HOW IS TUBERCULOSIS DIAGNOSED?
Mantoux Tuberculin Skin Test (TST): This test is used to check whether a person is infected with TB. A small needle is used to inject a liquid called tuberculin under the skin of the forearm. The injection causes a small, pale bump to appear under the skin. The swollen portion of this bump is called induration. After 48-72 hours (about 3 days), a health care professional examines the bump and measures the diameter of induration. They combine this measurement with a patient’s known risk for TB for the final test result. The cut off for a positive test is lower for people with a higher risk of TB. The TST cannot tell the difference between active infection and latent TB.
Interferon-Gamma Release Assays (IGRAS): This test is used to check whether a person is infected with TB. It is a blood test that measures the immune reaction to Mycobacterium tuberculosis. This test is important because if the person is infected with M. tuberculosis, interferon-gamma is released when mixed with antigens. A positive TB blood result means that TB bacteria are present in the blood, but it does not tell if it is an active infection or latent TB.
Chest Radiograph: If a person has a positive reaction to a TST or IGRAs, their doctor will use a chest x-ray to examine the lungs and see if there are any abnormalities or signs of previous TB infection. If there are no abnormalities, then the patient likely has latent TB. If there are signs of previous infection, the patient likely has latent TB but is at risk of developing secondary TB. If there are new abnormalities, it is possible that the patient has active TB. Chest x-rays cannot be used to definitively diagnose TB.
Acid-Fast-Bacilli (AFB) Tests: Acid-fast-bacilli is the category of bacteria that includes M. tuberculosis. AFB tests are usually performed on sputum, the thick mucus that is coughed up from the lungs.
To get sputum samples, the provider will have the patient cough deeply and spit into a sterile container. Because bacteria levels can vary day-to-day, the patient will have to do this multiple days in a row. If the patient cannot cough up enough mucus, the provider may have them breathe in a saline mist to help them cough more deeply. If they still cannot cough up enough sputum to sample, the provider may opt to perform a procedure called a bronchoscopy. During this procedure, the patient is put under anesthesia and a doctor collects samples from the lungs with a small lighted tube.
If the suspected TB infection is affecting areas other than the lungs, providers can take a tissue sample of the affected area and send it to the lab for AFB testing.
There Are Two Main Types of AFB Tests:
AFB Smear: This tests for the presence of any acid-fast-bacilli. The sample is “smeared” on a glass slide, stained with specific dyes, and examined under a microscope. The results will show if any acid-fast-bacilli are present in the sample, but the results cannot be used for a definite TB diagnosis. These tests return results in about 1-2 days.
AFB Culture: The sample is put into a special container that promotes bacterial growth. TB organisms are often slow-growing so it is monitored for up to 6-8 weeks to see whether bacteria grow. If they do, this can confirm a probable TB diagnosis.
Drug Susceptibility Testing (DST):
Because there is a growing rate of drug-resistant TB, labs often test samples to see which drugs work best to kill the bacteria. The sample is placed in two different environments: one that has been treated with drugs and one that has not. After a month, the growth in the two different environments are compared to see what drugs were the most effective.