A calculation of the anion gap is an essential part of practically every ABG interpretation question in the CICM fellowship exam. A profound understanding of the anion gap as a diagnostic tool is not an essential part of answering those questions. Only a crude workmanlike grasp is called for.
The anion gap is a mathematical test performed on laboratory electrolyte values to determine the cause of a metabolic acidosis. It is defined as the sum of serum anion concentrations subtracted from the serum cation concentrations. Specifically, by convention the cations are sodium and potassium, and the anions are bicarbonate and chloride. These measured anions account for only about 85% of the total anionic charge of the extracellular fluid. The gap affords us an estimate of the concentration of this unmeasured 15%, the "miscellaneous" anionic electrolytes which are present in the bloodstream. The "normal" expected value is around 12, though it is adjusted for the serum albumin, which - being a negatively charged protein - contributes to the anionic charge of the extracellular fluid.
What follows is a brief summary. A more detailed discussion of the merits and demerits of the anion gap is available elsewhere. Question 23 from the first paper of 2007 had asked for this sort of discussion as an answer, but the college has subsequently lost interest in such longform essays, because short ABG interpretation questions are substantially easier to mark. It is unlikely that the candidates will ever again be asked to hold forth extensively on this topic.
The anion gap represents several anionic species which could increase the numerical gap while not producing very much acidosis. Such situations would include:
Question 3.1 from the second paper of 2013 featured a negative anion gap (-2).
Figge, James, et al. "Anion gap and hypoalbuminemia." Critical care medicine 26.11 (1998): 1807-1810.
EMMETT, MICHAEL, and ROBERT G. NARINS. "Clinical use of the anion gap."Medicine 56.1 (1977): 38-54.
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.