The challenged urine bicarbonate excretion test in cystic fibrosis: A comprehensive analysis of urine acid/base parameters

Aim

Renal excretion of excess HCO₃⁻ depends on renal cystic fibrosis transmembrane conductance regulator (CFTR) and is impaired in people with cystic fibrosis (pwCF). Urine HCO₃⁻ excretion following oral NaHCO₃-loading may be a simple in vivo biomarker of CFTR function. In this study, we investigated changes in urine acid/base parameters following oral NaHCO₃-loading to comprehensively assess the physiological response to the test and evaluate HCO₃⁻ as the primary test result.

Methods

Urine acid/base parameters (titratable acid (TA), NH₄⁺, net acid excretion (NAE) and pH) were measured in bio-banked urine samples from controls (n = 10) and pwCF (n = 50) who completed the challenged urine HCO₃⁻ test. The association between urine acid/base excretion parameters and clinical CF disease characteristics and CFTR modulator therapy-induced changes were assessed.

Results

Before treatment, challenged urine acid/base excretion associated with important CF disease characteristics. TA excretion and NAE were lower in pwCF with residual function mutations, 7.9 and 16.6 mmol/3 h, respectively, and lower TA excretion and NAE associated with pancreatic sufficiency. A lower excretion of TA, NH₄⁺, and NAE associated with a higher percentage of predicted FEV₁ (1.3%, 2.5% and 0.8% per mmol/3 h higher, respectively). Modulator treatment decreased TA excretion and NAE (−2.9 and −5.3 mmol/3 h, respectively).

Conclusion

Following acute NaHCO₃-loading, increased base excretion is mirrored by decreased acid excretion. Urine HCO₃⁻ excretion sufficiently represents the additional urine acid/base parameters as test result. The observed changes in acid excretion support CFTR modulator-induced increase of CFTR-dependent type B intercalated cell HCO₃⁻ secretion and the use of the challenged urine HCO₃⁻ test as a possible CFTR-biomarker.