Value of examining three acid-fast bacillus sputum smears for removal of patients suspected of having tuberculosis from the "airborne precautions" category.

We examined the potential risk of tuberculosis transmission if we modified our policy for release of patients from the "airborne precautions" category from three negative acid-fast bacillus (AFB) smears to two, or even one. Over a 4-year period, respiratory cultures from 42 patients grew Mycobacterium tuberculosis. Of these, 36 patients (81%) had a positive AFB smear result on the first submitted specimen. One additional patient (2%) had a first smear-positive finding on the second submitted specimen, and no patients had a first smear-positive result on the third submitted specimen. Respiratory cultures from five patients (12%) grew M. tuberculosis without ever having a positive AFB smear result. These data indicate that in our institution, reducing the number of negative smears required before removal of patients from the airborne precautions category would pose little, if any, increase in the risk of spreading tuberculosis.