Supervised sputum collection reduces TB culture contamination

Sputum cultures are the clinical gold standard for diagnosing tuberculosis (TB) and for monitoring how patients respond to treatment, but they are not foolproof. In particular, cultures grown in MGIT liquid media are vulnerable to microbial contamination, which can make results uninterpretable and undermine both clinical care and research. Previous work has suggested that keeping samples cold during transport and supervising how patients provide sputum can cut contamination, but few studies have tracked contamination week by week through the early months of therapy. To address that gap, a team led by Nandi Niemand Wolhuter conducted a longitudinal study of patients on TB treatment. The researchers collected serial, weekly sputum specimens and set out to test two practical factors that might influence contamination: the time elapsed between when a specimen was produced and when it reached the laboratory, and whether the specimen had been collected under supervision or not. They also aimed to measure how contamination changed over the first 12 weeks of therapy and to look for clinical or behavioral predictors of contaminated cultures.

In this study the team gathered 3,155 sputum specimens from 301 participants undergoing TB treatment, taking samples repeatedly over the first 12 weeks. They evaluated associations between contamination and two main variables: the interval from specimen collection to laboratory processing, and whether the specimen was collected with supervision. Their findings show that contamination was lowest at the start of treatment (12.3%) but rose during the first weeks and then leveled off at roughly 30% from week 8 onward. Samples collected without supervision were significantly more likely to be contaminated at treatment initiation (p=0.048) and across the full 12-week sampling period (p=0.028). The study also documented an inverse relationship between smear grade and contamination risk throughout the sampling period, meaning higher smear grades were associated with lower contamination risk. These results highlight specific, measurable patterns in contamination across weekly cultures and link them to collection practices.

The findings point to practical steps that could make weekly culture monitoring more reliable. Because contamination rose over the course of treatment and stabilized around 30% after week 8, cultures taken later in therapy are more likely to be affected unless collection and transport practices are improved. The association between unsupervised collection and higher contamination suggests that supervising how sputum is produced — and maintaining advised sample handling such as cold chains — can preserve the clinical and research value of MGIT cultures. This is especially important for community-collected specimens, which are increasingly used to follow treatment response outside clinical settings. By identifying when contamination is most likely and linking it to modifiable behaviors, the study offers clear targets for clinics and studies that rely on serial sputum cultures to make treatment decisions or to evaluate new approaches to TB care.