#2534 Oral acid load for distal tubular acidosis diagnosis: are urinary ratio useful?

Abstract Background and Aims The acid-base balance plays a fundamental role in the homeostasis of the human body, regulated in part by the kidneys. Any disturbance in renal function will lead to an imbalance in the acid-base homeostasis leading to renal tubular acidosis. Distal renal tubular acidosis (RTA type 1) is defined by a decrease in proton excretion into the urine via the H+ ATPase transporter. Several tests are available to diagnose RTA like the oral acid load charge. Traditionally, ammoniuria and titratable acidity standards are expressed in mol/min/1.73 m². The aim of our research is to propose the urinary standards ratio to improve RTA diagnosis. Method We report a single-centre retrospective study. All patients who underwent an oral acid loads performed in the renal functional exploration department of the Edouard Herriot Hospital, HCL, France from May 2010 to December 2024 were included. Patients were admitted for a pre-test blood work-up, including blood and urine tests (at baseline and 1-hour after). The acid loading test was performed using oral ammonium chloride capsules at a dose of 1 mg/kg over 3 to 5 days before returning for post-test analyses. Good adaptation to the test is defined by a minimum urine pH (pH min) <5.5 and ammoniuria >39 µmol/min/1.73 m². The aim of the study was to establish mean urinary NH4/creatinine and urinary AT/creatinine ratios in patients with and without urinary acidification disorders. Results Fifty-three patients were included in our study; 30 patients were considered with an adapted response to acid load and 23 patients were considered with a non-adapted response. The classification of our patients between the adapted and non-adapted group seems correct with a higher ammoniuria in the adapted group after the test (before 31.92 ± 24.74 µmol/min/1.73 m², after in the adapted group 75.82 ± 36.20 µmol/min/1.73 m² and after in the non-adapted group 49.74 ± 38.90 µmol/min/1.73 m², p-value <0.05) and a higher AT in the adapted group after the test (before 32.23 ± 32.10 µmol/min/1.73 m², after in the adapted group 23.23 ± 10.09 µmol/min/1.73 m² and after in the non-adapted group 18.32 ± 7.51 µmol/min/1.73 m², p-value <0.05). There were no significant differences of AT/creatinine and NH4/creatinine ratio between the adapted and the non-adapted group after the oral acid load test for UT (first urinary sample) and U1 (second urinary sample one hour after the first one) except for the NH4/creatinine U1 ratio (6.43 ± 2.21 in the adapted group versus 4.86 ± 3.74 in the non-adapted group, p-value = 0.021). Conclusion Our study did not show any contribution of AT/creatinine and NH4/creatinine ratio in the interpretation of the acid load test. Renal acidification problems are still rare in France, making them difficult to study. The idea of ratio standards should continue to be explored with a larger population in order to gain statistical power.