Is Serum Lactate Necessary in Patients With Normal Anion Gap and Serum Bicarbonate?

Daniel Aronovich, DO; Maykel Trotter, MD; Cynthia Rivera, MD; Michael Dalley, DO; David Farcy, MD; Michel Betancourt, MD; Lydia Howard, MD; Sharon Licciardi; Luigi Cubeddu, MD, PhD; Robert Goldszer, MD, MBA


Western J Emerg Med. 2015;16(3):364-366. 

In This Article

Abstract and Introduction


Introduction There has been an increase in patients having serum lactate drawn in emergency situations. The objective of this study was to determine whether or not it was necessary to obtain a lactate level in patients with a normal serum bicarbonate level and anion gap.

Methods This is a retrospective chart review evaluation of 304 patients who had serum lactate and electrolytes measured in an emergency setting in one academic medical center.

Results In 66 patients who had elevated serum lactate (>2.2mmol/L), 45 (68%) patients had normal serum bicarbonate (SB) (greater than 21 mmol/L). Normal anion gap (AG) (normal range <16 mEq/l) was found in 51 of the 66 patients (77%).

Conclusion We found that among patients with elevated serum lactate, 77% had a normal anion gap and 68% had normal serum bicarbonate. We conclude serum lactate should be drawn based on clinical suspicion of anaerobic tissue metabolism independent of serum bicarbonate or anion gap values.


A variety of laboratory parameters can help identify patients with severely compromised or strained metabolism. Among these are the anion gap (AG), serum bicarbonate (SB), pH, and serum lactate (SL) levels. There are two possible strategies for the diagnostic detection of lactic acidosis. The first strategy is to order a lactate level upon any clinical suspicion of acidosis. The second strategy is to order routine chemistry and then if there is abnormality order follow up tests such as a serum lactate.

While the presence or absence of an AG has classically been used as a screening tool for lactic acidosis, there are some potential problems with this stepwise strategy.[1] Firstly, it has been recently suggestedthat the upper limit of a "normal" AG should be lowered to six because of a technological change in the process that measures electrolyte concentrations.[1] This is currently not accepted. Using a lower AG threshold would increase the number of subsequently ordered lactates. Secondly, lactic acidosis is a marker of life-threatening illness, and any delay between recognizing an increased AG level and then ordering and confirming a lactate level may add unnecessary risk to the patient. In one retrospective cohort study, Adams et al. evaluated all emergency department (ED) patients seen over a seven-month period in whom a lactate level was measured for any reason. The authors considered an AG >12 abnormal and conducted sensitivity analyses of the AG for detecting the presence of a lactate >2.5mmol/L. The AG was 52.8% sensitive, 81.0% specific, and with a negative predictive value of 89.7% for lactic acidosis.[1] Critically ill patients have impaired acid-base regulation and are thought to generate more unmeasured cations, such as magnesium and calcium, thereby affecting the AG. Furthermore, hypo-albuminemia affects the AG and is also prevalent in the ED population.[1,2] From these prior studies, it appears that the AG cannot be considered a surrogate for lactate testing.