Interferon Gamma Release Assays for Latent Tuberculosis: What Are the Sources of Variability?

Abstract

Interferon gamma release assays (IGRAs) are blood-based tests intended for diagnosis of latent tuberculosis infection (LTBI). IGRAs offer logistical advantages and are supposed to offer improved specificity over the tuberculin skin test (TST). However, recent serial testing studies of low-risk individuals have revealed higher false conversion rates with IGRAs than with TST. Reproducibility studies have identified various sources of variability that contribute to nonreproducible results. Sources of variability can be broadly classified as preanalytical, analytical, postanalytical, manufacturing, and immunological. In this minireview, we summarize known sources of variability and their impact on IGRA results. We also provide recommendations on how to minimize sources of IGRA variability.

FIG 1

Major sources of variability in IGRA results. (Republished from reference 1).

FIG 2

Between-run reproducibility of the QFT-GIT assay. (A and B) The TB response (IFN-γ concentration in antigen tube minus nil tube) was measured from a sample with a borderline negative result (A) and another sample with a borderline positive result (B) by an automated ELISA instrument over 20 consecutive runs. Samples were stored at 4°C between runs. Two to four ELISA runs were performed each day. The cutoff value for the standard QFT-GIT assay (dashed line) is shown for reference. Positive results are shown in red. Coefficient of variance (CV) and conversion and reversion rates are shown.