Fasting Plasma Glucose for Diabetes Diagnosis

Key Differences Between Plasma and Serum Glucose

The American Diabetes Association recommends using fasting plasma glucose over fasting serum glucose for diagnosing diabetes, as it is the validated method with established diagnostic thresholds 1, 2
Glucose concentrations are approximately 11% higher in plasma than in whole blood when hematocrit is normal, due to the higher water content in plasma 3, 4
Diagnostic criteria for diabetes are specifically established using plasma glucose measurements, not serum glucose, according to clinical guidelines 1, 2, 5

Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) is diagnostic of diabetes when confirmed by repeat testing, as recommended by the American Diabetes Association 1, 2, 5
Impaired fasting glucose is defined as fasting plasma glucose between 100-125 mg/dL (5.6-6.9 mmol/L) 2

Glycolysis prevention is crucial, and blood should be collected in tubes containing rapidly effective glycolytic inhibitors such as citrate buffer, as recommended by clinical guidelines 1, 3
If appropriate collection tubes are unavailable, the sample should be immediately placed in an ice-water slurry and centrifuged within 15-30 minutes 1

Analytical precision for glucose measurement should have analytical imprecision ≤2.4%, bias ≤2.1%, and total error ≤6.1%, according to quality standards 1
Plasma glucose should be measured in an accredited laboratory when used for diagnosis of or screening for diabetes, as recommended by clinical guidelines 1
Enzymatic methods (hexokinase or glucose oxidase) are used in virtually all analyses with good standardization across laboratories 3, 4

Using the wrong collection tube can lead to falsely low glucose values, and tubes without proper glycolysis inhibitors should be avoided 1, 3
Delayed processing can result in glycolysis and artificially lower glucose values, and samples should be processed promptly 3
Normal reference intervals (4.1-6.1 mmol/L or 74-110 mg/dL for adults) should not be confused with diagnostic criteria for diabetes, as recommended by clinical guidelines 4
Intraindividual variation in fasting plasma glucose can be 4.8-7.1%, which should be considered when interpreting results near diagnostic thresholds 4