Mycobacterium tuberculosis Culture (Bronchial Secretions)

This test is done to find out whether you have been infected with the bacteria that cause tuberculosis (TB). It is often recommended for people who have spent time around someone with active TB disease, who have symptoms such as a lasting cough, fever, or unexplained weight loss, or who need testing before starting certain jobs, school programs, or medicines that weaken the immune system.

Your healthcare provider may order this test if you have symptoms like persistent cough, fever, night sweats, or weight loss—especially if you live in an area with higher TB rates or have risk factors like weakened immunity. It’s also used to monitor treatment effectiveness or detect TB in people who don’t show classic symptoms.

The test checks whether your immune system reacts to TB bacteria. With the skin test, a small amount of tuberculin fluid is placed just under the skin on the inside of your forearm. If you have been infected, your immune system responds and a firm bump forms at the site within a couple of days. A provider measures the size of this bump to determine whether the result is positive or negative.

A sample of your respiratory secretions (e.g., sputum) is collected and sent to a lab. The sample is placed in a special medium to encourage bacterial growth. After several weeks, if *M. tuberculosis* is present, it will appear as colonies (tiny, fuzzy growths) under a microscope or in culture plates.

No special preparation is needed before the test. Tell your provider if you have ever received the BCG vaccine or had a previous positive TB test, since this can affect how the result is read. For the skin test, you will need to return within 48 to 72 hours so a nurse can examine your arm and measure any reaction. A TB blood test requires only one visit, and results are usually available within a few days.

No special preparation is needed, but you may be asked to avoid certain medications (like antibiotics) that could interfere with results. Results can take 2–8 weeks, depending on the lab’s methods. Some labs use rapid tests (like GeneXpert) for quicker preliminary results.

After a skin test, the injection site may feel slightly itchy or tender, which is normal. Try not to scratch or rub the area, and do not cover it with a bandage or apply lotions. Keep the arm clean and dry, and avoid rubbing the spot even if a bump forms. Your provider will examine the site at your follow-up visit, measure any reaction, and explain what the result means for you.

If the culture is positive, you’ll likely start TB treatment immediately. If negative, further testing (like a TB skin test or X-ray) may be needed. Follow-up cultures may be done during treatment to ensure the infection clears.