Determination of urinary bicarbonate with the Henderson-Hasselbalch equation

Abstract

The determination of urinary bicarbonate with the Henderson-Hasselbalch equation was compared using two methods: (1) correcting the pK in every urine sample according to ionic strength and using the solubility constant of CO₂ in urine (α=0.0309) and (2) using a fixed pK value (6.1) and a CO₂ solubility constant of 0.0301, which we use to calculate blood bicarbonate. Nine patients were studied and 29 determinations were performed. A high correlation was found between the methods ( r =0.99). Bicarbonate calculated with corrected pK was 24.3±6.6 mEq/l (95% confidence interval 11.4–37.2) and bicarbonate calculated with pK fixed at 6.1 was 25.6±6.6 mEq/l (95% confidence interval 12.7–38.5). For each urine sample, the Δ bicarbonate was calculated as the difference between the bicarbonate obtained with pK at 6.1 minus that obtained with the corrected pK (mean 1.25, standard error 0.83, P =0.15). This indicates that the difference between the methods was not significant. No difference was found whether pK was corrected or fixed (6.1). Therefore, our results suggest that it is valid to take the value shown by the equipment for blood gas determination as the urinary bicarbonate value. This would allow the rapid and accurate determination of urinary bicarbonate in patients with hyperchloremic acidosis, especially those with renal tubular acidosis.