Latent TB is the condition when a person is infected and there is a persistent immune response to that stimulation, what’s been proven by the positive result of the tuberculin skin test, whereas there are no other signs of the disease.
Tuberculosis is an infectious disease caused mainly by Mycobacterium tuberculosis, less commonly – Mycobacterium bovis. M.tuberculosis primarily affects lungs but may also disseminate throughout the body affecting various organs. It is considered that almost one-third of the world’s population is infected. Nevertheless, it was estimated that approximately 10% of infected individuals develop the disease at some point in their lifetime. In order to prevent the development of active tuberculosis a person who has the latent infection should undergo complete prophylactic treatment despite the fact that he/she is asymptomatic.
What is latent tuberculosis?
Individuals with latent tuberculosis do not have any symptoms of the disease, chest X-ray shows no abnormalities and they cannot infect the others, so they can go on with their normal everyday activities, although the preventive therapy is recommended in order to prevent the reactivation of bacteria. Latent tuberculosis is confirmed by the positive results of a tuberculin skin test.
It was estimated that the lifetime risk of developing active tuberculosis for a person with confirmed latent TB is about 5-10% with majority cases occurring within the first five years after acquiring the infection. The risk of developing active disease depends on the immune system of the infected person.
Tuberculin skin test
Tuberculin skin tests which include the Mantoux test and the Heaf test (which is no longer used) are performed to determine if a person is infected with M.tuberculosis or not. A positive result may be seen after 8-10 weeks after the person became infected.
For the Mantoux test tuberculin (purified protein derivative) which is a dead extracts of Mycobacteria is injected into the skin at a standard dose of 5 tuberculin units in order to estimate the immune response – namely, a positive result means that a person is infected whereas the negative result proclaims that a person is not infected with Mycobacteria.
The results of the test are estimated after 48-72 hours. An area of induration (a hardened area which may be palpated) is measured in millimeters to estimate the presence of TB infection.
- The result is interpreted as “0” if there is no induration at all.
- In HIV-positive individuals, persons who have been recently in contact with a person suffering from tuberculosis, immunosuppressed individuals, persons with abnormal X-ray scan the test is positive if the induration is 5 mm or more.
- In other individuals with increased risk of tuberculosis, the result is positive if the induration is 10 mm or more.
- In individuals with no risk factors, the test is considered positive if the lesion is 15 mm or more.
Nevertheless, sometimes the result of TST may be false-positive or false-negative.
The test has low specificity, therefore, most positive tests in individuals with low risk of tuberculosis are considered false-positive, probably due to nontuberculous mycobacteria infection, previous vaccination with BCG or allergic reaction to tuberculin.
In individuals with the suppressed immune system due to HIV-infection, immunosuppressive therapy, and infectious mononucleosis the test may be false-negative. The test becomes positive 8-10 weeks after the person has acquired infection. Respectively, in the case of recent TB infection, the Mantoux test may appear false-negative.
A tuberculin test conversion is a criterion used to make a conclusion about the tuberculosis infection. It is defined as an increase of test result of 10 mm or more within 2 years.
Another test used to identify infection is interferon-gamma (IFN-γ) test.
Groups of people who should be tested for latent tuberculosis
- Children younger than 5 years;
- People living with HIV regardless of age;
- Those who receive immunosuppressive treatment (preparing for organ transplantation, undergo cancer therapy);
- Those who receive dialysis;
- Those who are contacts of individuals with pulmonary tuberculosis;
- Prisoners, health care workers, homeless people, and intravenous drug users;
- A course of isoniazid for 6-9 months is recommended for treatment of latent infection and may be used for prevention of active tuberculosis. Vitamin B6 or pyridoxine should be given simultaneously to prevent adverse effects;
- Children may be given isoniazid and rifampicin for 3 months;
- Another option is the administration of rifampin for 4 months;
- All individuals who have HIV and receive antiretroviral therapy should also receive antituberculosis treatment.