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Scientific advances and the passage of time are causing us to review what constitutes a diagnosis of cystic fibrosis (CF). Over and above those with a typical presentation and laboratory findings, there have always been patients who fall in the grey area—“does she or doesn’t she have CF?” Since the discovery of the CFTR gene in 1989 and the ongoing discovery of mutations associated with CF (the mutation count now stands at over 800), genetic analysis has brought a new sophistication to the area but has not solved all the problems. A consensus conference on diagnostic criteria arranged by the North American Cystic Fibrosis Foundation has published the agreed criteria.1 In addition to those with typical features (sweat chloride levels above 60 mmol/l and the finding of two mutations of CFTR known to be associated with the disease), the consensus statement admits that approximately 2% have an atypical phenotype with chronic sinopulmonary disease, pancreatic sufficiency, and either borderline (40–60 mmol/l) or normal (<40 mmol/l) sweat chloride concentrations. The predominance of single features may still allow a diagnosis of CF if two CF mutations, or abnormal sweat electrolytes, or abnormal ion transport across the nasal epithelium are found.2 ,3 A truly borderline diagnostic situation may apply when congenital bilateral absence of the vas deferens (CBAVD) or another …