Borderline Sweat Chloride After Newborn Screening
A 2-month-old boy is referred after newborn screening showed an immunoreactive trypsinogen (IRT) above the 99th percentile. CFTR gene testing identified one F508del mutation; no second mutation was found on a 39-mutation panel. His sweat chloride is 32 mmol/L on 45 mg of sweat. He was born at term, weighs 5.2 kg (50th percentile), and has had no respiratory symptoms. He passes 3–4 seedy stools daily and is breastfed. His parents are unrelated and healthy. The family is anxious about the meaning of the borderline sweat test and asks whether he has cystic fibrosis. You review the sweat test and genetic results.
1. How do you interpret a sweat chloride of 32 mmol/L and a single F508del mutation? Does this infant have CF?
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A sweat chloride of 32 mmol/L is in the intermediate range (30–59 mmol/L) and is not diagnostic of CF. A single F508del mutation identifies the infant as a CF carrier, but CF requires two disease-causing CFTR mutations or other evidence of CFTR dysfunction. This infant could have CFTR-related metabolic syndrome (CRMS) or CF screen positive, inconclusive diagnosis (CFSPID). The normal growth and absence of symptoms are reassuring but do not exclude CF. The family should be told that the infant does not currently meet diagnostic criteria, but close follow-up is needed because some children with intermediate sweat chloride and one mutation later progress.
2. What further testing and follow-up would you recommend?
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Repeat the sweat chloride test in 1–2 months at a CF care center, ensuring adequate sweat volume. Send expanded CFTR sequencing with deletion/duplication analysis to look for a second mutation not on the 39-mutation panel. If available, nasal potential difference or intestinal current measurement can assess CFTR function. Monitor weight, stool pattern, and respiratory symptoms at each visit. Provide genetic counseling to parents. If the infant remains asymptomatic and testing is inconclusive, annual follow-up with a CF specialist is appropriate.
3. If the repeat sweat chloride is 45 mmol/L, how does that change your management?
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A repeat value of 45 mmol/L remains in the intermediate range and still does not diagnose CF. Management remains monitoring under the CFSPID umbrella: no pancreatic enzymes, no CF-specific airway clearance, and no CFTR modulators unless the child develops diagnostic criteria. Educate the family about red flags (poor weight gain, steatorrhea, persistent cough, nasal polyps) and avoid routine cross-infection with CF patients. Reassess growth and symptoms every 3–6 months and repeat sweat testing if clinically indicated. Some children eventually convert to a CF diagnosis, so continuity is essential.