Conference Abstract | Volume 8, Abstract NACNDC/19JASH075 (Poster C3) | Published: 01 Dec 2025
Clark Joshua Brianwong1,2,&, Noah Kiwanuka2,3, Simon Peter Kigozi2, Robert Kakaire3, Juliana Namutundu2
1Baylor Foundation, Kampala, Uganda, 2Makerere University School of Public Health, Kampala, Uganda, 3University of Georgia, United States
&Corresponding author: Clark Joshua Brianwong, Baylor Foundation Uganda, Email: clarkjoshua001@gmaill.com, ORCID: https://orcid.org/0009-0005-1548-2684
Received: 30 Aug 2025, Accepted: 20 Oct 2025, Published: 01 Dec 2025
Domain: Infectious Disease Epidemiology
Keywords: Latent tuberculosis, tuberculin skin test, Interferon Gamma Release Assay (IGRA), discordance, HIV, Uganda, diagnostic thresholds.
©Clark Joshua Brianwong et al. Journal of Interventional Epidemiology and Public Health (ISSN: 2664-2824). This is an Open Access article distributed under the terms of the Creative Commons Attribution International 4.0 License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Cite this article: Clark Joshua Brianwong et al., Optimal TST cut-off thresholds for agreement with IGRA positivity in serial testing: Insights from a high burden TB setting in Kampala, Uganda. Journal of Interventional Epidemiology and Public Health. 2025;8(ConfProc6):075. https://doi.org/10.37432/JIEPH-CONFPRO6-00075
Preventing the progression of latent tuberculosis infection (LTBI) to active disease is central to the End TB Strategy. However, LTBI diagnosis lacks a gold standard. In high burden TB/HIV settings, the tuberculin skin test (TST) is limited by fixed cut-offs and cross-reactivity with BCG vaccination, leading to discordance with interferon-gamma release assays (IGRAs), which also vary in serial testing, complicating clinical decisions. Defining context-specific thresholds could improve diagnostic accuracy and guide preventive therapy. This study evaluated optimal TST cut-offs aligning with IGRA positivity and IGRA thresholds concordant with standard TST positivity among initially TB-uninfected adults in Kampala, Uganda.
A retrospective cohort analysis used CAPTURE study data (2020–2024), including 838 adults with baseline negative TST and IGRA, followed quarterly for 18 months. We used Receiver Operator Characteristic (ROC) curves to evaluate the diagnostic performance of TST (induration modelled as a continuous variable) against the manufacturer’s IGRA threshold (>0.35 IU/mL), and conversely, IGRA values against TST thresholds of ≥10 mm (HIV-negative) and ≥5 mm (HIV-positive). For each timepoint (12 and 18 months) and pooled data, sensitivity, specificity, and AUC with 95% confidence intervals were calculated. Optimal thresholds were identified using Youden’s Index.
The pooled optimal TST threshold for predicting IGRA positivity was 3.35 mm, with poor sensitivity (30%) and low discriminatory power (AUC 0.64). Performance was worse among people living with HIV, with an 18-month threshold of 11.75 mm and sensitivity of 17%. Conversely, an IGRA threshold of 0.5 IU/mL aligned closely with standard TST positivity, showing high agreement overall (AUC 0.89; sensitivity 84%, specificity 95%). Alignment was stronger in HIV-negative individuals (AUC 0.91; sensitivity 86%) than in PLHIV (AUC 0.79; sensitivity 67%). Diagnostic performance improved over time.
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