Fasting Plasma Glucose versus Fasting Serum Glucose for Diabetes Diagnosis
For diagnosing diabetes, fasting plasma glucose is the recommended standard over fasting serum glucose, as it is the validated method specified in clinical guidelines with established diagnostic thresholds. 1
Key Differences Between Plasma and Serum Glucose
Sample Preparation: Plasma is obtained from blood collected in tubes containing anticoagulants, while serum is obtained after blood has clotted and been centrifuged without anticoagulants 1
Glucose Concentration: Glucose concentrations are approximately 11% higher in plasma than in whole blood when hematocrit is normal, due to the higher water content in plasma 1
Standardization: Diagnostic criteria for diabetes are specifically established using plasma glucose measurements, not serum glucose 1
Stability: Recent studies show slight differences between serum and plasma glucose values, with some reporting plasma glucose values approximately 0.2-0.9% higher than serum 1
Diagnostic Criteria Using Plasma Glucose
Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) is diagnostic of diabetes when confirmed by repeat testing 1
Impaired fasting glucose is defined as fasting plasma glucose between 100-125 mg/dL (5.6-6.9 mmol/L) 1, 2
For accurate diagnosis, blood for fasting plasma glucose should be drawn after at least 8 hours of fasting, preferably in the morning 1
Clinical Considerations for Sample Collection
Glycolysis prevention is crucial: To minimize glycolysis, blood should be collected in tubes containing rapidly effective glycolytic inhibitors such as citrate buffer 1
If appropriate collection tubes are unavailable: The sample should be immediately placed in an ice-water slurry and centrifuged within 15-30 minutes 1
Avoid relying on sodium fluoride alone: Tubes with only enolase inhibitors such as sodium fluoride should not be relied upon to prevent glycolysis 1
Quality Standards for Testing
Analytical precision: Based on biological variation, glucose measurement should have analytical imprecision ≤2.4%, bias ≤2.1%, and total error ≤6.1% 1
Laboratory setting: Plasma glucose should be measured in an accredited laboratory when used for diagnosis of or screening for diabetes 1
Standardization: Enzymatic methods (hexokinase or glucose oxidase) are used in virtually all analyses with good standardization across laboratories 1
Common Pitfalls to Avoid
Using the wrong collection tube: Using tubes without proper glycolysis inhibitors can lead to falsely low glucose values 1
Delayed processing: Failure to process samples promptly can result in glycolysis and artificially lower glucose values 1
Misinterpreting reference intervals: 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 1
Ignoring biological variation: Intraindividual variation in fasting plasma glucose can be 4.8-7.1%, which should be considered when interpreting results near diagnostic thresholds 1
Conclusion
When diagnosing diabetes, fasting plasma glucose is the established standard with clear diagnostic thresholds. While serum glucose measurements exist, clinical guidelines specifically recommend venous plasma for diagnosis, with proper attention to pre-analytical factors to ensure accurate results.