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Background and Aims Distal renal tubular acidosis (DRTA) is characterized by hyperchloremic metabolic acidosis, with normal anion gap and with the inability to acidify the urine to a pH lower than 5.3. DRTA can evolve without symptoms and systemic acidosis, this form being defined as incomplete DRTA, that necessitates the use of a urinary acidification test like the Furosemide and Fludrocortisone test for establishing the diagnosis. There are several cases of type 1 DRTA reported in patients with autoimmune diseases, especially Sjögren syndrome and systemic lupus erythematosus (SLE), most of them being diagnosed due to severe symptoms caused by the associated hypokalemia. The prevalence of the DRTA in patients with autoimmune diseases is unknown, especially if we refer to the incomplete form. Method We conducted an observational prospective study in a selected cohort of 21 patients diagnosed with autoimmune diseases, who presented for evaluation in our clinic during December, 2020 and January, 2021. We included patients diagnosed with SLE, Sjögren syndrome, ANCA vasculitis, cryoglobulinemic vasculitis, who were submitted to Furosemide/Fludrocortisone test in our nephrology department. The urinary pH was evaluated hourly for 6 hours after the test began.  The test was considered positive if the urinary pH did not decrease &amp;lt; 5.3. Results The study included 21 patients (16 females, 5 males, mean age 41.52 ± 17.58 years), diagnosed with SLE (13 patients, mean age 30.23 ± 10.34 years, eGFR 81.61±20.39 ml/min/1.73 m2), pANCA vasculitis (6 patients, mean age 60.83 ± 6.14, eGFR 40 ± 12.64 ml/min/1.73 m2), Sjogren syndrome (one 44 year-old patient, eGFR 39 ml/min/1.73 m2) and cryoglobulinemic vasculitis (one 69 year-old patient, eGFR 31 ml/min/1.73 m2). The test was positive for 4 patients out of 21 (3 females, one male; one with SLE, one with pANCA vasculitis, one with Sjogren syndrome and one with cryoglobulinemic vasculitis). Although 2 patients developed hypokalemia defined as a level of serum potassium lower than 3.5 mmol/l after the test and 1 patient augmented previous hypokalemia, there was not a significant change in kalemia (3.93 ± 0.32 mmol/l before the test vs 3.95 ± 0.49 mmol/l after the test, p=0.835). Although none of the patients developed metabolic alkalosis after the test, there was a significant increase in the level of serum bicarbonate (26.6 (2.2) mmol/l before the test vs 28.2 (2.7) mmol/l after the test, p=0.005) and also in the level of serum pH (7.36 ± 0.04 before the test vs 7.38 ± 0.04 after the test, p=0.018). None of the patients reported digestive or allergic side effects. It was interesting that the patients with vasculitis responded with delay to the treatment and urinary acidification under the pH of 5.3 occurred after a mean period of 3.2 hours in comparison to 1.5 hours in patients with SLE (p=0.014). Regarding the histological data, both the patients with vasculitis were elders, with an altered kidney function (both with a eGFR of 31 ml/min/1.73 m2) and severe tubular atrophy and interstitial fibrosis on kidney biopsy. The female patient with cryoglobulinemic vasculitis also had positive titers for antinuclear antibody, anti Ro-antibodies and anti-La antibodies. The patient with Sjögren syndrome was diagnosed with nephrocalcinosis and the kidney biopsy was not effectuated. The youngest patient with a positive test had preserved renal function, without tubular or interstitial lesions on kidney biopsy, but with a pattern of membranous lupus nephritis and with intense immunological activity (ANA, anti ds-DNA antibodies, anti RNP and Sm antibodies, antiphospholipid syndrome). Conclusion Incomplete DRTA was found in 4 out of 21 patients with autoimmune diseases, one with Sjogren syndrome and nephrocalcinosis, two with pANCA and cryoglobulinemic vasculitis with decreased eGFR and severe tubular atrophy and interstitial fibrosis and one young female with SLE.

MO003DISTAL RENAL TUBULAR ACIDOSIS IN PATIENTS WITH AUTOIMMUNE DISEASES