Tuberculosis laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Laboratory Findings
- Mantoux Tuberculin Skin Test and/or QuantiFERON®-TB Gold In Tube[2] Test: The Mantoux tuberculin skin test (TST) and the QuantiFERON®-TB Gold In Tube test (QFT-GIT) are used to test for M. tuberculosis infection. Additional tests are required to confirm TB disease. The Mantoux tuberculin skin test is performed by injecting a small amount of fluid called tuberculin into the skin in the lower part of the arm. The test is read within 48 to 72 hours by a trained health care worker, who looks for a reaction (induration) on the arm. The QFT-GIT is a blood test. It measures the patient’s immune system reaction to M. tuberculosis. Once the blood samples are taken, they must be processed within 16 hours. Interpretation of QFT-GIT results is influenced by the patient’s estimated risk for TB infection.
- Diagnostic Microbiology (sputum): The presence of acid-fast-bacilli (AFB) on a sputum smear or other specimen often indicates TB disease (at least 10,000c is needed on the smear to get a postive acid fast bacilli (AFB) stain). Acid-fast microscopy is easy and quick, but it does not confirm a diagnosis of TB because some acid-fast-bacilli are not M. tuberculosis. Therefore, a culture is done on all initial samples to confirm the diagnosis. However, a positive culture is not always necessary to begin or continue treatment for TB because cultures can take up to 3 weeks to yield definite results. A positive culture for M. tuberculosis confirms the diagnosis of TB disease. Culture examinations should be completed on all specimens, regardless of AFB smear results. Laboratories should report positive results on smears and cultures within 24 hours by telephone or fax to the primary health care provider and to the state or local TB control program, as required by law. A mycobacterium tuberculosis direct test (MTD) of nucleic acid amplification can also be performed to diagnose TB. An MTD test is similar to a polymerase chain reaction (pcr) and is very specific. The test is more sensitive than a smear but it is less senstive than a culture, and has the benefit of same day results.
- Diagnostic Microbiology (pleural fluid): A sample of pleural exudate can be analyzed by cytopathology or at a cell count lab. Samples are usually lymphocyte predominant, and cytopathology is more accurate than cell count labs at detecting lymphs. If there is more fluid present, then an AFB lab is more appropriate. A pleural exudate lab test may find sterile pyuria (especially in HIV positive patients), but overall this finding is fairly uncommon. Note: most extra-pulmonary TB is pauci-bacillary, so the yield of tests is very low. This means that negative cultures do not mean no disease (e.g. negative cerebrospinal fluid AFB or even MTD is not that sensitive).
- Drug Resistance: For all patients, the initial M. tuberculosis isolate should be tested for drug resistance. It is crucial to identify drug resistance as early as possible to ensure effective treatment. Drug susceptibility patterns should be repeated for patients who do not respond adequately to treatment or who have positive culture results despite 3 months of therapy. Susceptibility results from laboratories should be promptly reported to the primary health care provider and the state or local TB control program.