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Leann M Mikesh, David E Bruns, Stabilization of Glucose in Blood Specimens: Mechanism of Delay in Fluoride Inhibition of Glycolysis, Clinical Chemistry, Volume 54, Issue 5, 1 May 2008, Pages 930–932, https://doi.org/10.1373/clinchem.2007.102160
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To the Editor:
The recent report by Gambino(1) brought attention to the often overlooked fact that fluoride does not prevent loss of plasma glucose during the first 30–90 min (or longer) after blood collection(2). Although fluoride is effective in preventing later loss of glucose(1)(2), the mechanism of the delay in its action is a matter of some interest.
Fluoride acts primarily by inhibiting enolase in the glycolytic pathway. Fluoride strongly inhibits the enzyme in the presence of inorganic phosphate. The inhibitory species is the fluorophosphate ion, which when bound to magnesium forms a complex with enolase and inactivates the enzyme. The delay in fluoride’s prevention of glucose loss in blood samples is sometimes attributed to a postulated delay in the entry of fluoride ion into the blood cells in which the glycolytic enzymes reside. Several observations cast doubt on this explanation, however.
As part of a project for quality improvement of sample-handling requirements, we collected blood from a volunteer into four 5-mL Vacutainer (BD) tubes, 2 containing sodium fluoride/potassium oxalate, 10 mg/8 mg, and 2 with lithium heparin, 51 U, and plasma separating tube gel. The tubes were not centrifuged. After sample collection, 1 tube of each type was kept at room temperature, and another was put immediately into an ice water bath. The tubes were kept upright and mixed immediately before removing aliquots at 0, 15, 30, 45, 60, and 90 min. Each aliquot was centrifuged immediately for 1 min at 5585g in a microcentrifuge. The plasma was removed and lactate and glucose were measured on an Architect Analyzer (Abbott). The study was repeated on blood samples obtained from a second volunteer on a different day. The results for tubes at room temperature are summarized in Fig. 1 . In the tubes of blood stored in ice water, with or without fluoride, decreases in glucose and increases in lactate were ≤0.1 mmol/L even at 90 min (data omitted for clarity).
