Tuberculosis (TB) Testing: Understanding Your Report

Section Editor: Rodney E. Rohde PhD
June 4, 2026

Tuberculosis (TB) is an infection caused by a bacterium called Mycobacterium tuberculosis, along with a few closely related bacteria. A bacterium is a microorganism, a living thing too small to see without a microscope. TB usually affects the lungs, but it can also affect other parts of the body. It spreads through the air when a person with active TB in their lungs coughs or sneezes.

Testing for TB falls into two groups, and knowing which kind you had is the key to understanding your result. Some tests look for the TB bacteria directly to find an active infection. Other tests check whether your immune system has a memory of TB, which can indicate past exposure or a hidden (latent) infection. This article explains both kinds of tests and what the wording on your report means, so you can better understand a report you have received.

Latent and active tuberculosis

The difference between latent and active TB is central to understanding any TB test. When TB bacteria enter the body, the immune system often controls them without completely clearing them. This is called latent TB: the person has no symptoms, feels well, and cannot spread TB to others. Most people who are infected have latent TB.

In some people, the bacteria become active and multiply, causing active TB. A person with active TB in the lungs may have symptoms and can spread the infection to others. Latent TB can turn into active TB, especially in people whose immune system is weakened. Symptoms of active TB in the lungs can include a persistent cough, coughing up blood, chest pain, fever, night sweats, weight loss, and tiredness. This difference matters because both the tests and what happens next differ between latent and active TB.

Two kinds of TB tests

Tests for active TB look for the bacteria themselves in a sample taken from the body. Tests for latent TB instead look at the immune system’s response to see whether it has encountered TB before. A test for latent TB can show that you have been infected, but it cannot tell whether the infection is latent or active, so it is always interpreted alongside your symptoms and, often, with active TB tests.

Tests for active TB

These tests are usually done on sputum (mucus coughed up from deep in the lungs). Other fluids or a tissue sample (a biopsy), such as from a lymph node, may be used when TB is suspected elsewhere in the body.

Acid-fast bacilli (AFB) smear — A special stain makes TB bacteria appear red under the microscope, even after being washed with acid, which is where the name “acid-fast” comes from. A report of “AFB seen” or “acid-fast bacilli seen” is a positive result. The smear is fast, but it has two limits: other, related bacteria can also look acid-fast, so it is not specific to TB, and because TB bacteria are often few in a sample, a negative smear does not rule out TB.
Mycobacterial culture — Growing the bacteria from the sample. This is the most reliable test, often called the gold standard, and it also allows the laboratory to test which medicines will work, which matters because some TB is resistant to standard drugs. Its drawback is time: TB bacteria grow very slowly, so a culture can take several weeks, and a negative result is usually only confirmed after four to six weeks.
Molecular (PCR) test — Looks for the genetic material of TB directly in the sample, using the methods described in the article on PCR, antigen, and antibody tests. It is much faster than a culture, often giving a result within a day, and some versions can also detect resistance to a key TB drug. As with the smear, a negative molecular test does not completely rule out TB.

When TB is tested on a tissue sample, the pathologist may also see a pattern of inflammation called granulomatous inflammation, which can be a clue to TB. A chest X-ray is often part of looking for active TB in the lungs as well.

Tests for latent TB

These tests look for the immune system’s memory of TB rather than the bacteria themselves.

Tuberculin skin test (TST) — A small amount of harmless TB material is placed just under the skin of the forearm, and a health professional measures the reaction 48 to 72 hours later. A positive reaction usually means past exposure to TB. However, a person who received the BCG vaccine (a TB vaccine given in many countries) or who was treated for TB in the past may also test positive.
Interferon-gamma release assay (IGRA) — A blood test that measures how immune cells respond when they are exposed to parts of the TB bacteria. A positive result indicates past exposure. Unlike the skin test, the IGRA is not affected by the BCG vaccine, which is one of its advantages.

An important point applies to both of these tests: a positive result shows that your immune system has met TB, but it cannot tell whether the infection is latent or active. If active TB is a concern, the active TB tests above are also needed.

How to read your result

The meaning of your result depends on which test was done.

AFB smear: acid-fast bacilli seen — Bacteria that may be TB were seen under the microscope. Because other bacteria can look similar, this is interpreted with your symptoms and confirmed by a culture or a molecular test.
AFB smear: no acid-fast bacilli seen — None were seen, but because TB bacteria can be few in a sample, this does not rule out TB.
Culture positive or negative — A positive culture confirms TB and allows testing of which medicines will work; a negative culture is usually reported only after several weeks.
Molecular (PCR) test: detected or not detected — TB genetic material was, or was not, found. A “not detected” result does not completely rule out TB.
Skin test (TST) or blood test (IGRA) positive — Your immune system has been exposed to TB. This may be a latent infection or, in someone with symptoms, possibly active TB, so further testing may be needed.
Skin test (TST) or blood test (IGRA) negative — No immune memory of TB was found, which makes infection less likely.

What happens after these tests

TB testing describes what was found and informs the decisions you and your healthcare team make together, rather than dictating them on its own. Results are interpreted along with your symptoms, your history of possible exposure, and often a chest X-ray.

Active TB is treated with a combination of antibiotics taken for several months. Finishing the full course is essential, and treatment is often supported by public health programs. Because some TB is resistant to standard drugs, the susceptibility results from the culture guide which medicines are used. Latent TB may also be treated to prevent it from becoming active, especially in people at higher risk. TB is reportable to public health authorities, who help with treatment and with testing people who may have been exposed.

Questions to ask your doctor

Was I tested for active TB, latent TB, or both?
Does my result mean I have an active infection, a latent (hidden) one, or only past exposure?
If my skin test was positive, could it be from the BCG vaccine or a past infection?
Could a negative smear or molecular test still miss TB, and do I need more tests?
Am I contagious, and do the people around me need to be tested?
If I have TB, what does treatment involve, and how long does it last?
Could my TB be resistant to standard medicines?
If I have latent TB, should it be treated to prevent active disease?
Will my results be reported to public health, and what does that mean for me?
Who should I contact with questions about my results or treatment?