Tuesday, February 9, 2021

Anion gap

 Q: The normal anion gap with use of ion-selective electrodes is? (select one)

A) 12 +/- 4 mEq/L 

B) 8 +/- 4 mEq/L 

C) 6 +/- 3 mEq/L

Answer: C

The objective of this question is to introduce to the readers that the anion gap value largely depends on lab instruments. It is even more relevant than adjusting for albumin level. Although, this is true that traditionally and still 12 +/- 4 mEq/L has been used as a normal anion gap but labs now mostly use ion-selective electrodes for measurement. The formula for the anion gap is Serum AG = Na - (Cl + HCO3), and this method reports higher serum chloride concentrations. 

It has been more than ever important to know the method and normal range reported by the specific lab.



1. Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved? Clin J Am Soc Nephrol 2013; 8:2018. 

2. Jurado RL, del Rio C, Nassar G, et al. Low anion gap. South Med J 1998; 91:624. 

3. Winter SD, Pearson JR, Gabow PA, et al. The fall of the serum anion gap. Arch Intern Med 1990; 150:311. 

4. Paulson WD, Roberts WL, Lurie AA, et al. Wide variation in serum anion gap measurements by chemistry analyzers. Am J Clin Pathol 1998; 110:735. 

5. Pratumvinit B, Lam L, Kongruttanachok N, et al. Anion gap reference intervals show instrument dependence and weak correlation with albumin levels. Clin Chim Acta 2020; 500:172.

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