Shorter time to TB culture detection related to high transmission risk

A time-to-detection of tuberculosis of fewer than 9 days in culture was associated with an increased risk for transmission of the disease, according to researchers from the Heartlands Hospital in Birmingham, England.

“Where it is available, [time-to-detection] is superior to sputum smear microscopy for this purpose and will allow tuberculosis control services to better optimize the utilization of scarce resources,” the researchers wrote in Clinical Infectious Diseases. “However, it is important to stress that all contacts, irrespective of the [time-to-detection] of the index case, require screening.”

The researchers performed a retrospective review of consecutive TB cases at the Heartlands Hospital from 2010 to 2012. Among the 184 cases of pulmonary TB during this period, 111 were included in the analysis of the association between time-to-detection (TTD) and risk for transmission. The analysis also included 825 contacts who completed screening, of whom 165 were positive.

TTD was categorized as short (<9 days) or long (≥9 days). Among patients with a short TTD, the transmission rate to contacts was 0.32 (95% CI, 0.26-0.38). The transmission rate for patients with a long TTD was 0.15 (95% CI, 0.12-0.18). Short TTD was associated with a higher rate of transmission (adjusted OR=2.1; 95% CI, 1.37-3.24). Age also was associated with transmission: Index cases aged 20 to 39 years had the highest risk for transmission (adjusted OR=2.18; 95% CI, 1.31-3.64). Most transmission events were due to latent TB.

A separate analysis included 136 patients to evaluate a new chest radiograph grading scheme and its association with TTD. They found that a higher chest radiograph grade predicted a shorter TTD, after adjustment for smear-positivity, for grades 2 to 4 (adjusted HR=1.67; 95% CI, 0.88-3.17) and grades 5 and 6 (adjusted HR=3.01; 95% CI, 1.29-7.06).

“We believe that this novel grading system may be a useful tool in tuberculosis contact tracing, but that it requires further validation,” the researchers wrote.

Disclosure: The researchers report no relevant financial disclosures.