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Background: The concentration of plasma glucose is influenced by preanalytical variables such as delayed processing of whole blood. Glucose concentrations decrease by 5-7% per hour in whole blood due to glycolysis. This preanalytical loss decreases the diagnostic sensitivity of glucose when used to diagnose diabetes mellitus or gestational diabetes mellitus. Sodium fluoride (NaF) is commonly used to inhibit glycolysis in samples collected during oral glucose tolerance tests, but is inadequate as it does not stop glycolysis for two or more hours. Guidelines recommend blood samples be immediately immersed in an ice slurry and analyzed within 30 minutes of collection, but this is difficult to achieve in patient care settings. The objective of this study was to identify preferred processing methods for the preservation of glucose when samples are collected for oral glucose tolerance and postprandial glucose testing.

Methods: Whole blood samples were collected from three apparently healthy volunteers. One NaF tube, and five sodium-heparin gel separator tubes were collected from each volunteer. The NaF tube and three sodium-heparin tubes were kept at ambient temperature. Two sodium-heparin tubes were placed in an ice slurry immediately following collection. Within 10 minutes of collection, four 1 mL aliquots of whole blood from the NaF tube were incubated at ambient temperature for either 10, 60, 120, or 180 minutes after which the plasma was removed and stored at 4°C. The three sodium-heparin tubes at ambient temperature were centrifuged 10, 30, and 60 minutes after collection and stored at 4°C. The two sodium-heparin tubes on ice were centrifuged and processed similarly 30 and 60 minutes after collection. Plasma samples were analyzed in duplicate for glucose using the same Dimension Vista (Siemens Healthineers) chemistry analyzer and the mean result calculated. The glucose concentration in the samples centrifuged 10 minutes after collection were considered to be the baseline result. Absolute and relative differences to baseline were determined for all other timed samples. A total allowable error of 6 mg/dL or 10% was defined as the quality goal. The study was approved by the WCG Institutional Review Board.

Results: Glucose results in sodium-heparin tubes from one volunteer were excluded due to mechanical error during centrifugation of the samples. Samples collected in NaF tubes had mean differences in glucose concentrations of -4.7, -11, and -10 mg/dL (-4, -10.2, and -9.4%) at 60, 120, and 180 minutes, respectively. Ambient temperature sodium-heparin samples had mean differences in glucose concentration of -4, and -5.5 mg/dL (-3.4 and -4.7%) at 30 and 60 minutes, respectively. Plasma from sodium-heparin samples kept on ice had mean differences in glucose concentration of -3.5 and -1.5 mg/dL (-2.8 and -1.4%) at 30 and 60 minutes, respectively.

Conclusion: The decrease in glucose exceeded the allowable error 120 minutes after collection in NaF tubes. While glucose decreased in samples collected in sodium-heparin tubes, the allowable error was never exceeded in samples stored at ambient temperature or on ice. Whole blood collected into plasma separator tubes and processed within 60 minutes yields acceptable glucose results for oral glucose tolerance testing.

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