In short, base excess (BE) represents the requisite amount of base required to titrate a blood sample to physiologic pH (7.35 to 7.45). Prior to the advent of rapidly available lactate concentration measurements, providers utilized BE as a surrogate marker for build up of organic acids, especially lactic acid.
BE has been largely supplanted by the ready availability of lactate testing, and thus is skipped over by most emergency medicine providers and toxicologists. However, there may be a reason to take a closer look at this laboratory value, especially when assessing poisoned patients.
How the test is performed:
- Performed on standard blood gas analyzer
- In the old days machine dilutes blood to 5 g/dL and standardizes pCO2 to 40 mm HG (note that the reason for dilution to 5 g/dL is that this is thought to account for fluids in the extracellular space); however, now this is done via a series of calculations performed by the machine. Our shop uses the Radiometer ABL800 Flex. See Derranged Physiology’s image of the equation here.
- The sample is then titrated with either base or acid to a pH of 7.4
- Output values represent the number of mmol/L of base required to titrate the sample to a pH of 7.4
Interpreting the test:
- The more negative the result, the more base is required to normalize the sample; so, a more negative base deficit implicates that more acid is present in the sample
- A positive base excess suggests a metabolic alkalosis is present
Potential benefits
- Results are available rapidly, so if you are trending an acidosis you can have an answer in minutes versus having to wait about hour or more for the result of a BMP
- Can be rapidly used in lieu of DeWinters formula because the blood gas result gives you pCO2 and BE. So, you can rapidly identify whether there is a respiratory acidosis or alkalosis AND a metabolic acidosis or alkalosis.
Examples:
- A patient has a blood gas demonstrating a pH7.45, pCO2 of 20, BE of -20, and LA of 2.0. This patient has a respiratory alkalosis and a primary metabolic acidosis that is not likely attributable to a mild lactic acidosis. In a toxicologic setting, this patient’s blood gas is suggestive of salicylism.
- A patient has a blood gas demonstrating a pH7.25, pCO2 of 60, BE of +4, and LA of 1.0. This patient has a respiratory acidosis with likely metabolic compensation, which suggests that the patient has a a chronic respiratory acidosis.
- A patient has a blood gas demonstrating a pH7.51, pCO2 of 40, BE of +8, and LA of 1.5. This patient has a normal respiratory status and clearly has a primary metabolic alkalosis or may be on a sodium bicarbonate infusion.
Limitations:
- Does not identify species of acids contributing to the acid-base status of the patient
- Does not allow for deduction of mixed metabolic acidoses or combine metabolic acidosis/alkalosis; so, if the patient is compensating for a subacute acidosis, the BE may be higher.
- Does not function for patients with a HGb of less than 5 gm/dL
- Should be correlated with concomitant BMP’s to calculate the AG, other objective data (including vital signs), as well as the clinical history.
References:
Berend K. Diagnostic use of base excess in acid-base disorders. N Engl J Med. 2018;378(15):1419-1428.