Recent Thymic Emigrant Levels in Inborn Errors of Immunity: Is Their Diagnostic Value Greater Than We Think?

We aimed to evaluate the sensitivity and specificity of Recent Thymic Emigrant (RTE) levels for diagnostic discrimination among various Inborn Errors of Immunity (IEI) subgroups. We analysed 205 paediatric patients diagnosed with IEI at the Paediatric Immunology and Allergy Department of Başakşehir Çam and Sakura City Hospital in Istanbul between January 2021 and January 2024. An age- and sex-matched control group consisting of 31 healthy children was also included for comparison. Demographic, clinical and laboratory data were evaluated. Reduced RTE levels were identified in 39% of all patients. Low RTE levels were most pronounced in patients with syndromic combined immunodeficiency (77%) and immune dysregulation (100%), and were also observed at high frequencies in combined immunodeficiency (CID) (60%) and autoinflammatory disorders (57.1%). For the syndromic CID group, the diagnostic performance of RTE yielded an area under the curve (AUC) of 0.80, with a sensitivity of 92% and a specificity of 52%. In the immune dysregulation group, the AUC was 0.79, with a sensitivity of 83.5% and a specificity of 57.1%. Assessment of RTE levels represents a valuable diagnostic biomarker, particularly for patients within the syndromic CID and immune dysregulation categories. This parameter may contribute meaningfully to clinical practice by facilitating early diagnosis, guiding genetic testing and enabling assessment of thymic function. Incorporating thymic output assessment into routine immunophenotyping panels may shorten the diagnostic timeline for IEI and allow earlier initiation of appropriate therapies.