Complete Blood Count (CBC)

An automated CBC with differential white blood cell count can determine 32 different hematological parameters. The complete blood count (CBC) includes: hemoglobin, hematocrit, red blood cell (RBC) count, white blood cell (WBC) count, RBC indices, platelet count, mean platelet volume (MPV), and red cell distribution width (RDW). 

Hemoglobin, WBC count, RBC count, mean corpuscular volume (MCV), and platelet count are measured electronically by an automated hematology analyzer. Hematocrit, mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC) and RDW are calculated.

CBC calculated values

• Hct = RBC x MCV
• MCH=Hgb/RBC
• MCHC=Hgb/Hct

RBC Parameters

Modern cell counters measure the number of RBCs within a given blood volume by electrical impedance. RBC count is usually expressed as the number of RBCs per microliter or liter. 

Hemoglobin concentration is measured by converting hemoglobin to hemiglobincyanide and spectrophotometrically measuring its light absorption at 540 nm. Hemoglobin is an indicator of anemia or polycythemia. 

Hematocrit is the percentage of whole blood volume represented by packed red blood cells. Automated cell counters calculate hematocrit by multiplying RBC count and MCV.

The Rule of 3 states that hematocrit must match the Hb x3 +/-3.  Samples not complying with this rule should be evaluated by morphologic examination. Disagreement is seen with abnormal RBCs, cryoglobulinemia, lipemia, giant platelets, and very high WBC counts. 

Hemolysis can cause a discrepancy between hemoglobin and hematocrit. Hemoglobin is measured after lysis of RBC, so it includes both RBC and plasma hemoglobin. Hemoglobin may appear normal even though hematocrit and RBC count are decreased.

Mean cell volume (MCV) is a measure of the mean size of RBCs. Individual RBC volumes are measured directly by electrical impedance. The values are plotted on a histogram that is used to calculate mean cell volume. MCV is useful in classifying anemia a normocytic, microcytic or macrocytic.

Red cells swell after 6 to 8 hours exposure to EDTA anticoagulant. MCV is artifactually increased after prolonged storage of a specimen. 

Mean corpuscular hemoglobin (MCH) is the average hemoglobin content of a single RBC. It is the calculated by dividing hemoglobin by the RBC count. This parameter does not consider red cell size. MCH is a more sensitive indicator of iron deficiency than MCV, although neither one is very good. 

Mean corpuscular hemoglobin concentration (MCHC) is the average red cell hemoglobin concentration expressed as a percent. It is calculated by multiplying hemoglobin by 100 and dividing the product by the hematocrit. MCHC is a more useful RBC index than MCH and correlates with the degree of central pallor in red blood cells. MCHC is useful in the differential diagnosis of anemia. Elevated MCHC can be caused by spherocytosis, hyponatremia, cold agglutinin, lipemia or strongly discolored plasma.

If red cells lyse during collection or transportation, red cell count and hematocrit decrease, causing an increase in the calculated values for MCH and MCHC. 

Red cell distribution width (RDW) is a measure of variability in the size of RBCs. RDW corresponds to the morphological term, anisocytosis. Hematology analyzers measure red cell size and calculate the RDW based on either the coefficient of variation in, or the standard deviation of, the red cell population. 

The most commonly calculated RDW is based on the coefficient of variation of the red blood cell distribution volume. RDW-CV is calculated using the formula: 

RDW-CV=1SD x 100/MCV

In adults, the reference range of RDW-CV is typically 11.0 to 15.0%. 

RDW is helpful in formulating the differential diagnosis of anemia. Normally, red cells are uniform in size and a single narrow Gaussian-type histogram is produced. In anemia due to nutritional deficiency anemias (iron, folate, or vitamin B12), homozygous hemoglobinopathies and red cell fragmentation RDW is elevated because of the increasing variability in red cell size.

Automated cell counters incorporate a method for flagging white blood cell (WBC), red blood cell (RBC) and platelet abnormalities. The flagged abnormalities prompt a medical scientist to perform a manual peripheral smear review and differential to verify the abnormality flagged as well as allow identification of additional morphological abnormalities.

Hematology Analyzer Flags

An automated cell counter can be programmed to flag the following abnormalities:

WBC Abnormalities

• Abnormal lymphocytes/lymphoblasts
• Atypical lymphocytes
• Basophilia
• Blasts
• Eosinophilia
• Immature granulocytes
• Left shift
• Leukocytopenia
• Leukocytosis
• Lymphopenia
• Monocytosis
• Neutropenia
• Neutrophilia
• Nucleated red blood cell abnormal scatter
• WBC abnormal scatter

RBC Abnormalities

• Fragments (schistocytes)

Platelet Abnormalities

• Platelet abnormal scattergram
• Platelet clumps
• Thrombocytopenia
• Thrombocytosis

If specimens are flagged for review, a clinical laboratory scientist examines a peripheral blood smear for the following RBC morphological abnormalities: polychromasia, acanthocytes (spur cells), ovalocytes (elliptocytes), spherocytes, sickle cells (drepanocytes), target cells (codocytes), schistocytes, stomatocytes, rouleaux and basophilic stippling. RBC inclusions (Howell Jolly bodies), granulocyte abnormalities (toxic granulation, Dohle bodies, Pelger Huet, hypersegmented polymorphonuclear leukocytes) and giant platelets are also reported if present. 

Slides with newly identified blasts, prolymphocytes, plasma cells or unusual WBC and RBC morphology are submitted by the clinical laboratory scientist to a pathologist for review.  In addition, healthcare providers may request a pathologist review of a peripheral blood smear if further morphological review is deemed clinically necessary.  In these cases, the indication for the pathologist review should be clearly stated on the requisition form.

Reference Ranges

Adult reference ranges and critical values are:

Reference range for the WBC differential count is:

Total WBC count and absolute differential counts are higher in newborns and decrease until five to ten years of age. There are no significant differences between males and females regarding WBC and differential counts. 

Specimen requirement for a CBC is one lavender top (EDTA) tube.