Methods: Data of the QFT assays conducted in health care workers (HCW) at Houston Methodist Hospital (HMH; Houston, TX) between 08/2008 and 04/2017 were analyzed and stratified by the season when the blood samples were drawn. Multivariate generalized linear models (GLM) controlling for age, gender and ethnicity were used to estimate IFN-γ measured in the nil and mitogen controls per season.
Results: Data from 10,089 QFT assays were included in the analysis. The tested HCW were primarily between the ages of 18 to 49 years (76.5%), female (65.9%), and non-Hispanic (77.0%). A significantly higher level of IFN-γ was found in the mitogen stimulated blood (Phytohemaglutinin) in the summer (June – August) (estimate: 0.19 IU/mL; p <0.001) compared to the other season, and a significantly lower level of IFN-γ was found in the fall (September – November) (estimate: -0.27 IU/mL; p <0.001) compared to the other seasons. The IFN-γ level was significantly lower in unstimulated blood (estimate: -0.02 IU/mL; p: 0.038) but not in the antigen stimulated blood samples drawn in the winter (December – February) compared with those drawn in other seasons.
Conclusion: We observed a seasonal variation of the IFN-γ level measured in unstimulated and antigen-stimulated blood samples drawn for the QFT assays, in which seasonal factors such as airborne antigens like pollen may play a role. Clinicians should take into account the possible seasonal variation when interpreting positive QFT results, especially those on the borderline of the assay’s diagnostic cut-offs. Re-testing or implementing additional diagnostic tools should be considered if necessary. Further research would be needed to identify the specific seasonal factors that may influence the QFT results.
E. A. Graviss, None