Repeatability of QuantiFERON-TB gold plus testing utilizing microparticle chemiluminescence

Detection of latent Mycobacterium tuberculosis (LTBI) in patients is important to prevent active infection and the spread of disease, particularly in vulnerable patient populations. In 2020, a kit on the high throughput Liaison XL (DiaSorin) became commercially available for the analysis of QuantiFERON-TB Gold Plus assay (Qiagen). Pilot testing indicated suboptimal repeatability of some samples with this assay. This study provides an extensive assessment of repeatability with DiaSorin system.

Repeat testing of 481 IGRA positive samples, demonstrated substantial variability upon repeat analysis. Repeat results for TB1 and TB2 tubes, showed 73.73% and 72.82% concordance with initial results, respectively. TB1 and TB2 tube values minus the nil (IU/mL) were significantly higher in samples that were repeat positive (p < 0.001). Repeat results had better concordance with initial results if both TB1 and TB2 tubes were positive. Samples with TB1 tube values minus the nil (IU/mL) ≥ 4.54 and TB2 tube values minus the nil (IU/mL) ≥ 4.78 were found to always repeat positive. Assigning a threshold of 1.55 IU/mL for the TB1 tube value minus the nil and 1.45 IU/mL for the TB2 tube value minus the nil yielded a positive predictive value ≥95%.

These results identified a potential role for retesting of select IGRA positive samples on the Diasorin Liaison XL platform due to the high proportion of samples that show a lack of repeatability. Additionally, we identified a threshold that would determine samples most likely to repeat test positive and which samples should be retested.