Furosemide/Fludrocortisone Test and Clinical Parameters to Diagnose Incomplete Distal Renal Tubular Acidosis in Kidney Stone Formers

Background: Distal renal tubular acidosis (dRTA) is characterized by an impairment of urinary acidification resulting in metabolic acidosis, hypokalemia, and inappropriately elevated urine pH. If not treated, this chronic condition eventually leads to nephrocalcinosis, nephrolithiasis, impaired renal function, and bone demineralization. dRTA is a well-defined entity that can be diagnosed by genetic testing of 5 genes known to be disease-causative. Incomplete dRTA (idRTA) is defined as impaired urinary acidification that does not lead to overt metabolic acidosis and therefore can be diagnosed if patients fail to adequately acidify urine after an ammonium chloride (NH4Cl) challenge or furosemide and fludrocortisone test. It is still uncertain whether idRTA represents a distinct entity or is part of the dRTA spectrum and whether it is caused by mutations in the same genes of overt dRTA. Methods: In this cross-sectional study, we investigated a group of 22 stone formers whose clinical features were suspicious of idRTA. They underwent an NH4Cl challenge and were found to have impaired urinary acidification ability. These patients were then analyzed by genetic testing with sequencing of 5 genes: SLC4A1, ATP6V1B1, ATP6V0A4, FOXI1, and WDR72. Results: Two unrelated individuals were found to have two different variants in SLC4A1 that had never been described before. Conclusions: Our results suggest the involvement of other genes or nongenetic tubular dysfunction in the pathogenesis of idRTA in stone formers. However, genetic testing may represent a cost-effective tool to recognize, treat, and prevent complications in these patients.

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P0058THE ROLE OF GENETICS IN INCOMPLETE DISTAL RENAL TUBULAR ACIDOSIS IN NEPHROLITHIASIS

Nephrology Dialysis Transplantation ◽

10.1093/ndt/gfaa142.p0058 ◽

2020 ◽

Vol 35 (Supplement_3) ◽

Author(s):

Viola D'Ambrosio ◽

Eugenio Sangiorgi ◽

Bernhard Hess ◽

Giovanni Gambaro ◽

Pietro Manuel Ferraro

Keyword(s):

Metabolic Acidosis ◽

Genetic Testing ◽

Family History ◽

Renal Tubular Acidosis ◽

Kidney Stones ◽

Distal Renal Tubular Acidosis ◽

Urinary Acidification ◽

Stone Formers ◽

History Of ◽

Renal Tubular

Abstract Background and Aims Distal renal tubular acidosis (dRTA) is characterized by an impairment of urinary acidification resulting in a hyperchloremic non-anion gap metabolic acidosis, hypokalemia and inappropriately elevated urine pH. If not treated, this chronic condition eventually leads to nephrocalcinosis and recurrent nephrolithiasis, impaired renal function and bone demineralization due to reabsorption of bicarbonate and phosphate complexed with calcium from the bone as a buffer for metabolic acidosis. Distal RTA is therefore a well-defined entity that can be diagnosed by genetic testing of five genes known to be disease-causative (ATPV1B1 and ATPV0A4, FOXi1, SLC4A1 and WDR72). Incomplete distal renal tubular acidosis (idRTA) is defined as impaired urinary acidification that does not lead to overt metabolic acidosis and therefore can be diagnosed if patients fail to adequately acidify urine after an ammonium chloride (NH4Cl) challenge or other acid load tests. It is observed in 10-20% of calcium stone formers. It is still uncertain whether idRTA represents a distinct entity or it is part of the dRTA spectrum and whether it is caused by mutations in the same genes of overt dRTA. Heterozygous ATP6V1B1 pathogenic variants have been linked to idRTA, as well as mutations in SLC4A1. Method In this cross-sectional study we investigated a group of 23 stone formers whose clinical features were suspicious of idRTA: a history of nephrolithiasis or nephrocalcinosis with morning urinary pH > 5.8 in the absence of overt metabolic acidosis. They therefore underwent a simplified NH4Cl challenge and were found to have impaired urinary acidification ability. These patients were then analyzed by genetic testing with sequencing of 4 genes: SLC4A1, ATP6V1B1, ATP6V0A4 and FOXi1. Results Two unrelated individuals were found to have mutations in SLC4A1: 2 different variants in heterozygosis that had never been described before. The first patient was a 47-year old man, recurrent stone former (mainly apatite, but also some calcium oxalate), with hypercalciuria but normal bone mineral density (BMD). His family history revealed 1 uncle with kidney stones. The second patient was a 56-year old woman with a diagnosis of osteogenesis imperfecta, a history of reduced BMD and severe restrictive ventilation disorder and passed 1 stone (35% apatite, 65% CaOx). She was found to have bilateral nephrocalcinosis and hypocitraturia. Her family history revealed a sister with kidney stones. Conversely, 21 patients did not show any mutations for the genes sequenced, leading to a prevalence of genetic mutations of 8.7%. This is a much lower figure compared with overt dRTA, in which only 30% of patients with a clinical diagnosis of hereditary dRTA have no identified causative mutations in the currently known genes. Conclusion This suggests the involvement of other genes (WDR72 or others) or non-genetic tubular dysfunction in the pathogenesis of idRTA in stone formers.

