Traditional guidelines for removal of patients from respiratory isolation with suspected active pulmonary Mycobacterium tuberculosis (Mtb) infection are three consecutive respiratory specimens negative by smear for acid-fast bacilli (AFB) staining. However, collection of three specimens may be delayed for various reasons, while the patient remains in a high-cost airborne isolation room. Early identification of patients with either negative culture or low organism load and thus low transmissibility of disease would reduce the time spent in respiratory isolation. OShea et al. recently showed that a time to detection in liquid culture (TTD-LC) of greater than or equal to 9 days predicts low transmissibility of disease.
We propose that patients whose first respiratory specimen has a combination of negative AFB smear and negative nucleic acid amplification testing (NAAT) will have predictably long TTD-LC if Mtb grows in culture, and therefore low transmissibility of active tuberculosis. We have retrospectively reviewed all Mtb culture-positive respiratory specimens at University of California Irvine Health (UCI) and Orange County Healthcare Agency (OCHA) over a two-year period. We analyzed the first culture-positive specimen from 94 patients using a combination of NAAT (Cepheid Xpert), TTD-LC (MGIT), and AFB smear grade (0 to 4+).
The figure presents TTD-LC versus sputum AFB grade by smear and NAAT for 94 culture-positive first respiratory specimens (60 from OCHA, 34 from UCI). Of these 94 specimens, 61 (65%) had a positive AFB stain. Of the remaining 33 specimens, 21 (22% of total) had positive NAAT. The key issue involves the remaining 12 specimens (13% of total) with a false-negative AFB smear and NAAT; all had TTD-LC greater than 9 days (range 13 to 40, mean 23 days).
Among patients with suspected active pulmonary Mtb infection whose first respiratory specimen is negative for both AFB by stain and NAAT, the subset who are Mtb culture positive have low transmissibility of disease. Therefore, these patients can be considered for removal from respiratory isolation if clinically appropriate.
M. Ghajar, None
M. Zhowandai, None
S. Prabhu, None
R. Alexander, None
J. Low, None
E. Peterson, None
L. Thrupp, None