Introduction: Who Should Undergo Diagnostics

Not everyone needs to be tested for tuberculosis, but certain situations make getting checked very important for your health and the health of those around you. If you’ve been spending a lot of time with someone who has active TB disease, especially someone you live with, work with, or care for, you should seriously consider getting tested. The TB bacteria spread through the air when someone with active lung TB coughs, sneezes, talks, or sings, so close and prolonged contact increases your risk of infection.^[1]

You should also seek testing if you’re experiencing symptoms that could point to tuberculosis. These symptoms typically develop gradually and might seem mild at first, which is why many people wait too long before seeing a doctor. A cough that persists for more than three weeks is one of the most important warning signs. If you’re also coughing up blood or thick mucus from deep in your lungs, you should get tested right away. Other symptoms that suggest you might need TB testing include chest pain, ongoing tiredness or weakness, losing weight without trying, losing your appetite, fever, chills, and waking up drenched in sweat at night.^[1][2]

Certain groups of people face higher risk and should consider getting tested even if they don’t have symptoms yet. This includes people born in or who frequently travel to countries where TB is common, such as parts of Asia, Africa, and Latin America. People who live or work in settings where TB spreads more easily—like homeless shelters, correctional facilities, nursing homes, or hospitals—should also be vigilant about testing. Healthcare workers who might be exposed to TB patients need regular screening as part of their occupational health requirements.^[1]

People with weakened immune systems need to be especially careful about TB testing. This includes individuals living with HIV, those who have received organ transplants, people undergoing chemotherapy for cancer, and those taking medications that suppress the immune system such as steroids or treatments for autoimmune diseases. People with diabetes, kidney disease, or other chronic health conditions also face higher risks. Young children under five years old and older adults are more vulnerable to developing active TB disease after infection, so testing becomes more important for these age groups.^[1][2]

Diagnostic Methods for Identifying Tuberculosis

Initial Screening Tests

The journey to diagnosing tuberculosis typically begins with screening tests designed to detect whether TB bacteria are present in your body. There are two main types of screening tests, and your healthcare provider will decide which one is most appropriate for you based on your situation. These tests are particularly useful because they can detect TB infection even when you have no symptoms and the bacteria are lying dormant in your body.^[1]

The Mantoux tuberculin skin test, also called a TST, has been used for over a century and remains a common screening method today. During this test, a healthcare worker injects a small amount of a substance called tuberculin just under the skin on the inside of your forearm. The injection creates a small bump that looks like a mosquito bite. You then need to return to the clinic within 48 to 72 hours so a trained healthcare professional can examine your arm. They’re looking for a raised, hard area at the injection site, and they measure its size carefully. The size of this raised bump helps determine whether your test is positive or negative.^[10][2]

A positive skin test doesn’t automatically mean you have active TB disease—it means your immune system has recognized the TB bacteria because you’ve been exposed to or infected with them at some point. People who received the BCG vaccine (a TB vaccination given in some countries) might test positive even if they’ve never been infected with TB, which is one limitation of this test. A negative test usually means you haven’t been infected, but there are exceptions. If you were very recently exposed to TB, your body might not have developed an immune response yet, leading to a false negative result.^[10][2]

The newer option is a blood test called an interferon-gamma release assay, or IGRA. This test became available in the 2000s and offers some advantages over the skin test. A sample of your blood is taken and sent to a laboratory, where special immune system cells in your blood are tested to see whether they “recognize” tuberculosis bacteria. One major benefit of the blood test is that it requires only one visit to the healthcare facility—you don’t need to return for someone to read the results. Another advantage is that the BCG vaccine doesn’t affect IGRA results, making it more accurate for people who were vaccinated. A positive IGRA test, like a positive skin test, indicates that you have either inactive TB infection or active TB disease, and further testing is needed to determine which.^[3][2]

Confirming Active TB Disease

If your screening test comes back positive, or if you have symptoms of TB disease, your healthcare provider will order additional tests to determine whether you have active TB disease. These tests look for evidence that TB bacteria are actively multiplying in your body and causing illness, rather than just sitting dormant. The specific tests you need depend on where the TB might be located in your body.^[4]

For suspected TB in the lungs or throat—which is the most common location—doctors typically start with imaging tests. A chest X-ray is usually the first step. This painless test creates a picture of your lungs and can show irregular patches or shadows that are typical of active TB disease. The X-ray might also reveal scarring from past TB infections. Sometimes a chest X-ray looks normal even when someone has TB, especially early in the disease, so additional tests are often necessary. In some cases, your doctor might order a CT scan of your chest, which provides more detailed images than a regular X-ray and can help identify TB in difficult-to-see areas.^[10][2]

The most definitive way to diagnose active TB in the lungs is through sputum testing. Sputum is the thick mucus you cough up from deep inside your lungs—not regular saliva from your mouth. Your healthcare provider will ask you to produce sputum samples, often first thing in the morning when it’s easier to bring up material from your lungs. You might need to provide samples on three different days to increase the accuracy of testing. These samples are examined in laboratories using several different methods.^[10][5]

