The oldest and time tested test to detect Tuberculosis infection in an individual is TST(Tuberculin skin test),also known as Mantoux test.It is being done with PPD RT -23.The standard strength for its use is 2TU(tuberculin unit).

But this strength is not available in the market and pathologists are compelled to use 5TU,the cut off value of induration for which is not standarized.

Moreover ,there may be false positive results, due to cross reactivity with Mycobacteria other than Mycobacterium tuberculosis (NTM)and BCG Vaccination particularly if the vaccination has been done after the infantile period.

The latest test which will be available in market very shortly is C-Tb skin test.

C-Tb skin test

C-Tb test- This test is done in a similar manner as Mantoux test.Instead of PPD-RT23,in this test, antigens either ESAT6 or CFP 10 are used ,which are also used in Quaniferon Gold test.

This is the reason,it does not cross react with non tuberculous Myobacterium(NTM) as well as with BCG.At the same time, the test is not as complicated as Quantiferon Gold as it does not require sophisticated laboratory.

In the same way as in Mantoux test,0.1 ml of antigen is inoculated intradermally and the induration is meassured after 48-72 hours with ball pen and scale method. The cut off point has been established at 5mm for all age groups,irrespective of HIV status,irrespective of BCG Vaccine status.

The positive result indicates only the infection specifically to Mycobacterium tuberculosis but not the active disease

IGRA TEST

IGRA TEST:This test is interferon gamma release assay test.This is based on the principle that when T lymphocytes infected with Mycobacterium tuberculosis is exposed to specific antigen of M.Tb, in the form of either ESAT6 or CFP10 ,they release interferon gamma which is then measured.There are two tests,

QUANTIFERON  TB GOLD ;In this test total interferon gamma which are released is measured

and

T spot TB. -It is a variant of Ellispot .In this test the number of effector T cells are measured which release interferon gamma upon exposure to specific antigens.

So ,this test is specific for Mycobacterium tuberculosis and it is not positive in cases of NTM(non tuberculosis mycobacteria) or in cases of BCG vaccination.

There is no need of a technique of intradermal inoculation and the measurement of induration as this test uses blood withrawn from patients.

There is no need of different cut off point of induration for HIV and non HIV patients

This test indicates only the infection,not the active disease

But ,this is not reliable below the age of 5 years.

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