A negative AFB smear could indicate that there is no infection, that symptoms are caused by something other than mycobacteria, or that mycobacteria were not present in sufficient numbers to be detected under the microscope. Typically, three samples are collected to enhance the likelihood of detecting the organisms.
If AFB smears are negative but there is still a high suspicion of a mycobacterial infection, more samples may be taken and analysed on different days. A smear negative sample may nonetheless grow mycobacteria because the culture media permits small amounts of bacteria that cannot be seen under a microscope to multiply and be discovered.
AFB smears that are positive indicate a possible mycobacterial infection. A culture, however, is required to establish a diagnosis and identify the species of mycobacteria present.
A positive AFB smear or culture several weeks after starting medication treatment may indicate that the treatment regimen is ineffective and should be modified. It also signifies that the person is still contagious and can spread the mycobacteria to others by coughing or sneezing.
A negative culture indicates that the person examined does not have an active AFB infection or that mycobacteria were not present in that specific sample or were present in too few numbers to be detected. Cultures are kept for six to eight weeks before being deemed negative. The person tested may have a latent infection that resulted in a positive TB screening test but does not have active TB.
A negative culture many weeks following treatment shows that the tuberculosis infection is responding to treatment and that the patient is no longer infectious.