Discuss the advantages and limitations of the anion gap in the evaluation of acid-base disturbance
Definition AG -a derived variable for the evaluation of metabolic acidosis to
determine the presence of unmeasured anions.
AG = [(Na + K)-(Cl + HC03)], normal reference range: 8-12 meq/ L
Utility: A raised AG is seen with elevated lactate, ketoacidosis, salicylates, alcohol
poisonings, and pyroglutamate
Advantages of the anion gap:
a) A simple measure to quantify and evaluate acid-base disturbance
b) Can be· easily done at the bedside
1) Reduced unmeasured anions such as hypoalbuminemia (frequently seen in critical
illness) will reduce the AG and may mask an elevated AG
2) UnmeasUred cations such as elevated Li and hyperglobulinemia will reduce AG.
3) Hypercalcemia and hypermagnesemia will also reduce the AG.
4) Calculation of AG involves measurement of electrolytes and therefore depends on
the accuracy of the measurement process.
To overcome the effects of the hypoalbuminemia on the AG, the corrected AG can be
used which is AG + (0.25 * (40-albumin) expressed in G/L
The article by Kraut and Nicolaos is an excellent longform answer to this question.
The question itself is not even a "critically evaluate" type of question. It asks simply for the advantages and limitations.
Thus, there they are.
Advantages of the anion gap
Limitations of the anion gap
EMMETT, MICHAEL, and ROBERT G. NARINS. "Clinical use of the anion gap."Medicine 56.1 (1977): 38-54.
Figge, James, et al. "Anion gap and hypoalbuminemia." Critical care medicine26.11 (1998): 1807-1810.
Salem, Mahmoud M., and Salim K. Mujais. "Gaps in the anion gap." Archives of internal medicine 152.8 (1992): 1625-1629.
Kraut, Jeffrey A., and Nicolaos E. Madias. "Serum anion gap: its uses and limitations in clinical medicine." Clinical Journal of the American Society of Nephrology 2.1 (2007): 162-174.