One type of sputum test is called sputum smear microscopy. Laboratory technicians spread your sputum sample on a glass slide, treat it with special stains, and examine it under a microscope to look for TB bacteria. This test can provide results relatively quickly, sometimes within a day, but it requires a fairly large number of bacteria to be present in the sample to give a positive result.^[4]

More sensitive than microscopy is sputum culture. In this test, your sputum sample is placed in a special environment that encourages TB bacteria to grow. Because TB bacteria grow very slowly, culture results can take several weeks to come back. However, this test is extremely accurate and has the added benefit of allowing doctors to perform drug susceptibility testing. This means they can test whether the TB bacteria in your body are resistant to certain antibiotics, which is crucial information for choosing the right treatment.^[10]

Modern laboratories now use advanced techniques called nucleic acid amplification tests, or NAATs, which include molecular diagnostic tests. The World Health Organization recommends these as initial tests for people showing signs and symptoms of TB. These tests can detect the genetic material of TB bacteria very quickly—sometimes within hours—and are extremely accurate. Some of these tests can also identify whether the bacteria are resistant to certain medications at the same time they detect the presence of TB. This rapid turnaround is invaluable because it means treatment can start sooner, reducing the time someone might be contagious to others.^[4][3]

When TB is suspected in parts of the body other than the lungs—what doctors call extrapulmonary TB—different samples and tests are needed. For example, if TB might be affecting your kidneys, doctors will test your urine. If it might be in your brain or spinal cord, they might perform a lumbar puncture to collect spinal fluid. TB in bones or lymph nodes might require a biopsy, where a small piece of tissue is removed and examined in a laboratory. The same types of laboratory tests used for sputum—microscopy, culture, and molecular tests—can be applied to these other body fluids and tissues.^[4][5]

Monitoring Tests During and After Treatment

Once you begin treatment for active TB disease, your healthcare provider will order regular tests to monitor how well the medications are working. These follow-up tests are essential for ensuring that the TB bacteria are being killed and that you’re responding well to treatment. Most commonly, you’ll provide sputum samples regularly—often weekly at first—until tests show that live TB bacteria are no longer present in your lungs. This is called sputum conversion, and it’s an important milestone that indicates you’re likely no longer contagious to others.^[9]

Your doctor will also monitor you for medication side effects. TB medications can sometimes affect your liver, so blood tests to check your liver function are typically done before you start treatment and periodically while you’re taking the medications. If you’re taking ethambutol, one of the TB drugs, you’ll need regular vision tests because this medication can rarely affect eyesight. These might include tests of your visual acuity (how clearly you see) and your ability to distinguish between red and green colors.^[2][13]

Diagnostics for Clinical Trial Qualification

When researchers conduct clinical trials to test new treatments or diagnostic methods for tuberculosis, they need to use standardized tests to determine who can participate in the study. These qualification criteria ensure that all participants truly have the condition being studied and that results from different trial sites can be compared fairly. The specific diagnostic requirements vary depending on what the clinical trial is testing, but there are some common patterns.^[3]

Most TB clinical trials require confirmation of active TB disease through culture testing before someone can enroll. This is because culture remains the gold standard for definitively proving that someone has TB, even though it takes longer than newer molecular tests. Culture testing also allows researchers to perform drug susceptibility testing, which is crucial for trials studying treatments for drug-resistant TB. Some trials might require that TB bacteria be detected in sputum samples using both microscopy and culture, or that a certain amount of bacteria be present, to ensure participants have a level of disease activity that will allow researchers to measure whether a treatment is working.^[12]

Clinical trials often require chest X-rays showing evidence of TB disease as part of their enrollment criteria. Researchers might specify that X-rays must show certain types or patterns of lung damage characteristic of TB. This imaging requirement helps ensure that participants have pulmonary (lung) TB if that’s what the trial is studying, and it provides a baseline for measuring improvement during treatment.^[13]

For trials testing treatments for inactive TB (also called latent TB infection), enrollment criteria typically require a positive tuberculin skin test or IGRA blood test, combined with absence of symptoms and normal or stable chest X-rays. This combination confirms that someone has TB bacteria in their body but not active disease. Some trials might specify a certain size of skin test reaction or a particular result threshold for blood tests to standardize who qualifies.^[9]

Many clinical trials exclude people who have certain other health conditions or who are taking certain medications that might interfere with TB treatment or make it harder to interpret study results. For instance, trials might require tests showing normal liver function at the start, since both TB and TB medications can affect the liver. Kidney function tests, blood counts, and HIV testing might also be required before someone can join a trial, depending on what’s being studied. These additional diagnostic tests help researchers ensure participants’ safety and understand how different factors might influence treatment outcomes.^[13]