Lactate and anion gap in sepsis

As a global marker of tissue oxygenation serum lactate measurement has proven its usefulness in monitoring the critically ill. It has also proved useful as a screening tool for sepsis in the emergency room. 

However, lactate measurement is not always readily available. Increased lactate is a common cause of raised anion gap, a much more readily available parameter derived by calculation from serum electrolyte results, and this has led to the suggestion that anion gap could serve as a surrogate measure of lactate concentration. 

This suggestion is tested in a recent study whose precise aim was to determine if anion gap could be used as a surrogate marker for abnormal lactate in patients admitted to the emergency room who are at risk of sepsis. During a 9-month period, 1419 adult patients admitted to the emergency department of a Boston hospital were recruited to the study. 

All patients had signs and symptoms suggestive of possible infection, i.e. they were at risk of sepsis. All had blood sampled on admission for lactate and electrolyte estimation, the latter allowing calculation of anion gap. Mean lactate of the study population was 2.1 mmol/L (SD 1.3) and mean AG was 11.8 (SD 3.6). For the purposes of the study lactate >4.0 mmol/L and anion gap >12 were defined as abnormal. 

Of the 1419 patients studied, 108 had an abnormal lactate (>4.0 mmol/L). Anion gap was abnormal (>12) in 86 of these patients. Thus raised anion gap was found to have a sensitivity of 80 % for predicting raised lactate. Of the remaining 1311 patients whose lactate was normal (<4.0 mmol/L), 900 also had a normal anion gap so that specificity of raised anion gap for predicting raised lactate was 69 %. 

Further analysis revealed that patients with a raised AG have a 7.3-fold increased risk of having a lactate of >4.0 mmol/L. The authors conclude that ‘AG is a good but not excellent screening test to help identify elevated lactate in an emergency department population at risk of sepsis’. 

They advise that in emergency-room patients with evidence of infection a raised anion gap should prompt the attending physician to order a lactate test, but if this is not available, aggressive resuscitative efforts, as required for patients with sepsis, should be considered.