First Reported Case of a Pyrophosphate Kidney Stone in a Human

Urolithiasis composed of pyrophosphate salts has only been reported in animals, in the form of potassium magnesium pyrophosphate. However, there have been no reports of pyrophosphate stones in humans. Hypophosphatasia is an inherited disease characterized by low alkaline phosphatase activity and elevated levels of pyrophosphate in blood and urine. Urolithiasis is a part of the hypophosphatasia phenotype. The role of elevated urine pyrophosphate levels in the formation of stones in hypophosphatasia is unknown. Here, we report a case of a 60-year-old man with recurrent urolithiasis. The patient's most recent presentation was gross hematuria and his computed tomography scan showed bilateral kidney stones. Stones were removed via retrograde intrarenal surgery. Stone analysis revealed a composition of potassium magnesium pyrophosphate. The patient also has a long history of fracturing bone disease which led to the consideration of hypophosphatasia as the cause of both his bone disease and pyrophosphate stones. Hypophosphatasia was confirmed by genetic analysis. Pyrophosphate has been of interest in the fields of mineral metabolism because of its action as a crystallization inhibitor. However, pyrophosphate at elevated concentrations in the presence of divalent cations can exceed its solubility. Nephrocalcinosis and stone disease have been described in hypophosphatasia; stones have been assumed to be calcium phosphate but no compositional analysis has been reported. This is the first report of human stones composed of pyrophosphate salts, which led to the subsequent diagnosis of hypophosphatasia in this patient.