Blood samples collected for plasma glucose estimation are mixed with sodium fluoride. Why does fluoride prevent the falsely low glucose values that would otherwise develop during transport to the laboratory?

• A It inhibits hexokinase, preventing glucose entry into erythrocytes
• B It chelates magnesium ions required by all glycolytic kinases
• C It inhibits enolase, blocking glycolysis beyond the 2-phosphoglycerate step ✓
• D It denatures pyruvate kinase irreversibly

Explanation

Fluoride ion inhibits enolase by binding its essential magnesium cofactor, converting 2-phosphoglycerate metabolism to a halt at this step. This stops continued glycolytic consumption of glucose by erythrocytes and leukocytes in the tube, preventing spurious lowering of measured glucose. It does not act on hexokinase, does not broadly chelate magnesium at kinase steps, and is not an irreversible denaturant.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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