Contamination rates in serially sampled sputum specimens obtained during tuberculosis treatment to capture culture conversion

Sputum cultures are the gold standard for tuberculosis (TB) diagnosis and treatment monitoring. However, cultures in MGIT liquid media are susceptible to microbial contamination, often rendering them uninterpretable. Research has shown that maintaining strict cold chains and supervised sample collection can reduce contamination rates, but few longitudinal studies with weekly sampling have explored this. Here, we evaluated whether (i) the time between specimen collection and laboratory processing and (ii) unsupervised specimen collection are associated with contamination rates. Additionally, we estimated contamination rates over the first 12 weeks of treatment and assessed the clinical and behavioral predictors of contamination. We collected 3,155 sputum specimens from 301 participants undergoing TB treatment. Contamination was lowest (12.3%) at treatment initiation, increased over the first few weeks, and stabilized around 30% from week 8 onwards. Samples collected without supervision were more likely to be contaminated at treatment initiation ( P = 0.048) and over 12 weeks ( P = 0.028). We observed an inverse relationship between smear grade and contamination risk throughout the sampling period. These findings underscore the importance of supervised sputum collection to reduce contamination and provide ways to enhance the clinical and research values of weekly cultures, particularly those collected later in treatment. This is especially relevant for community-collected specimens used in monitoring treatment response.

IMPORTANCE

It is essential to understand how we can minimize sputum specimen contamination rates, as culture contamination may lead to false negative or indeterminate results that require repeat sampling and testing, increasing the burden on healthcare systems and potentially delaying treatment initiation. This research underscores the importance of maintaining a stringent cold chain and highlights the need for participant education and supervision during sample collection. The findings from this study have important implications for TB diagnosis and treatment outcome programs in low- and middle-income countries where biosafety level 3 facilities are located within centralized national reference laboratories or tertiary care hospitals.