Evaluations of 1,270 patients with recurrent nephrolithiasis in an outpatient setting were analyzed for the purpose of updating the classification of nephrolithiasis. All but 4% had abnormal urinary biochemistry that placed them into one or more of 20 etiologic categories. A single diagnosis was documented in 41.3% of patients. The remaining 58.7% had more than one diagnosis. Hypercalciuric calcium (Ca) nephrolithiasis, encountered in 60.9% of patients, comprised six variants-absorptive hypercalciuria Type I and II, renal hypercalciuria, primary hyperparathyroidism, and unclassified hypercalciuria (renal phosphate leak and fasting hypercalciuria). Hyperuricosuria Ca nephrolithiasis (HUCN) and gouty diathesis (GD) accounted for 35.8% and 10.0% of patients, respectively. Distinguishing features were hyperuricosuria and normal urinary pH in HUCN, and normal urinary uric acid and low urinary pH (<5.5) in GD. Hyperoxaluric Ca nephrolithiasis, occurring in 8.1% of patients, was subdivided into enteric, primary, and dietary variants. Hypocitraturic Ca nephrolithiasis affected 28% of patients in its idiopathic variant. Many of these patients' problems were probably dietary in origin, while some could have had incomplete renal tubular acidosis. Hypocitraturia due to renal tubular acidosis or chronic diarrheal syndrome affected only 3.3% of patients. Hypomagnesiuric Ca nephrolithiasis, infection stones, and cystinuria were uncommon, accounting for 6.8%, 5.9%, and 0.9% of patients, respectively. The acquired problem of low urine volume (<1 L/d) was found in 15.3% of patients. The remaining 3.5% of patients were difficult to classify despite the presence of abnormal urinary biochemistry.
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