Microscopic examination of sputum smears

Sputum microscopy is the most efficient way of identifying a tuberculosis infection. It is the primary tool used for diagnosing TB and for monitoring the progress of treatment until the patient is cured. You are expected to oversee the collection of sputum samples during initial diagnosis, and at various times during drug treatment to monitor the effectiveness of the treatment.

You will recall from Study Session 13 that three sputum samples will be collected over two consecutive days, and that one of the sputum samples is collected in the morning. The samples are sent to a laboratory and Figure 14.1 illustrates a health professional examining a sputum smear using a microscope. The smear is treated with chemicals that reveal the presence of TB bacteria and these can be seen using a microscope (they cannot be seen with the naked eye). The examined specimens are classified as being either smear-positive pulmonary TB or smear-negative. However, you should know that a smear-negative result could either mean the patient has TB but it is not showing in the sputum, or that the person does not have TB.

An example of a positive smear, seen under a microscope, is shown in Figure 14.2. The smear has been stained with chemicals to reveal the presence of TB bacteria (they appear as purple rod-shaped bacteria).

A diagnosis of TB is made if at least two out of the three sputum smears are positive for TB bacteria. TB is also confirmed if one sputum specimen is positive for bacteria, and there is also evidence of abnormalities in a chest X–ray. Finally, in people living with HIV (or in the presence of a strong clinical suspicion of HIV infection), only one positive smear result is necessary to make a diagnosis of smear-positive pulmonary TB.