Laboratory Tests: Blood Gases, Anion Gap, and Strong Ion Gap

Blood gas measurements provide the intensivist with diagnostic information on many organ systems. The anion gap (AG) and the strong ion gap (SIG) exploit the principle of electroneutrality to quantify the net balance of unmeasured ions in plasma. The AG should be corrected for abnormalities in plasma albumin and phosphate concentrations. The AG and SIG can be used to narrow the differential diagnosis of acid-base disorders, and an increased corrected AG or SIG is diagnostic of a metabolic acidosis (i.e., an acidifying process) irrespective of plasma pH or bicarbonate. The SIG is often perceived as more complex but frequently yields more precise results in critically ill patients. In addition, the strong ion model can be used to guide fluid management because it acknowledges that electrolyte changes are causal mechanisms of acid-base disorders.