Assessing urine ammonium concentration by urine osmolal gap in chronic kidney disease

Acidemia is one of the risk factors for end-stage kidney disease and increases the mortality rate of patients with chronic kidney disease (CKD). Although urinary ammonium (U-NH₄ ⁺ ) is the crucial component of renal acid excretion, U-NH₄ ⁺ concentration is not routinely measured. To estimate U-NH₄ ⁺ , urine osmolal gap (UOG = urine osmolality - [2(Na⁺ + K⁺ ) + urea + glucose]) is calculated and the formula (U-NH₄ ⁺  = UOG/2) has traditionally been used. However, the usefulness of this formula is controversial in CKD patients. We assessed the relationship between U-NH₄ ⁺ and UOG in patients with CKD. Blood and spot urine samples were collected in 36 patients who had non-dialysis-dependent CKD. The mean ± SD age of patients was 72.0 ± 14.8 years, and the mean ± SD serum creatinine and U-NH₄ ⁺ were 2.7 ± 2.3 mg/dl and 9.3 ± 9.2 mmol/L, respectively. A significant relationship was found between UOG/2 and U-NH₄ ⁺ (r = .925, p < .0001). U-NH₄ ⁺ estimated using the UOG was on average higher by 4.7 mmol/L than the measured one. Our results suggested that UOG could be a useful tool in clinical settings, especially in patients with moderate to severe CKD.