Because TB is not easy to diagnose, the path to diagnosing TB usually involves a number of clinical investigations and tests, which together make up the evidence of TB infection. Some of these investigations provide hints that the body might be infected with TB (for example by showing signs on a chest x-ray), and others will then provide proof that this is the case (by finding Mycobacterium tuberculosis). It is important to know all the investigations that usually take place, to know how they work and what they can and cannot prove.

When TB bacilli are inhaled, they rapidly pass through the mouth and nose and pass into the lowest and smallest parts of the airways. They move into the terminal bronchioli and alveoli of the lung. The terminal bronchioli are the smallest part of the bronchi, the structure that guides air from the upper airways (nose, mouth and trachea) into the lung tissue. Alveoli are part of the lung tissue and are the place where the oxygen from the inhaled air is usually used by the body, and transferred into the blood to be carried to the organs that need it.

Pulmonary tuberculosis is defined as an active infection of the lungs (latin pulmo = lung). It is the most important TB infection, because an infection of the lungs is highly contagious due to the mode of droplet transmission. It can be life-threatingly dangerous to the patient: if left untreated, more than 50% of patients with pulmonary tuberculosis die. Worldwide, 87% of all tuberculosis cases that were reported in 2004 were either only pulmonary TB or included pulmonary TB. 

While the majority of TB infections affects the lungs, TB can infect any place in the body: the inner organs, the bone, the brain, the spine etc. If TB infection is not primarily located in the lungs, it is called extrapulmonary TB (extra = outside of, pulmonary = affecting the lung). Worldwide, it is estimated that between 10 to 25% of TB infections occur extrapulmonarily, outside of the lungs.

This article explains how TB is spread and what can be done to prevent it.

In 1882, the German microbiologist Robert Koch discovered the cause of tuberculosis. By doing so, he confirmed that tuberculosis was a disease that can be transmitted from one person to another. He named the agent that he had discovered tubercle bacillus. A few years later it was discovered that the cause for leprosy resembles this tubercle bacillus, and the two of them were categorised together into a group called Mycobacterium.

The MGIT system is another way to test for the resistance of TB bacteria to certain TB drugs. Unlike the Genotype MTBDRplus and the INNO-LiPA tests, it uses another way to examine TB bacteria for resistance to drugs.

The Genotype MTBDRplus and INNO-LiPA Rif.TB assays are tests that can detect if a TB bacterium is resistant to two of the most common TB drugs, rifampicin and isoniazid.

This article descibes the tests that are used to diagnose active TB.