Samuel Baumgart¹, Ruvanika Rajapakse¹, Kartik Naidu¹, Juliette Holland^1
1Laverty Pathology, 60 Waterloo Road, North Ryde, Australia

Introduction: Interferon gamma-release assays (IGRA) screen for exposure to Mycobacterium tuberculosis. The QuantiFERON®-TB Gold Plus assay separates results qualitatively into positive, indeterminate and negative categories, however various ‘borderline’ ranges have been employed1,2. Laverty Pathology’s low positive criteria is TB1 and TB2 antigen both ≥0.35 and <2.00.  

Aim: Ascertain proportions of results in each category, the reasons for indeterminate results, and whether our low positive range is appropriate.

Method: Audit of all IGRA tests performed in 2023 including subsequent results. 

Results: 26157 samples were analysed. 1461 (5.59%) were low positive and 1218 (4.66%) positive. All subsequent reversions to negative results occurred only in those with prior low positive results. Indeterminate rates were low (195/26157, 0.75%) and mainly due to inadequate mitogen response. Follow-up samples were only received in 27/195 (13.9%) indeterminate cases with the majority (17/27, 63%) becoming negative. 

Conclusions: A cutoff value of 2.00 for TB1 and TB2 antigens separated ‘low positive’ results that are more likely to revert to negative from true positive results, however further research needs to be performed to delineate the ideal borderline range. Low rates of indeterminate results suggest good pre-analytical and analytical processes however obtaining follow-up samples may be an area for quality improvement. 

References:
1. Wikell A, Jonsson J, Dyrdak R, Henningsson AJ, Eringfalt A, Kjerstadius T, et al. Latent Tuberculosis Screening under Routine Conditions in a Low-Endemicity Setting. J Clin Microbiol. 2021;59:e01370-21. doi: https://doi.org/10.1128/JCM.01370-21 

2. Wang MS, Li-Hunnam J, Chen YL, Gilmour B, Alene KA, Zhang YA, Nicol MP. Conversion or reversion of interferon gamma release assays for Mycobacterium tuberculosis infection: a systematic review and meta-analysis. Clin Infect Dis. 2024;ciae357. doi: 10.1093/cid/ciae357 

Notes:
1. This work was the original idea of the authors, and oversight was provided by JH, RR and KN. SB performed the data collection and analysis. 

2. Generative AI was NOT used at any stage in the research process including the writing process.