Assessment of Imprecision in Gamma Interferon Release Assays for the Detection of Exposure to Mycobacterium tuberculosis

New gamma interferon (IFN-gamma) release assays (IGRAs) to detect an exposure to Mycobacterium tuberculosis have recently been launched. The majority of the studies in temperate-climate countries agree that these methods have superior specificity and equal or even superior sensitivity over tuberculin skin tests (TSTs) in the diagnosis of latent tuberculosis (TB) infection (LTBI). However, reproducibility data of IGRAs are virtually missing. We assessed within-run, between-run, and total imprecision of two commercial IGRAs by testing samples from subjects with a stable state of TB infection or treated pulmonary TB, a sample from a healthy volunteer, and internal quality control samples. We calculated coefficients of variance (CV% s) to describe assays variability and compared the obtained results to the reported CV% s for other commercial immunodiagnostic methods. We illustrate an example of assay variability near the cutoff zone to demonstrate the necessity of a gray zone. Due to the strict adherence to the standard operation procedures (SOP) adopted in our laboratory, the total imprecision of enzyme-linked immunospot (ELISPOT)- and enzyme immunoassay (EIA)-based IGRAs was at a maximum CV% of 37.8% for the samples with moderate and high reactivities. Imprecision of testing samples with very low reactivity levels or nonreactive samples may, however, exceed 100%. In conclusion, despite multiple steps of the method performance, the analytical imprecision of IGRAs, which in our study design included also between-lot variability and had a component of normal biological variation, was well in accordance with the reported imprecisions of other manual immunodiagnostic tests. The recognition of the variability around the cutoff point advocates the use of a gray zone to avoid ambiguous result interpretations.