Objective

This study evaluates the performance of a new prototype interferon-gamma release assay (IGRA) for the determination of T-cell-mediated immunity against Mycobacterium tuberculosis (MTB).

Methods

Blood samples from 34 adults with active tuberculosis (TB) and from 30 children with active TB (n=1), latent tuberculosis infection (TBI, n=12) or without TB (n=17) were analyzed using the prototype Quan-T-Cell TB (EUROIMMUN) on the DSX Automated ELISA System (Dynex Technologies). The results were compared to the clinical diagnosis. Additionally, 29 of the pediatric samples were measured using the established QuantiFERON-TB Gold Plus assay (Qiagen) to determine inter-assay concordance. Both assays use stimulatory antigens that are based on the mycobacterial proteins ESAT-6 and CFP-10.

Results

The prototype Quan-T-Cell TB differentiated patients with active TB or TBI from cases without TB with a sensitivity of 91.3% at a specificity of 93.8%, while the positive and negative predictive values were 97.7% and 78.9%, respectively. Comparison between the two IGRAs showed positive, negative and overall agreement rates of 100%, 93.8% and 96.3%, respectively, with a kappa score of 0.924 indicating almost perfect agreement. Qualitative inter-assay discrepancies were found in three samples (1 TBI, 2 without TB) that were negative by QuantiFERON-TB but borderline or very weakly positive by the prototype Quan-T-Cell TB.

Conclusion

The results suggest very good performance of the prototype Quan-T-Cell TB. Unlike the QuantiFERON-TB, the Quan-T-Cell TB has a borderline range (grey zone), which is advantageous as it may help to differentiate non-specific variations near the cut-off value from true conversions or reversions. Moreover, reduction to one MTB-specific stimulation tube minimizes the required sample volume and maximizes the cost-effectiveness of laboratory testing. Using only 1.5 ml blood per analysis (QuantiFERON-TB: 4 ml) is helpful particularly in pediatric applications. Further optimization of the Quan-T-Cell TB is underway, requiring additional evaluation studies including larger, age- and gender-matched patient and control cohorts.