How can glucose stability be maintained in a blood sample from a patient with diabetes or impaired glucose regulation?

Glucose Stability in Blood Samples

Use blood collection tubes containing citrate buffer, sodium fluoride, and EDTA (citrate/fluoride/EDTA tubes) to maintain glucose stability in blood samples, as these tubes immediately block glycolytic enzymes through acidification and prevent glucose degradation far more effectively than fluoride-only tubes. 1

The Problem with Glycolysis

Glucose degrades rapidly in blood samples through glycolysis at an average rate of 5-7% (approximately 0.6 mmol/L or 10 mg/dL) per hour, with variation based on glucose concentration, temperature, white blood cell count, and other factors. 1 This degradation leads to falsely low glucose readings and can result in missed diagnoses of diabetes, particularly in patients with glucose concentrations near diagnostic cut points. 1

Why Traditional Fluoride Tubes Are Inadequate

Sodium fluoride tubes alone should NOT be relied upon to prevent short-term glycolysis. 1 Although sodium fluoride (2.5 mg/mL blood) inhibits the enzyme enolase and provides long-term stability, it has critical limitations:

• Glucose decline rates in the first hour are virtually identical in tubes with and without fluoride 1
• Glycolysis continues for up to 4 hours in fluoride-only tubes 1
• After 4 hours, glucose becomes stable for 72 hours at room temperature 1
• High white blood cell counts increase glycolysis even with fluoride present 1

Optimal Collection Methods

First Choice: Citrate/Fluoride/EDTA Tubes

Citrate-containing tubes (citrate buffer + sodium fluoride + EDTA) provide superior glucose stabilization through immediate acidification (pH 5.3-5.9) that blocks glycolytic enzymes. 1 Performance data shows:

• Only 0.3% glucose decrease at 2 hours when stored at 37°C 1
• Only 1.2% glucose decrease at 24 hours at 37°C 1
• Minimal glycolysis (<3.8%) by 24 hours at room temperature 2

These tubes are commercially available in Europe but were not available in the U.S. at the time of the 2023 guidelines, though manufacturers are strongly encouraged to make them available worldwide. 1

Alternative Methods When Citrate Tubes Unavailable

If citrate/fluoride/EDTA tubes are not available, use one of these two classic approaches:

Option 1 - Immediate Separation: 1

• Immediately separate plasma from blood cells after collection
• Glucose remains stable for 8 hours at 25°C in separated, non-hemolyzed serum without fluoride 1
• Glucose remains stable for 72 hours at 4°C in separated serum 1

Option 2 - Ice-Water Slurry: 1

• Place blood tube immediately in ice-water slurry after collection
• Separate plasma from cells within 30 minutes 1
• This method inhibits glycolysis through cold temperature (4°C) 2

Important Clinical Considerations

Sample Type

Plasma is recommended for diagnosis over whole blood or serum. 1 Plasma glucose concentrations are approximately 11% higher than whole blood values when hematocrit is normal, because water concentration in plasma is 11% higher than in whole blood. 1

Centrifugation Advantage

When comparing collection methods in field studies, promptly centrifuged samples (using clot activator and gel separator tubes, centrifuged 20 minutes after collection) showed glucose readings 0.196 mmol/L (4.2%) higher than sodium fluoride tubes when both were assayed 2-4 hours later. 3 This demonstrates that even with fluoride, significant glycolysis occurs before the inhibitor becomes fully effective. 3

Common Pitfalls to Avoid

• Do not rely on fluoride-only tubes for samples that cannot be processed within 4 hours 1
• Do not use EDTA-only tubes - these show 85% glycolysis by 24 hours 2
• Avoid delays in processing - the traditional methods (immediate separation or ice-water slurry) are often impractical and not widely used in routine practice 1
• Be aware of leukocytosis - very high white cell counts increase glycolysis even with fluoride present 1

Diagnostic Implications

The widespread adoption of citrate-containing tubes will likely increase diabetes detection rates while decreasing cases of artifactual hypoglycemia. 1 This is because elimination of glycolysis substantially reduces measurement variability attributable to inconsistent sample handling in both routine care and research studies. 1