Real-world evidence on the utility of extended lymphocyte subsets in screening inborn errors of immunity: Experience from a referral laboratory in India

Background: Inborn errors of immunity (IEI) are a heterogeneous group of disorders. Severe combined immunodeficiency (SCID) requires early diagnosis and treatment, but newborn screening by T-cell receptor excision circles is not widely available in India. Flow cytometry-based lymphocyte subset analysis remains the first-line test. Aim: The aim of the study was to evaluate whether adding naïve T-cell estimation to routine lymphocyte subset analysis improves SCID detection in a real-world setting. Methods: We conducted a prospective study (January 2024–June 2025) including 1374 patients referred for IEI screening. Flow cytometry was performed using an extended 8-color panel (T, B, NK cells and naïve/memory subsets). Demographic data, lymphocyte subsets and cost implications were analysed. A waiver of consent was sought as the study involved analysis of anonymised patient data. Results: Of 1374 patients, 1074 (78%) were children <8 years; 499 (36%) were <1 year. T-cell lymphocytopenia was present in 7.4% of children <2 years. Among 21 children with markedly reduced naïve CD4+ T-cells, 6 (28.5%) had normal total T-cell counts, suggesting SCID variants that would have been missed with routine panels. The extended assay increased costs by 56% compared with the basic panel but remained 61% cheaper than the EuroFlow PID orientation tube panel. Conclusion: Naïve T-cell estimation significantly improves early SCID detection, even when total T-cell counts appear normal. Incorporating naïve T-cell analysis into first-line screening offers a cost-effective, practical solution for resource-limited settings.