New recommendations for the duration of respiratory isolation based on the time to detect M. tuberculosis in liquid culture

¹ Dept of Microbiology, LabPLUS, Auckland City Hospital
² Green Lane Respiratory Services, Auckland City Hospital
³ Auckland Regional Public Health Service

^* To whom correspondence should be addressed. E-mail: sritchie{at}adhb.govt.nz.

	Abstract

We hypothesised that time to detect Mycobacterium tuberculosis in liquid culture of sputum from patients with pulmonary tuberculosis may be a better indicator for the duration of respiratory isolation than sputum smear status.

We reviewed pre-treatment and during-treatment sputum acid-fast bacilli smear and culture results in 284 patients with pulmonary tuberculosis. The time to detect M. tuberculosis in liquid culture (TTD-TB) was the number of days from inoculation of the Mycobacterial Growth Indicator Tube to culture detection and visualisation of acid-fast bacilli.

The median TTD-TB for smear group 0 (no bacilli seen) was 14 days (interquartile range 12-20). This value was used as the standard at which release from isolation could be permitted. In smear group 4 (>9 AFB·hpf^-1 in sputum specimens before treatment) patients the TTD-TB exceeded 14 days after a median of 25 days of treatment.

We recommend that patients in smear groups 1 and 2 (1-9 AFB/100 hpf and 1-9 AFB/10 hpf in sputum specimens before treatment) receive treatment in respiratory isolation for 7 days, provided the risk of drug resistance is low. Smear group 3 (1-9 AFB·hpf^-1) patients should receive treatment in respiratory isolation for 14 days; and smear group 4 patients for 25 days. These criteria would have reduced the duration of respiratory isolation by 1516 days in our 146 study participants with sputum smear positive pulmonary tuberculosis.

Provided clinical and radiographic criteria are satisfactory, use of TTD-TB could enable the duration of respiratory isolation to be predicted from the pre- treatment sputum smear grade. Our recommendations enable isolation to end well before sputum becomes smear negative, with considerable benefits to patients and health-care providers.