Tuberculin skin testing (TST) has been used for years as an aid in diagnosing latent tuberculosis infection (LTBI) and includes measurement of the delayed type hypersensitivity response 48–72 hours after intradermal injection of PPD.
Tuberculin (also called purified protein derivative or PPD) is a standardized dead extract of cultured TB, injected into the skin to measure the person’s immune response to the bacteria.
Infection with M. tuberculosis bacterium produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium.
The components of the organism from TB cultures are extracted as the core elements of the classic tuberculin PPD (also known as purified protein derivative).
It is PPD material that is used for skin testing for tuberculosis.
T cells, sensitized by prior infection, are attracted by the immune system to the skin site where they release chemical messengers, lymphokines.
These lymphokines induce indurated margins at and around the injection site through local vasodilation leading to fluid deposition, fibrin deposition, and attraction of other types of inflammatory cells to the area.
Two to 12 weeks of infection incubation is usually necessary after exposure to the TB bacteria in order for the PPD test to be positive.
The standard recommended tuberculin test, known as the Mantoux test.
It is administered by injecting a 0.1 mL of 5 TU (tuberculin units) of PPD intradermally, immediately under the surface of the skin of the forearm.
The skin test determines if someone has developed an immune response to the bacterium that causes tuberculosis (TB).
It usually takes 8 to 10 weeks after exposure before the TB test is positive, that would show if someone had become infected.
TB skin test indirectly tests for TB by measuring immune sensitization to Mycobacterium tuberculosis antigens.
The most common TB skin test administered uses purified protein derivative to elicit immune reactions.
After 48-72 hours, indurations are measured and reactions greater than 5-15 mm are diagnosed as positive.
Grade 0: no reaction, or induration of 3 or less puncture points;
Grade 1: induration of four or more puncture points;
Grade 2: induration of the six puncture points coalesce to form a circle;
Grade 3: induration of 5 mm; or more
Grade 4: induration of 10 mm or more, or ulceration.
A positive response can occur if someone currently has TB, if they were exposed to it in the past, or if they received the BCG vaccine against TB.
Estimates indicate that one-third of the world’s population has latent TB, and around 1.3 million people worldwide die of TB each year.
TB skin test can result in false positive because of BCG vaccine and non-tuberculosis mycobacteria.
Longitudinal studies on TB skin test find that most conversions from negative to positive with false positive, with 53.7% of positive TB skin test results reverting back to negative 6 months later.
If MMR vaccine is given on the previous day or earlier, the TST can reduce the reactivity of the skin test because of mild suppression of the immune system.
BCG vaccinations may result in positive reactions from purified protein derivative for 10 or more years after vaccination.
Non-tuberculosis mycobacterial infections, commonly produce false positives.
In a study of US born healthcare workers and medical students it was noted that up to 50% of TB positive skin test results were a result of non-tuberculous mycobacteria.
A longitudinal study of TB skin tests for latent TB infection in healthcare workers found that most conversions from negative to positive test were false positive with more than 53% of the positive skin test results reverting back to negative 6 months later.
False positive TB skin test among healthcare workers can lead to misdiagnoses and a necessary treatment.