A re-evaluation of the urinary parameters of acid population and excretion in patients with chronic renal acidosis

Abstract

We have studied acid-base balance in 32 patients attending the renal clinic of Mount Sinai Hospital. The parameters of acid-base balance measured included acid production measured as urinary sulfate and organic anions, net acid excretion measured as urinary ammonia plus titratable acid minus bicarbonate, and net GI absorption of alkali measured by a new method utilizing urinary electrolytes. Net GI absorption of alkali by urinary electrolytes measures alkali addition to the body from the GI tract as well as from any other sources, including bone. All patients had a creatinine clearance less than 80 ml/min and they were divided into two groups: those with normal serum bicarbonate (Group 1; N = 12) and those with subnormal serum bicarbonate (Group 2; N = 20). Hydrogen ion balance was -0.6 ± 9 mEq/day in the first group, while those in the second group had a hydrogen ion balance of +16 ± 5 mEq/day. A group of 8 normal controls had a hydrogen ion balance of -0.3 ± 5 mEq/day. When the sum of all cations was compared with the sum of all anions in the urine, a cation gap of exactly the same magnitude as the positive hydrogen ion balance was found in patients with low serum bicarbonate. In conclusion, our data show that patients with decreased GFR and low serum bicarbonate appear to have a significantly positive hydrogen ion balance. However, we believe that the positive hydrogen ion balance is only apparent, but not real for the following reasons. If the positive hydrogen balance shown by the previous investigators were due to bone buffering, the positive balance should have disappeared in our study since we included the contribution of bone buffers in the calculation of acid-base balance. Moreover, the positive hydrogen ion balance in our study would indicate the presence of a cation gap, a theoretical impossibility. The most likely explanation for the apparently positive hydrogen ion balance, shown only in the low bicarbonate group, is a systematic technical error in the current methods for measurement of parameters of acid-base balance, either an underestimation of net acid excretion or an overestimation of acid production. Further studies are needed to delineate the sources of technical errors.