Question

Why sodium flouride is added as a preservative while collecting blood samples for estimation of glucose levels?

• A. Fluoride ions block citric acid cycle.
• B. Fluoride ions inhibit enolase, which is an enzyme of glycolysis and thus stops metabolism of glucose in blood cells.
• C. Fluoride ions act as good anticoagulants.
• D. Fluoride ions do not allow bacterial growth to occur in blood collected for glucose estimation.

Hint:

Remember the gray-top tube biochemistry:
Fluoride = Antiglycolytic agent (inhibits enolase).
Oxalate = Anticoagulant (precipitates calcium).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
When blood is collected for clinical biochemistry analysis, living cells (such as erythrocytes and leukocytes) continue to consume nutrients.
If glucose levels are to be measured accurately, this ongoing cellular metabolism must be stopped immediately after collection to prevent false low readings.

Step 2: Detailed Explanation:
In an untreated blood sample, red blood cells consume glucose via anaerobic glycolysis at a rate of approximately $5\%$ to $7\%$ per hour.
To prevent this, sodium fluoride (NaF) is added to blood collection tubes (typically gray-top tubes).
The fluoride ion ($F^{-}$) acts as a potent inhibitor of the glycolytic pathway.
Specifically, fluoride binds with magnesium and phosphate to form a fluorophosphate complex.
This complex coordinates to the active site of the enzyme enolase, displacing the essential magnesium ($Mg^{2+}$) cofactor.
Enolase catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate.
By inhibiting enolase, glycolysis is halted, preserving the concentration of glucose in the tube for accurate measurement.
Sodium fluoride is often paired with an anticoagulant like potassium oxalate, which prevents clotting by chelating calcium ions.
While fluoride has mild antimicrobial properties, its primary purpose in glucose vials is metabolic preservation through enolase inhibition.

Step 3: Final Answer:
Sodium fluoride preserves blood glucose levels by inhibiting enolase, which stops glycolysis in red blood cells.