Date Published: August 2, 2017
Publisher: Public Library of Science
Author(s): Priya B. Shete, Resmi Ravindran, Emily Chang, William Worodria, Lelia H. Chaisson, Alfred Andama, J. Lucian Davis, Paul A. Luciw, Laurence Huang, Imran H. Khan, Adithya Cattamanchi, Juraj Ivanyi.
Improved systematic screening of high-risk groups is a key component of the tuberculosis (TB) elimination strategy endorsed by the World Health Organization (WHO). We used a multiplex microbead immunoassay to measure antibody responses to 28 M. tuberculosis (M.tb) antigens, and assessed whether combinations of antibody responses achieve accuracy thresholds required for a TB screening test.
A random selection of plasma samples obtained from consecutive HIV-negative adults who were admitted to Mulago Hospital in Kampala, Uganda with cough ≥2 weeks’ but <6 months’ duration were analyzed for serological response to 28 M.tb antigens using an in-house multiplex microbead immunoassay. We compared the median difference of the antibody response to each antigen between patients with and without culture-confirmed TB, ranked each antigen according to variable importance (VIM), and assessed the sensitivity and specificity of combinations of antibody responses using an advanced classification algorithm, SuperLearner. Among the 237 patients included in the analysis, 119 (50%) were female, median age was 32 years (IQR 25, 46), and 113 (48%) had TB. Median antibody levels to eight antigens were significantly different between patients with and without TB. A panel including eight of the top ranked antigens had a sensitivity of 90.6% (95% CI 89.4, 93.8) and a specificity of 88.6% (95% CI 78.2, 97.6) (Ag85B, Ag85A, Ag85C, Rv0934-P38, Rv3881, BfrB, Rv3873, and Rv2878c). With sensitivity constrained to be >90%, specificity remained close to 70% with as few as 3 antigens included in the panels.
Measuring antibody responses to combinations of antigens could facilitate TB screening and should be further evaluated in populations being targeted for systematic screening.
In order to meet ambitious tuberculosis (TB) elimination targets, the World Health Organization (WHO) now recommends systematic screening of high-risk groups. Screening for active disease has several benefits, including improved patient outcomes and reduced transmission through detection and treatment of TB at an earlier stage. To facilitate screening, the WHO target product profile for a TB screening test recommends a minimum sensitivity of 90% and minimum specificity of 70% [3, 4]. These targets were selected to minimize the number of false-negative results in those with TB, and to limit the need for unnecessary and costly diagnostic testing in those without TB. In addition, the target product profile calls for a low-cost and simple-to-perform assay that could be performed by front-line health workers at community health centers[4, 5]. Unfortunately, the lack of a screening strategy that meets all of these criteria is a major challenge for uptake of the systematic screening guidelines.
Of 777 HIV-negative patients enrolled into the parent study, 556 met eligibility criteria and 261 were randomly selected for analysis based on the available budget for antibody response testing (Fig 1). Of the 261 patients selected, 24 (9.6%) were excluded for high response to the negative BSA control bead set. Of the 237 patients remaining, 119 (50%) were female, median age was 32 years (IQR 25, 46), and 113 (48%) had TB. Of those patients diagnosed with TB, 97 (86%) were sputum smear positive and 16 (14%) were sputum smear negative.
A simple, low-cost and accurate biomarker-based screening test for active TB is among the highest priorities for TB diagnostics. In this study, we analyzed antibody responses to 28 M.tb antigens and identified at least eight (Ag85B, Ag85A, Ag85C, Rv0934-P38, Rv3881, BfrB, Rv3873, and Rv2878c) that showed potential for utility in TB screening. These eight antigens were the top-ranked antigens based on variable importance and five of them also demonstrated median levels that were >10-fold higher in TB patients than in patients without TB. With sensitivity constrained to ≥90%, specificity was approximately 70% for a panel consisting of the 3 top-ranked antigens and increased to approximately 90% for a panel consisting of all 8 top-ranked antigens. Thus, the antigen panels identified here meet or exceed the minimum accuracy targets for a TB screening test[3, 4] and should be further evaluated in populations targeted for systematic TB screening.
We have identified panels of antibody responses to three to eight antigens that show promise for TB screening. Accuracy approaches minimum recommended accuracy thresholds with as few as three antigens, and is greater than that reported for TB screening using symptoms or chest radiography. If further validated, multiplex serologic testing could facilitate uptake of systematic TB screening of high-risk populations.