- The prevalence of tuberculous infection may be estimated by the use of the tuberculin test in contradistinction to the clinical disease.
- The positive test indicates that the individual has been infected by the tubercle bacillus sometime without necessarily showing overt signs or symptoms of disease.
- A standardized tuberculin is available now through the World Health Organization.
- It is available in the form of a purified protein derivative of Myco. tuberculosis (PPD/RT 23) with Tween 80 as a stabilizer.
- It is the intra-cutaneous injection of the measured amount (usually 1 to 3 tuberculin units) which allows the comparison of the results obtained at different times and in different countries.
- It is also known as Mantoux test.
- A positive test appears as a delayed hypersensitivity tissue reaction, manifesting as an area of oedema (or induration) with a wider erythematous zone.
- The diameter of the indurated area is measured after three days and the reading is expressed in millimeters.
- In many warm climate countries where non-specific tuberculin reactions, presumably related to infection with mycobacteria other than mammalian tubercle bacilli, are common.
- Only reactions of 10 or more mm are likely to indicate previous tuberculous infection.
- The risk of misinterpreting tuberculin positive reactions in these communities may be minimized by dual tuberculin testing.
- This is done with the PPD-S (or human) tuberculin plus tuberculin prepared from a non-mammalian species, e.g. PPD-B (or Battey).
- Non-specific reactions are less common in most countries with temperate or colder climate where a positive test measuring 6 to 8mm is suggestive of tuberculous infection.
- An alternative method of testing for hypersensitivity is done by a multiple puncture apparatus which pricks the skin through a film of stronger tuberculin.
- This method is called Heaf test.
- This method may also be used with undiluted BCG vaccine instead of tuberculin.
- The Heaf test is read as grades 1 to 4 according to the degree of reaction.
- While reading is done, grade 1 probably indicates a non-specific response.
- This test has the advantage of speed, acceptability and reproducibility but is not so easily standardized as the Mantoux test.
Application and uses of tuberculin test
- Tuberculin tests to measure the prevalence of infection may be applied to the children at school entry (5 to 6 years) or at 10 to 13 years.
- In Britain it is used as the preliminary screening test before BCG vaccination.
- This test is used in contact tracing of infectious cases of tuberculosis in the families and close relatives of young children who give positive reactions.
- It is also used as an aid to the clinical diagnosis of suspected infection in young children.
- It is also used as an indication of early clinical infection in older children who give strongly positive reactions (15mm or more). These kinds of children require regular follow up with X-ray examinations for 2 to 3 years.
- It is also used as a post-vaccination check on the efficacy of BCG vaccination.
Tuberculin Skin test (Mantoux test)