To determine which features of incomplete or “nonclassic” forms of cystic fibrosis (CF) are associated with deleterious CF transmembrane conductance regulator gene (CFTR) mutations, and to explore other etiologies for features not associated with deleterious CFTR mutations.

Clinical features were compared between 57 patients with deleterious mutations in each CFTR and 63 with no deleterious mutations. The Shwachman Bodian Diamond syndrome gene (SBDS) was sequenced to search for mutations in patients with no deleterious CFTR mutations and steatorrhea to determine if any had unrecognized Shwachman-Diamond syndrome (SDS).

The presence of a common CF-causing mutation, absence of the vas deferens, and Pseudomona aeruginosa in the sputum correlated with the presence of two deleterious CFTR mutations, whereas sweat chloride concentration, diagnostic criteria for CF, and steatorrhea did not. However, sweat chloride concentration correlated with CFTR mutation status in patients infected with P aeruginosa. One patient had disease-causing mutations in each SBDS.

Presence of a common CF-causing mutation, absence of the vas deferens and/or P aeruginosa infection in a patient with features of nonclassic CF are predictive of deleterious mutations in each CFTR, whereas steatorrhea in the same context is likely to have etiologies other than CF transmembrane conductance regulator (CFTR) dysfunction.