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False-Negative Sweat Chloride Testing in a Child With Cystic Fibrosis and Undiagnosed Hypohidrotic Ectodermal Dysplasia

A 4-year-old male presented for profound failure to thrive and chronic cough. He was underweight (weight < 3rd percentile) since a year of age, and recently his height percentile had fallen from the 10th to below the 5th percentile (Figure 1). The patient was of Caucasian and Hispanic descent without consanguinity. Prenatal and birth history were unremarkable. The child was born full term, there was no history of intrauterine growth restriction, and his birth weight was appropriate for gestational age. He had no history of hospitalizations, sinopulmonary disease, or fevers. Review of systems was notable for chronic constipation since infancy but no diarrhea or steatorrhea. Physical exam revealed a small child in no distress. The only remarkable finding was delayed dental eruption, and the teeth that were present were malformed and conical in shape (Figure 2). The child was referred to a pediatric endocrinologist for failure to thrive. Extensive laboratory evaluation for causes of poor growth included comprehensive metabolic panel, complete blood count, erythrocyte sedimentation rate, thyroid-stimulating hormone, FT4, celiac panel, insulin-like growth factor 1, and insulin-like growth factor binding protein 3 were normal. However, due to persistent constipation and poor weight gain, sweat chloride testing was also performed. After 3 unsuccessful attempts to obtain adequate sweat samples for analysis at a certified cystic fibrosis (CF) center, his first valid samples showed sweat chloride concentrations of 28 and 34 mmol/L, respectively. Follow-up testing showed a sweat chloride concentration of 43 mmol/L, an intermediate result. Given the high suspicion for CF, CFTR gene sequencing was performed (Ambry Genetics, Aliso Viejo, CA), which revealed 3 different mutations. The first, p.R668C (c.2022C>T), has been reported in 2 men with azoospermia and congenital bilateral abscess of the vas deferens (CBAVD). Although a missense mutation in exon 14 of the CFTR gene, this mutation has been reported in combination with other mutations to cause mild symptoms consistent with a CFTR-related disorder. The second, p.A1067V (c.3200C>T), is also an exon mutation but has been reported in multiple individuals with CBAVD and in combination with other mutations would be expected to result in a milder phenotype with variable expression. The third, p.R1162L (c.3485G>T), has been reported in a patient with atypical CF when combined with a pathologic mutation. Given these genetic testing results and the clinical presentation with failure to thrive and chronic cough, the patient was diagnosed with cystic fibrosis. Because of the difficulty in collecting sweat samples, delayed dental eruption, and malformed teeth, he was sent for additional genetics evaluation. During this evaluation further history revealed that during activity he would 518426 CPJXXX10.1177/0009922813518426Clinical PediatricsWelsh et al research-article2014