Can the anion gap be used to identify patients with lactic acidosis?

Measurement of blood lactate provides clinicians with a valuable means of identifying and monitoring critically ill patients who are suffering clinical shock as a result of severe trauma, sepsis or cardiogenic shock. Lactic acid accumulates in the blood of these patients due to the tissue hypoxia that results from inadequate perfusion. 

Early recognition of lactic acidosis in the critically ill is important for a favorable outcome. Rather than proceed directly to measurement of blood lactate in all patients suspected of suffering lactic acidosis, an alternative approach is to first calculate the anion gap using plasma electrolyte (sodium, chloride and bicarbonate) results and only proceed to lactate measurement if the calculated anion gap is elevated. 

This approach is of course only valid if the anion gap is a reliable screening test for lactic acidosis; a proposition that was tested in a recently published retrospective study of emergency department patients.

The authors of this study revisited the case notes of all 440 patients admitted to their emergency department during a seven-month period, who had blood lactate measured. Those whose case notes indicated they had been suffering any condition other than lactic acidosis that would cause an increased anion gap (e.g. ketoacidosis) were excluded. 

Patients were also excluded if blood for sodium, chloride and bicarbonate (the parameters required for calculation of anion gap) was sampled more than 60 minute apart from the blood used to measure lactate. After exclusion there were 303 study patients each with a lactate result and contemporaneous biochemical data required for calculation of anion gap.

Of the 303 patients, lactate measurement had revealed lactic acidosis in just 47 (15.5 %). Using an anion gap cut-off of >12, the sensitivity of detecting lactic acidosis was 58.2 %; specificity was 81.0 % and negative predictive value 89.7 %. 

Using an anion gap cut-off of >6 greatly improved the sensitivity (93.2 %) of detecting lactic acidosis, but this improvement was at the expense of specificity, which was only 17.2 %. The authors conclude that using anion gap to screen for lactic acidosis "may be inappropriate in emergency department patients". They recommend that all patients suspected of suffering lactic acidosis should have blood lactate measured.