Glucose, Plasma

Measurement of plasma glucose concentration can detect hyperglycemia and hypoglycemia.

Type 2 diabetes is the most common cause of hyperglycemia, affecting u to 14% of the global population. It is associated with preventable long-term complications such as cardiovascular disease, kidney failure, vision loss, and increased mortality. Diabetes becomes clinically evident when elevated blood glucose levels cause glycosuria with resultant polyuria, polydipsia, fatigue, blurry vision, and unintentional weight loss. 

Historically, the diagnosis of diabetes was based on fasting plasma glucose and 2 hour plasma glucose level after a 75 gram oral glucose tolerance test (OGTT). 

The U.S. Preventive Services Task Force (USPSTF) and The Expert Committee on Diagnosis and Classification of Diabetes Mellitus recognized an intermediate group of individuals whose glucose or HbA1c levels are higher than normal but do not meet the diagnostic criteria for diabetes. People with fasting glucose levels between 100-125 mg/dL were defined as having impaired fasting glucose (IFG). Individuals who have a 2 hour glucose level between140-199 mg/dL were defined as having impaired glucose tolerance (IGT). Individuals with any of these intermediate laboratory values were referred to as having prediabetes. People with prediabetes are at higher risk of developing diabetes and cardiovascular disease. These patients often have abdominal obesity, dyslipidemia with low HDL cholesterol and high triglycerides and hypertension.

Random plasma glucose <140 mg/dL is considered normal while a result >200 mg/dL indicate diabetes. A random value between 140 and 199 mg/dL suggests IFG or IGT. 

Fasting plasma glucose level <100 mg/dL is considered normal while a value >126 mg/dL can mean diabetes. A fasting glucose level between 100 and 125 mg/dL can mean IFG or IGTl. 

An oral glucose tolerance test result <140 mg/dL is considered normal while a result >200 mg/dL can mean diabetes. An OGTT result between 140 and 199 mg/dL is suggestive of IFG or IGT.

For more information, see the article entitled: “Diabetes Mellitus.”

Hypoglycemia

Hypoglycemia is defined by the presence of Whipple’s triad: plasma glucose concentration less than 50 mg/dL, symptoms of nervousness, anxiety and sweating, and relief of symptoms with administration of glucose.  It occurs infrequently in hospitalized patients. The most common causes of hypoglycemia in inpatients are diabetes mellitus, renal failure, liver disease, infection, pregnancy, metastatic cancer, and burns. 

Hypoglycemia is also rare in outpatients. The best way to diagnose this condition is to instruct the patient to eat a meal similar to the one that produces symptoms and then have their blood drawn for a plasma glucose level when they become symptomatic. If plasma glucose is not <50 mg/dL, then the person does not have hypoglycemia. 

If a person has an abnormal screening test, they should be further evaluated with a 72 hour fast unless an obvious cause is indicated in the medical history, physical exam or laboratory tests. The 72 hour fast attempts to document Whipple’s triad under controlled conditions. This test is difficult to perform correctly.  Criteria for discontinuing the fast are strict: blood glucose level must be lower than 45 mg/dL and the patient must be symptomatic.  All nonessential medications must be discontinued and only water, black decaffeinated coffee, and diet sugar-free sodas can be consumed. 

For more information about low plasma glucose levels, see the article entitled: “Hypoglycemia”. 

Specimen requirement is one grey top tube of blood for glucose. Fasting is defined as no consumption of food or beverage other than water for at least 8 hours before testing.

References

The International Expert Committee; International Expert Committee Report on the Role of the A1C Assay in the Diagnosis of Diabetes. Diabetes Care 1 July 2009; 32 (7): 1327–1334.

American Diabetes Association; Standards of Medical Care in Diabetes—2011. Diabetes Care 1 January 2011; 34 (Supplement_1): S11–S61.

Kalyani R et al. Diagnosis and Treatment of Type 2 Diabetes in Adults: A Review, JAMA, published online June 23, 2025, doi:10.1001/jama.2025.5956