have this appearance. Renal tubular cells are difficult to identify in cellular casts. The cells are elongated or columnar and have large eccentric nuclei with sparse granular cytoplasm. Their presence indicates renal tubular damage.
When cellular casts remain in the nephrons for some time before being flushed into the bladder, the cells may degenerate to a coarsely granular cast, later to a finely granular cast, and eventually, to a waxy cast. Granular and waxy casts are believed to be derived from renal tubular casts. Granular casts are non-specific and may be present in a variety of kidney disorders. Waxy casts are associated with advanced kidney disease and chronic kidney failure. They are opaque, easy to see and have broken off ends, as well as notched parallel margins.
Granular casts are semitransparent cylinders containing fine or coarse granules. Distinction between finely and coarsely granular casts is not clinically important. The granules are the result of cellular breakdown and protein aggregation. They have little clinical significance, unless present in very large numbers. Increased numbers are seen after strenuous exercise, fever, dehydration and stress. Large numbers are associated with renal glomerular or tubular disease.
Calcium oxalate crystals occur as the common dehydrate form and the less common monohydrate form. The dehydrate form appears as small colorless octahedrons. The monohydrate form is dumbbell-shaped or ovoid rectangles. Both forms are found in acid or neutral urine. They are seen more commonly in urine in individuals who eat foods rich in oxalic acide such as tomatoes, asparagus, oranges and rhubarb. Large numbers are associated with kidney stone formation, severe chronic renal disease, ethylene glycol poisoning and methoxyflurane toxicity. They may also be increased in patients with Crohn’s disease and after small bowel resection.
Triple phosphate (ammonium magnesium phosphate) crystals differ from calcium oxalate in that they are colorless 3 to 6 sided prisms, called coffin lids. Occasionally, sheets, flakes, flat or fern leaf forms are seen. Their formation increases if urine is left at room temperature. They may develop a feathery appearance as they breakup. Triple phosphate crystals occur in neutral to alkaline urine and do not have clinical significance.
Calcium pyrophosphate dehydrate crystals appear as rhomboids, rods or rectangles and tend to be small, measuring between 1 and 20 um. They are smaller than cholesterol crystals and lack the corner notch. They differ from monosodium urate in that they form very small rhomboids, short rods, or rectangles. With polarized light, they are weakly birefringent and appear yellow when aligned with the compensator axis. This polarization pattern is the opposite of monosodium urate crystals. They are associated with degenerative arthritis.
Uric acid crystals are only seen in acid urine and have a wide variety of forms including rhombic, four-sided plates, rosettes, wedges and lemon shapes. Needle shaped crystals of monosodium urate are rarely seen in urine. They usually appear yellow or red-brown and form in acid urine. Uric acid crystals are considered to be a normal constituent of urine. Large numbers may be seen with gout, increased cell turnover associated with chemotherapy and Lesch Nyhan syndrome.
Cholesterol crystals usually form colorless plates with a notched corner. Cholesterol crystals are rarely seen in urine from patients with normal renal function. Cholesterol crystals, fatty casts and fat droplets are most commonly seen in patients with nephrotic syndrome.
Tyrosine crystals are rarely seen. They form clusters of needles that are colorless or black in acidic urine. Tyrosinuria accompanies liver disease or hereditary hypertyrosinemia.
Mucus is a normal finding in urine sediment and is more often seen in specimens from females. Mucus forms colorless threads or long ribbon-like structures with undefined edges and pointed or frayed ends. Mucus threads may be confused with hyaline casts.
The presence of contaminants such as fiber indicates poor collection technique or contamination from clothing or diapers. They can also occur as a result of fecal contamination. Fibers need to be distinguished from casts. Fibers have dark edges and a flat appearance. Fibers are usually longer and more refractile than casts. Unlike casts, fibers polarize brightly.
Corpora amylacea are small, amorphous nodules or concretions that accumulate in the prostate gland with age. They can be seen in the urine of older men with benign prostatic enlargement, as well as in urine specimens obtained after prostatic manipulation.
