FASCIA Method in the Assessment of Lymphocyte Mitogen Responses in the Laboratory Diagnostics of Primary Immunodeficiencies.

Lymphocyte responses to mitogens constitute a key part of the diagnostics of combined immunodeficiency (CID). Currently, mostly radioactive thymidine incorporation and carboxyfluorescein diacetate succinimidyl ester (CFSE) dilution methods are used. Flow-cytometric assay for specific cell-mediated immune-response in activated whole blood (FASCIA) has been put forth as an easy-to-perform option for the measurement of lymphocyte responses with the advantage of recognizing different lymphocyte subtypes and avoiding the use of radioactive reagents. Our aim was to analyze retrospectively the usefulness of FASCIA in the diagnostics of CID. We included all lymphocyte stimulation tests done with FASCIA in HUSLAB (Helsinki, Finland) between February 2015 and September 2018 in our analysis. The cohort was divided into two groups according to the patients' final diagnoses: CID (n = 30) or non-CID (n = 159). We evaluated the stimulation responses with a combined FASCIA score (the average of all mitogen responses). The FASCIA score was significantly lower among the CID group compared to the other patients (p = 0.002), and in the ROC analysis, the AUC was 0.75 (p < 0.001) for the FASCIA score. When the three mitogens were analyzed separately, phytohemagglutinin (PHA) was best in separating patients with CID from non-CID (in the ROC analysis AUC 0.71, p = 0.001). Immunosuppressive medication affected the FASCIA result significantly and needs to be considered when evaluating the results. In conclusion, FASCIA can reliably detect the CID patients in the absence of immunosuppressive medication. It emerges as a method with many benefits compared to tests requiring radioactive reagents or the complicated CFSE staining.