Abstract
A 16-year-old female had mutations in both alleles of the gene encoding her sodium-chloride cotransporter; one of these mutations is newly described. Her clinical findings were not typical because of the absence of hypocalciuria in 24-h urine samples, her maximum urine osmolality (U(osm)) was only 802 mosmol/kg H(2)O, and her plasma magnesium (Mg) concentration (P(Mg)) was easily maintained in the normal range with oral Mg supplements for 1 month. In detailed studies, the calcium/creatinine ratio in spot urines with a U(osm) >700 mosmol/kg H(2)O was very low, except during Mg therapy. Renal medullary function did not appear to be compromised because she had a non-urea U(osm)of approximately 600 mosmol/kg H(2)O, reflecting a very high non-urea osmole excretion rate (due to KCl supplements). At age 18 years, her P(Mg) became persistently low despite Mg therapy. We conclude that the clinical criteria for a provisional diagnosis of Gitelman syndrome should be revised. Hypocalciuria may only be evident initially in concentrated spot urine samples. Urine concentrating ability should include an analysis of the non-urea U(osm), especially when patients are taking large KCl supplements.