Starch granules are a frequent contaminant in urine due to contamination with the powder used in exam or surgical gloves. They appear spherical or oval and are highly refractile. They typically have a dimpled or indented center that resembles a maltese cross. They may be several times larger than a red blood cell. They can be confused with crystals or parasites.
Exogenous fatty material from creams or lubricants can contaminate urine and appear as fat globules. They are not associated with fatty casts. Oil droplets are round like a ball and can easily be mistaken for red blood cells.
Radiographic contrast material may produce flat colorless crystals, but is usually associated with very high specific gravity.
Sperm may be seen in male urine or female urine that has been contaminated with vaginal secretions.
Trichomoniasis is one of the most common sexually transmitted diseases, mainly affecting sexually active women. T. vaginalis appears in urine as a result of vaginal contamination. Often the organism is motile and will swim rapidly across the microscope slide.
Yeast are ovoid, colorless and have smooth thick walls that give them a refractile appearance. They may show branching, or hyphae. Yeast are smaller than red blood cells, measuring only 2 mm in diameter. Yeast in urine may be a contaminant from air or skin or may be due to a urinary tract infection. Large numbers of white blood cells will also be present in the latter case. The most important type is Candida. Infections are most common in diabetic and immunocompromised patients.
Pinworms may be seen in urine, especially if the urine was obtained very early in the morning, because the adult pinworm usually deposits its eggs in the perianal area during the night. They can also be seen in urine specimens with fecal contamination.
Filariasis is a term applied to several parasitic infections caused by microscopic, thread-like worms (e.g., Loa loa, Onchocerca volvulus). Typically, the adult worms live in the human lymph system. During their life cycle, the adults produce microfilariae measuring 250 to 300 μm by 6 to 8 μm, which are sheathed and demonstrate diurnal periodicity (organisms circulate in the bloodstream during the day and are non-circulating in the night). Microfilariae have been recovered from urine, as well as spinal fluid and sputum.
Schistosomiasis (bilharziasis) is one of the world's most common parasitic infections. The life cycle of the organism has been extensively studied. Eggs from feces, when deposited in a fresh water site, hatch and infect an appropriate snail host. After about four weeks, cercarial forms emerge from the snail and swim in search of the next host, humans. After skin penetration, the organisms enter the host’s bloodstream, pass through the lungs, and lodge as adult forms in the portal-mesenteric vascular system. The image is of S. haematobium, whose eggs can be found in urine. The eggs are laid in the venous system of the bladder.
Specimen requirement is 10 mL from a random urine collection. A first morning void (overnight) specimen is preferred because it is more concentrated and can be assumed to have been in the bladder for a number of hours. A dilute specimen is more likely to yield a false negative result.
Urine should be tested as soon as possible after collection. Some determinations such as urobilinogen, bilirubin and pH are only valid if obtained on a fresh specimen. Prolonged length of time between specimen collection and analysis can result in false-positive urine nitrites and false-negative glucoses. Likewise, bacterial overgrowth in non-preserved, non-refrigerated specimens leads to false-positive or contaminated urine culture results. Bacterial growth, cellular degradation, precipitation of amorphous material, and increasing pH due to urease producing bacteria adversely affect protein measurements. A urine more than 24 hours old, even if refrigerated should not be tested. If urine has been refrigerated, it should be allowed to return to room temperature before testing. Refrigeration may cause an increase in specific gravity and precipitation of amorphous phosphates and urates. Glucose sensitivity is adversely affected by not testing at room temperature.
Microscopic Reference Range
Microscopic examination is considered normal if all of the following criteria are met:
- 0 to 6 erythrocytes per high power field
- 0 to 6 leukocytes per high power field
- <3 hyaline or <1 granular cast per low power field
- Absence of any other casts
- Absence of significant crystals (i.e. cystine, leucine, tyrosine)