Editor—In the course of neonatal screening for cystic fibrosis, a boy had raised immunoreactive trypsin values 112 pg and 73 μg/l (normal reference values <70 μg/l) at 6 and 28 days of age. His birthweight had been 3.6 kg (50th centile) at 41 weeks of gestation, and at 6 weeks his weight was 3.77 kg (< third centile). His length was 54 cm (25th centile) and head circumference 37.5 cm (25th–50th centile). He was alert and cheerful, not dehydrated or wasted. Respiratory and gastrointestinal systems were functioning normally.

Initial investigations showed a serum sodium of 163 mmol/l, urea 8.7 mmol/l, potassium 4.7 mmol/l, and bicarbonate 24.4 mmol/l; creatinine was 53 μmol/ml and urinary sodium, 17 mmol/l. Matched serum/urinary osmolalities were: 322/70 and 318/53 mosmol/kg, and there was no response to 10 mg of nasal desmopressin (matched serum/urinary osmolalities 317/58 mosmol/kg). An initial sweat test at 6 weeks obtained only 10 mg of sweat, but at 20 weeks the sweat chloride was 15 mmol/l and sodium 15 mmol/l (288 mg sweat). Faecal chymotrypsin was 149 μg/g (normal 120). DNA analysis was negative for DF508, G551D, and R553X mutations. Chest x ray picture and renal ultrasound scan were both normal. Serial measurements of urinary osmolality and urinary arginine–vasopressin (AVP) showed low osmolalities with inappropriately high AVP values (fig 1), with a flat dose response curve, confirming the diagnosis of nephrogenic diabetes insipidus.

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Figure 1

Dose response curve of urinary osmolality (mmol/kg) to log urine AVP excretion rate, corrected for osmolality clearance (pg/min/Cosm).

Raised serum immunoreactive trypsin in infants who do not have cystic fibrosis has been reported in DF508 heterozygotes,1 and in hypoxic insult to the fetal pancreas, congenital viral infections, trisomy 18 and 13, renal insufficiency, congenital heart disease and spina bifida.2 Nephrogenic diabetes insipidus is yet another cause for falsely high serum values, but the mechanism for this is unclear. Dehydration does not seem to be the entire explanation as the immunoreactive trypsin value had become almost normal by day 28, when the serum osmolality was still high.