Refining CFTR-Related Metabolic Syndrome (CRMS)/Cystic Fibrosis Screen Positive, Inconclusive Diagnosis (CFSPID) Diagnosis: Impact of CFTR2 Variant Classifications.

An unintended consequence of cystic fibrosis (CF) newborn screening (NBS) is the identification of infants with a positive NBS who do not meet the diagnostic criteria for CF (two CF-causing variants and/or sweat chloride > 60 mmol/L). This indeterminate diagnosis is called cystic fibrosis transmembrane conductance regulator (CFTR)-related metabolic syndrome (CRMS) or CF screen positive, inconclusive diagnosis (CFSPID). CRMS/CFSPID occurs when it is not clearly known whether variants are disease-causing. In 2024, the CFTR2 classification of many variants was changed from unknown significance to either CF-causing variants or variants of varying clinical consequences (VVCCs). We conducted a meta-analysis of CRMS/CFSPID cases from manuscripts to describe how the diagnoses would change using two different variant panels: (1) only CF-causing variants (Panel) and (2) CF-causing variants and VVCCs (Panel). Using the Panel, 8.7% had two CF-causing variants (reclassified as CF), while 91.3% had less than two CF-causing variants (reclassified as Undetected). Using the Panel, 51.4% had either two VVCCs or one VVCC with one CF-causing variant detected (reclassified as CRMS/CFSPD), 39.9% had less than two CF-causing variants detected (reclassified as Undetected), and 8.7% had two CF-causing variants (reclassified as CF). In conclusion, using the updated CFTR2 classification of variants significantly decreases the number of children with CRMS/CFSPID and gives a definitive diagnosis of CF to some children while not detecting as many children who are unlikely to develop CF.