Geographic differences in time to culture conversion in liquid media: Tuberculosis Trials Consortium study 28. Culture conversion is delayed in Africa

Abstract

Background: Tuberculosis Trials Consortium Study 28, was a double blind, randomized, placebo-controlled, phase 2 clinical trial examining smear positive pulmonary Mycobacterium tuberculosis. Over the course of intensive phase therapy, patients from African sites had substantially delayed and lower rates of culture conversion to negative in liquid media compared to non-African patients. We explored potential explanations of this finding.

Methods: In TBTC Study 28, protocol-correct patients (n = 328) provided spot sputum specimens for M. tuberculosis culture in liquid media, at baseline and weeks 2, 4, 6 and 8 of study therapy. We compared sputum culture conversion for African and non-African patients stratified by four baseline measures of disease severity: AFB smear quantification, extent of disease on chest radiograph, cavity size and the number of days to detection of M. tuberculosis in liquid media using the Kaplan-Meier product-limit method. We evaluated specimen processing and culture procedures used at 29 study laboratories serving 27 sites.

Results: African TB patients had more extensive disease at enrollment than non-African patients. However, African patients with the least disease by the 4 measures of disease severity had conversion rates on liquid media that were substantially lower than conversion rates in non-African patients with the greatest extent of disease. HIV infection, smoking and diabetes did not explain delayed conversion in Africa. Some inter-site variation in laboratory processing and culture procedures within accepted practice for clinical diagnostic laboratories was found.

Conclusions: Compared with patients from non-African sites, African patients being treated for TB had delayed sputum culture conversion and lower sputum conversion rates in liquid media that were not explained by baseline severity of disease, HIV status, age, smoking, diabetes or race. Further investigation is warranted into whether modest variation in laboratory processes substantially influences the efficacy outcomes of phase 2 TB treatment trials or if other factors (e.g., nutrition, host response) are involved.

Trial registration: ClinicalTrials.gov NCT00144417.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Probability of sputum culture conversion among TBTC Study 28 protocol-correct patients, by week of treatment, region (Africa vs. non-Africa) and culture media type (solid vs. liquid).

Figure 2. Probability of sputum culture conversion in liquid media by baseline sputum smear, African/non-African site and week of treatment, TBTC Study 28 protocol-correct patients.

Figure 3. Probability of sputum culture conversion in liquid media by baseline aggregate cavity size, African/non-African site and week of treatment, TBTC Study 28 protocol-correct patients.

Figure 4. Probability of sputum culture conversion in liquid media by baseline radiographic extent of involvement, African/non-African site and week of treatment, TBTC Study 28 protocol-correct patients.

Figure 5. Probability of sputum culture conversion in liquid media by baseline number of days to detection (DTD) of growth in liquid media, African/non-African site and week of treatment, TBTC Study 28 protocol-correct patients.

Figure 6. Probability of sputum culture conversion among African patients by HIV status by media type and week of treatment, TBTC Study 28 protocol-correct patients.

Figure 7. Probability of sputum culture conversion in liquid media by race (black and non-black), African/non-African site and week of treatment, TBTC Study 28 protocol-correct patients.

Figure 8. Probability of sputum culture conversion in liquid media by NaOH final concentration used for specimen decontamination, African/non-African site and week of treatment, TBTC Study 28 protocol-correct patients.