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Diagnosing Tuberculosis

NYU Langone physicians are experienced at recognizing tuberculosis, an infectious disease caused by the bacterium Mycobacterium tuberculosis. Tuberculosis usually affects the lungs, but can sometimes occur in other parts of the body, such as the lymph nodes and bones. It is one of the most common infectious diseases in the world, but is relatively rare in the United States. 

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Pulmonary tuberculosis—in which the disease occurs in the lungs—is contagious. It spreads when a person inhales infectious droplets, which are released into the air when an infected person coughs, sneezes, or talks. Because the droplets can spread through the air, the disease is easily transmitted. 

In most people, the immune system is able to fight off tuberculosis bacteria without treatment. Sometimes, however, if the immune system is compromised because of certain health conditions, such as HIV, the body may not be able to suppress an infection. 

People with type 2 diabetes are also vulnerable to tuberculosis infection, because diabetes can weaken the immune system. Having leukemia, a cancer of the blood, or lymphoma, a cancer of the lymphatic system, also increases a person’s risk of tuberculosis because the body’s lymphocytes—immune system cells that fight disease—are compromised. Doctors may recommend tuberculosis screening for people at increased risk. 

In most people who have been infected with the bacterium that causes tuberculosis, the disease is latent. This means that the immune system prevents bacteria from multiplying or making a person ill. However, sometimes the infection may become active after lying dormant for months or years. 

Symptoms of active tuberculosis include a very high fever, a cough that persists for three weeks or longer, chills, sweats, and weight loss. Occasionally, coughing up blood is a symptom.

To diagnose tuberculosis, an NYU Langone pulmonologist takes a medical history and conducts a physical exam. He or she asks about your symptoms, health, lifestyle, and travel history. Your doctor may also perform several tests. 

Sputum Culture 

A sputum culture is the primary test used to diagnose tuberculosis. Your doctor asks you to cough deeply to produce mucus from the lower airways, which is called sputum. After you spit the mucus into a container, specialists examine the sample for bacteria. 

The tuberculosis bacterium can be identified immediately in about two-thirds of people who have the infection. In others, it can be identified only after the Mycobacterium tuberculosis organism grows in the laboratory culture, for up to two months. 

Polymerase Chain Reaction Test

If a sputum culture appears to test positive, doctors perform a polymerase chain reaction (PCR) test to confirm that the organisms seen are tuberculosis bacteria. In some cases, doctors may perform the PCR test on a sputum culture that appears to be negative. This can reveal the presence of tuberculosis genetic material faster than waiting for the culture to grow, although it does not confirm a diagnosis.

Chest X-ray

In a chest X-ray, electromagnetic radiation forms an image of the organs in your chest, such as your heart and lungs. An X-ray can detect damage in the lungs, which may indicate tuberculosis. 

CT Scan

If a chest X-ray does not produce a clear enough image or is not definitive, your doctor may order a CT scan. A series of X-rays is taken from different angles to form clear images of the bones and soft tissues in your body. A CT scan can detect more subtle signs that might indicate tuberculosis.


Rarely, a doctor might suspect a person has tuberculosis despite negative sputum tests and negative PCR test results. In this case, he or she may perform a bronchoscopy. During this exam, the doctor inserts a bronchoscope—a long, thin, flexible tube with a light attached to it—through the nose or mouth and into the airways of the lungs. You are given sedation to help you relax during the procedure.

Images of the lungs show up on a video monitor. Your doctor examines them for signs of inflammation in the lungs. About 80 percent of people with tuberculosis have a type of chronic inflammation known as granulomatous inflammation.