Patients eventually shown to have pulmonary tuberculosis often grow both Mycobacterium avium complex (MAC) and Mycobacterium tuberculosis (MTB) from sputum. If the MAC grows first, patients may erroneously be assumed to have MAC disease, with discontinuation of isolation, contact investigation and alterations in chemotherapy. In patient with compatible disease, it would be helpful to know the likelihood of recovering both MAC and MTB as well as the predictive value of a positive sputum smear.
During 2012 and 2013 the Tuberculosis Control Program of the Philadelphia Department of Public health evaluated the relative incidence of MAC versus MTB in patients with suspected pulmonary tuberculosis, the time to culture growth from the initial smear for MTB and MAC, and determined the positive predictive value of a positive acid fast smear.
In this setting, 22% of patients that were culture positive for MTB also grew MAC, with 78% of them growing MAC before MTB. The positive predictive value of a positive acid fast smear was 89% for MTB versus MAC.
There is a relatively high frequency of recovering MAC in TB patients Therefore, the recovery of MAC in a patient with a positive acid fast smear and compatible disease should not rule out MTB until cultures are final.