AG with K

Q: If potassium is added/counted in Anion gap (AG) formula, the anion gap will? (select one)

A) decrease

B) increase

Answer: B

The objective of this question is to highlight the fact that each laboratory has its own value of normal AG, depending on the method they use. Labs which use ion-selective electrodes usually have lower range of AG, somewhere at 6±3 mEq/L. This is due to the fact that ion-selective electrodes counts higher serum chloride concentrations leading to lower value of AG.

Many labs now routinely use potassium in AG calculation with the formula: (Na + K) - (Cl + HCO3). This increases the AG on average by 4 mEq/L.

#acid-base

#electrolytes

References:

1. Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007; 2:162.

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

3. 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.

4. 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.