How can tuberculosis bacilli be identified?

The most accurate way of diagnosing tuberculosis is to identify TB bacilli. Unfortunately this is not always possible in a child and the diagnosis often has to be made on the history, clinical examination, chest X-ray and Mantoux skin test alone. TB bacilli may be identified by either:

  • Seeing TB bacilli
  • Growing TB bacilli

In adults, TB bacilli are usually identified in a stained sample (smear and direct microscopy) of sputum (not saliva). A special stain is used to identify the bacteria as TB bacilli. The sputum is collected at a TB clinic and the test done at the nearest laboratory. The result can be obtained in a few hours. Patients who have TB bacilli identified by a ‘positive TB stain’ on a smear of their sputum are called ‘smear positive’.

When staining for TB bacilli with Ziehl-Nielsen stain, the stained bacilli ares said to be ‘acid-fast’. Therefore, TB bacilli are often called ‘acid-fast bacilli’ or ‘AFBs’. The greater the concentration of AFBs in the sputum, the greater is the infectious risk to others. Sometimes TB bacilli can also be seen in other body fluids such as CSF or a pleural aspirate, in fine needle aspirates of a lymph node, or in histological specimens obtained through open biopsy.

TB bacilli can also be grown (cultured). Unfortunately this is more difficult and expensive and may take many weeks. Therefore, in most poor countries tuberculosis in adults is confirmed with a sputum smear. TB culture and sensitivity testing is important in communities where multidrug resistance is common.

A definite diagnosis of pulmonary tuberculosis in adults is usually made by identifying TB bacilli in their sputum.

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