Marked discrepancy in bicarbonate values explained – a case history

Plasma bicarbonate concentration (HCO₃^-), an essential parameter for the assessment of patient acid-base status, is routinely generated during blood gas analysis by calculation from measured pH and pCO₂. It is also directly measured by chemical analyzers in clinical laboratories that offer bicarbonate as one component of the urea and electrolyte (U&E) profile. 

Although the blood-gas-calculated bicarbonate concentration usually approximates fairly closely to the U&E-measured bicarbonate concentration (generally within 1-3 mmol/L), marked discrepancy does occasionally occur. In a recently published report the authors describe such a case that concerns a 72-year-old lady who was admitted to intensive care with multiple organ failure secondary to severe acute pancreatitis. 

The results of the U&E profile from this lady included a measured bicarbonate result of 16 mmol/L. Point-of-care arterial blood gas analysis at the same time revealed a calculated bicarbonate of just 6 mmol/L. Clinical details, including worsening metabolic acidosis (pH 7.01), suggested that the blood gas result (6 mmol/L) was more likely to be the correct result and that the measured bicarbonate (16 mmol/L) was artifactually raised. 

Further laboratory investigation described in this report confirmed this to be the case. It transpired that the falsely raised measured bicarbonate was due to positive interference in the enzymatic bicarbonate assay by lactate dehydrogenase (LD) present at high concentration in the patient’s serum; her serum LD was grossly raised, reaching a peak level of 14,200 IU/L within 24 hours of admission. 

The authors of this report caution that very high levels of LD can interfere with some enzyme methods used to measure bicarbonate in the laboratory, and that as a result of this interference clinically significant acidosis may be masked. The report provides evidence of one more mechanism that should be considered if there is discordance between calculated and measured bicarbonate.