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Laboratory Tests

Latent TB infection
Testing for Mycobacterium tuberculosis begins with a skin or antibody test for latent TB infection. This is not used as a general screen, but is targeted at those who are at a high risk for contracting the disease and at those who work or live with high-risk patients. These tests may also be done as part of a physical examination prior to starting school or a new job. Positive results may indicate a latent TB infection and should be followed by other investigations such as chest X-rays to look for signs of active disease.

Active Tuberculosis
To diagnose TB of the respiratory tract, 3 - 5 sputum specimens are collected first thing in the morning when they are most likely to contain the most TB bacteria. If extrapulmonary TB is suspected, samples are collected based upon where in the body the infection is likely to be. Multiple samples of gastric washings/aspirates or urine may be collected and submitted to the laboratory. Sometimes cerebral spinal fluid (CSF), biopsied tissue, or other body fluids are also collected.

A presumptive diagnosis of TB can be made by examining a smear of the patient's specimen under the microscope after it has been stained with a special stain to detect acid fast bacteria (AFB). Positive AFB smears are likely to indicate a TB infection, since M. tuberculosis is the most common acid-fast bacillus, but the smears cannot distinguish between the different species of "acid-fast" bacilli.

A genetic probe or molecular TB test can add additional information. It amplifies/replicates genetic components of TB and can narrow the identification to a group of mycobacteria (of which M. tuberculosis is the most common). While AFB smears and genetic tests may provide results the same day that the samples are submitted, both positive and negative results must be confirmed by culture, which can take 6-12 weeks dependant on sample type.

AFB cultures are set up using decontaminated, digested, and concentrated body samples. Nutrients and incubation provide a supportive environment for the slow growing mycobacteria. The results of cultures are definitive: They can tell your doctor what organisms are present and what drugs are likely to kill them but they take time - days to several weeks for positive samples, up to six to twelve weeks to confirm negative results.

Once M. tuberculosis has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.

Non-Laboratory Tests

X-rays are often used as a follow-up to positive TB skin and antibody tests to look for signs of mycobacteria growth and to help determine whether someone has active tuberculosis or a latent TB infection. Infection with TB can cause a number of characteristic findings on x-rays, including cavities (holes) and calcification in organs such as the lungs and kidneys. More information on radiological tests can be found at The Royal College of Radiologists.

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