Biochemical evaluation in renal stone disease

Renal stone disease may ensue from either derangements of urine biochemistries or anatomic abnormalities of kidneys and urinary tract. Genetic, environmental and dietary factors may also cooperate in the pathophysiology of nephrolithiasis. An adequate metabolic evaluation should focus on the urinary excretion of promoters and inhibitors of stone formation as well as on the occurrence of systemic diseases potentially related to secondary nephrolithiasis (i.e., endocrine disturbances, malabsorption, bone diseases). Moreover, metabolic investigations should provide reliable information on patient's dietary habits, guide towards the best therapeutic approach and enable the physician to verify patient's compliance to prescribed therapies. An extensive metabolic evaluation is recommended in patients with active stone disease (namely, at least one new stone within the last two years), or in those having had a single stone episode occurred in peculiar conditions: familial history of disease, childhood, menopause, pregnancy, systemic diseases. Simplified protocols may be adequate in non-active nephrolithiasis or in patients with single stone and no relevant risk factors. In our Stone Centre, a so-called "first level screening" is performed by routine, in order to assess urinary supersaturation with stone forming salts and evaluate the excretion of dietary-related metabolites in urine. Relative blood and urine determinations are reported below. In venous blood: urea, creatinine, uric acid, Na, K, total and ionised Ca, Mg, P, Cl, alkaline phosphatase, gas analysis. In 24-hr urine samples: urea, creatinine, uric acid, Na, K, Ca, Mg, P, Cl, oxalate, inorganic sulphate, citrate, pH, ammonia and titratable acidity. In fasting urine samples: Ca, citrate, creatinine, hydroxyproline, Brand's test for cistinuria, urine sediment, urine culture. If the first-level evaluation suggested an abnormal bone turnover, then further determinations are warranted, namely, calciotropic hormones (blood Vitamin D and PTH), markers of bone resorption (urine pyridinium crosslinks, serum crosslaps) and formation (serum osteocalcin) bone mineral density. Eventually, more sophisticated investigations are required to improve the diagnosis of peculiar diseases: serum oxalate and glycolate, urine glycolate and L-glycerate, hepatic AGT activity (primary hyperoxalurias); genetic tests (hereditary nephrolithiasis); acidification tests (renal tubular acidosis).