Discordance between measured and calculated bicarbonate - a case study

Plasma bicarbonate concentration, a parameter generated during arterial blood gas analysis, is essential to the assessment of patient acid-base status. Blood gas analyzers do not have the capacity to directly measure bicarbonate; instead, it is calculated from measured pH and pCO₂(a), using the Henderson-Hasselbalch equation that relates all three parameters. 

Bicarbonate is also one component of the much more frequently requested urea and electrolyte (U&E) chemical profile. In this case bicarbonate is directly measured as total carbon dioxide. Since patients who require blood gases will frequently also require U&E, it is common for there to be two contemporaneous bicarbonate results on the same patient, one calculated and one measured. 

Usually there is clinically acceptable agreement between the two values, but it is by no means rare for the two results to be wholly discordant with values differing by as much as 10-15 mmol/L. A number of studies over the years have been directed at seeking to establish the frequency, magnitude and causes of discordance between measured and calculated bicarbonate. 

Of course, for the attending doctor faced with discordant results, the only pressing problem is: which of the two results is correct? The perhaps understandable assumption when faced with discordant results is that the measured bicarbonate result is more likely to be correct than a calculated value, but as a recently published case study demonstrates, such an assumption is misplaced. 

The case concerns a 74-year-old man whose hospital admission U&E profile results included reduced total CO₂ (bicarbonate) – 10 mmol/L, and consequent raised calculated anion gap. The assumed high anion gap metabolic acidosis that these results indicated fitted nicely with evidence of decreased renal function (raised urea and creatinine). 

However, venous blood gas analysis the next day revealed calculated bicarbonate within the reference range (24 mmol/L), inconsistent with the diagnosis of acidosis. During a 5-month stay in hospital (made necessary by diagnosis and treatment of cancer), this marked discrepancy between measured and calculated bicarbonate persisted. 

Mean of 107 measured bicarbonate results was 12.4 mmol/L (±2.7) and mean of 17 calculated bicarbonate results obtained during arterial blood gas analysis was 24.0 mmol/L (±0.9). Following investigation, it transpired that measured results were spuriously low due to interaction between an unidentified substance (possibly a paraprotein) in the patient’s serum and the reagents used in the enzymatic method employed to estimate total CO₂. 

In this case calculated bicarbonate (the blood gas result) was correct. In discussion of the case history the authors reflect on more general issues surrounding discordance between measured and calculated bicarbonate, including its many potential causes; an interesting, instructive case study report.