ECFS statement on theratyping and theranostics in the context of rare and ultrarare CFTR variants in people with CF.

Genotype-based drug development has yielded highly effective therapies, notably the triple combinations elexacaftor/tezacaftor/ivacaftor (ETI) and vanzacaftor/tezacaftor/deutivacaftor (VTD), now approved in Europe for people with CF having at least one non-class I variant. However not all these people with CF will respond to ETI or VTD, and a few not under the label may respond. Facilitating opportunities to access for patients with rare variants has required a shift in paradigm toward functional testing-based access. This approach assesses the potential benefit of modulator therapy using in vitro functional assays, either in engineered systems expressing defined CFTR variants (theratyping) or in patient-derived tissues (theranostics). We review theranostics as a critical tool for personalized medicine in CF, highlighting its validation in in vitro models derived from patients' own cells such as human intestinal organoids and human nasal epithelial cells. We discuss the current regulatory landscape regarding modulator approval and propose strategies for improving equitable access to effective treatments for all people with CF. Importantly, we advocate for functional assays to be accepted as standalone evidence of drug efficacy for patients with rare variants. Theranostic approaches remain critical when theratyping has not been achieved, and genetic data is not available or clearly interpretable. Indeed, theranostics has emerged as an essential pillar of CF drug access, complementing genotype-driven strategies. As the field advances, continued validation, standardization, and regulatory integration of in vitro functional assays will be key to ensuring that every person with CF-regardless of their genotype-has the opportunity to benefit from precision therapies.