Cystic fibrosis mimicking Bartter's syndrome

A four-month-old female presented with dehydration and an unexplained hypoelectrolytaemia and metabolic alkalosis in association with hyperreninaemia and hyperaldosteronism. These findings suggested Bartter's syndrome. Sweat sodium concentrations were within normal limits. Total skin water loss was estimated and found to be sufficient for the sodium loss in the sweat to exceed the dietary intake. Reinvestigation at three years revealed normal plasma electrolytes, plasma renin activity and aldosterone values but sweat sodium concentrations were in excess of 80 mmol/l, indicating a diagnosis of cystic fibrosis.