The cystic fibrosis gut microbial dysbiosis index (CF-GMDI): a quantitative measure of gut microbial imbalance in children with cystic fibrosis.

BACKGROUND: Gut dysbiosis is a hallmark of cystic fibrosis (CF), typically characterised using non-specific diversity metrics and study-specific taxonomic lists, limiting comparability across studies. We aimed to develop a metagenomic, species-level CF Gut Microbial Dysbiosis Index (CF-GMDI) to standardise measurement of gut microbial imbalance in children with CF (cwCF) METHODS: CF-GMDI was derived using stool metagenomic data from the PEARL-CF study (70 cwCF; 67 healthy controls (HC); 0-6 years). Differentially abundant taxa were identified between cwCF and HC using MaAsLin2. The index was calculated as the log10 ratio of the summed relative abundances of taxa enriched in CF vs HC to those depleted in CF vs HC. Reproducibility was assessed in the independent EARTH cohort (56 cwCF; 56 HC; 0-18 years). Responsiveness to therapy was evaluated using publicly available metagenomic data from an Elexacaftor/Tezacaftor/Ivacaftor (ETI) study (39 cwCF; 6-18 years). RESULTS: CF-GMDI was significantly higher in cwCF than HC (p < 0.001), inversely correlated with species richness (ρ = -0.74, p < 0.001), and higher in pancreatic-insufficient vs pancreatic-sufficient cwCF in the PEARL-CF cohort (p = 0.01). Key ecological and clinical associations were replicated in the EARTH cohort. In the ETI study, CF-GMDI decreased significantly at 6 and 12 months post-treatment, whereas alpha diversity remained unchanged. CONCLUSIONS: CF-GMDI is a CF-associated metric that captures clinically relevant gut microbiome restructuring not detected by standard diversity measures in cwCF (0-18 years). It differentiates disease and pancreatic status and tracks therapeutic modulation, supporting its use as a novel endpoint in CF intervention studies.