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Prospective long-term evaluation of incomplete distal renal tubular acidosis in idiopathic calcium nephrolithiasis diagnosed (treated) by low-dose NH4CL loading – gender prevalences and impact of alkali treatment

Journal of Nephrology ◽

10.1007/s40620-021-01207-7 ◽

2022 ◽

Author(s):

Juri Sromicki ◽

Georg Kacl ◽

Malin Föhl ◽

Bernhard Hess

Keyword(s):

Calcium Phosphate ◽

Renal Tubular Acidosis ◽

Alkali Treatment ◽

Distal Renal Tubular Acidosis ◽

Urine Ph ◽

Calcium Stone ◽

Stone Formers ◽

Alkali Therapy ◽

Renal Tubular

Abstract Purpose Prospective evaluation of the prevalence of incomplete distal renal tubular acidosis (idRTA) in idiopathic calcium stone formers (ICSF) diagnosed by half-dose ammonium chloride loading (NH4Cl, 0.05 g/kg body weight/day) and impact of alkali treatment of idRTA. Methods Evaluation of 386 consecutive idiopathic calcium stone formers (ICSF) (280 males, 106 females) for idRTA. If screening fasting urine pH was > 5.80, 1-day NH4Cl loading was performed without severe adverse effects. Normally, urine pH falls below 5.45. Results Sixty-four idiopathic calcium stone formers exhibited idRTA, one complete dRTA. Prevalence was higher in women (25.4%) than in men (13.6%). Thus, for more equilibrated comparisons, we formed pairs of 62 idiopathic calcium stone formers (ICSF) with and 62 without idRTA, matched for gender, age, BMI and serum creatinine. Idiopathic calcium stone formers with idRTA more often had hypercalciuria (p < 0.025) and urine citrate < 2 mmol/d (p < 0.05), formed calcium phosphate stones more frequently, exhibited higher numbers of stones/year (1.4 ± 1.5 vs. 0.9 ± 0.8, p = 0.034) and 2.5 times more intrarenal calcifications (4.6 ± 5.9 vs. 1.8 ± 3.6, p = 0.002). All idiopathic calcium stone formers with idRTA were recommended chronic alkali therapy. After 4–15 years of follow-up, stone events /years follow-up (stone passage or urologic intervention) were higher in patients non-adherent to alkali therapy (0.61 ± 0.92) than in patients adherent to treatment (0.11 ± 0.21, p = 0.006). Conclusion Incomplete distal renal tubular acidosis is 1.8-fold more prevalent among female idiopathic calcium stone formers, predicts more stone recurrences, predisposes to calcium phosphate stones and is associated with 2.5 times more intrarenal calcifications vs. non-idRTA patients. Chronic alkali treatment reduces clinical stone recurrences by 5.5 times. Graphical abstract

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