Guidance for Studies Evaluating the Accuracy of Sputum-Based Tests to Diagnose Tuberculosis

Abstract

Tests that can replace sputum smear microscopy have been identified as a top priority diagnostic need for tuberculosis by the World Health Organization. High-quality evidence on diagnostic accuracy for tests that may meet this need is an essential requirement to inform decisions about policy and scale-up. However, test accuracy studies are often of low and inconsistent quality and poorly reported, leading to uncertainty about true test performance. Here we provide guidance for the design of diagnostic test accuracy studies of sputum smear-replacement tests. Such studies should have a cross-sectional or cohort design, enrolling either a consecutive series or a random sample of patients who require evaluation for tuberculosis. Adults with respiratory symptoms are the target population. The reference standard should at a minimum be a single, automated, liquid culture, but additional cultures, follow-up, clinical case definition, and specific measures to understand discordant results should also be included. Inclusion of smear microscopy and Xpert MTB/RIF (or MTB/RIF Ultra) as comparators is critical to allow broader comparability and generalizability of results, because disease spectrum can vary between studies and affects relative test performance. Given the complex nature of sputum (the primary specimen type used for pulmonary TB), careful design and reporting of the specimen flow is essential. Test characteristics other than accuracy (such as feasibility, implementation considerations, and data on impact on patient, population and health systems outcomes) are also important aspects.

Keywords: WHO End TB strategy; diagnostics; study design guidance; target product profiles; tuberculosis.

Figure 1.

Precision of accuracy estimates as function of sample size. The lines show the precision of accuracy estimates as a function of sample size, when sensitivity (blue line) and specificity (red line) are fixed at the minimum targets (60% sensitivity among smear-negatives, 98% specificity) established by the target product profile (TPP). The y-axis shows total width of the 95% confidence interval (CI) (ie, upper limit of the 95% CI minus the lower limit of the 95% CI) for sensitivity and specificity for a given sample size. The x-axis shows the number of smear-negative tuberculosis (TB) cases and non-TB cases needed to achieve a given precision for sensitivity and specificity, respectively. Sensitivity among smear-negative TB patients is shown here, rather than overall sensitivity, because (1) sensitivity for detecting this group is a crucial performance target in the TPP and (2) this group represents a small subset of all patients enrolled and thus drives sample size needs. Studies of novel smear-replacement tests should aim to enroll ≥60 smear-negative, culture-positive TB patients [23]. Assuming 30% smear-negative, culture-positive TB prevalence, 200 culture-positive TB cases (assuming no losses or exclusions) would be required to obtain a sensitivity estimate with a 24% 95% CI width. This figure also shows that increasing the sample size beyond 60 smear-negative culture-positive TB cases, yields diminishing returns in terms of narrowing the CI width.

Figure 2.

Example of a sample flow diagram for diagnostic accuracy studies a smear-replacement tests. This figure shows an example sputum specimen flow diagram. Studies evaluating the diagnostic accuracy of novel sputum-based tuberculosis (TB) diagnostics should include sputum specimen flow diagrams in their reporting to allow readers to contextualize accuracy estimates. Flow diagrams should include when sputum was collected (spot vs morning), sputum processing methods, and the type and number of TB tests performed from a single specimen. In this hypothetical study, 3 sputum specimens were collected from all patients (2 spot specimens on day 1 and 1 morning specimen on day 2). Sputum 1 underwent fluorescence microscopy (FM) smear before undergoing glass bead homogenization. The homogenized sample was then split for Xpert MTB/RIF testing and the index test. Sputum 2 and 3 undergo identical processing methods and TB testing (FM smear, solid culture, liquid culture); Mycobacterium tuberculosis (MTB) culture isolates are then sequenced for target single nucleotide polymorphisms (SNPs). LJ, Lowenstein-Jensen; MGIT, Mycobacterial Growth Indicator Tube liquid culture; NTM, nontuberculous mycobacteria; PBS, phosphate-buffered saline; SR, Xpert sample reagent.