Interesting question there Rahul. A number of researchers have given them a head to head comparison and here are the following results;

1. "For young children presenting in a community setting after exposure to tuberculosis or with symptoms suggesting tuberculosis, T-SPOT.TB cannot be used to exclude active disease. The sensitivity of this assay may be impaired for very young children."Nicol MP et al. Comparison of T-SPOT.TB assay and tuberculin skin test for the evaluation of young children at high risk for tuberculosis in a community setting. Pediatrics 2009 Jan;123(1):38-43. doi: 10.1542/peds.2008-0611.

2. "The sensitivity and NPV of the TST were 38.3 and 60.8%, respectively. The T-SPOT.TB specificity (80.9%) and positive predictive value (81.3%) were lower than those of TST (95.7 and 90.0%, respectively)." Simsek H et al.  Comparison of tuberculin skin testing and T-SPOT.TB for diagnosis of latent and active tuberculosis.  Jpn J Infec disease. 2010 Mar;63(2):99-102.

3. T-SPOT.TB is comparable to the TST in the diagnosis of

tuberculosis disease and identification of high-risk children with tuberculosis

infection and is more specific than the TST in children who

have received the BCG vaccine. Cruz AT. Pediatrics 2011;127:e31–e38

Dear Rahul,

This might be quite a difficult question to answer since what is expected from a test does not always correlate with what happens in vivo.

Neither TSpot nor TST detect the causative organism or components of the organisms. Rather, they rely on the immunologic response to different antigens (ESAT-6 and CFP-10 for TSpot and PPD for TST). As a result, their specificity and sensitivity will vary depending on what is being tested (example latent TB versus active disease), the population being tested (example patients living in countries with high burden of non-tuberculous environmental mycobacteria or those having been vaccinated with BCG)

Some studies have been looking at these as can be seen with Thabisa's response. Pooled studies have shown similar specificity (88% for TSpot and 86% for TST)

I suggest you have a look at the CDC guidelines on using IGRAs from 2010. It can be accessed at www.cdc.gov/mmwr/PDF/rr/rr5905.pdf. Especially have a look at the tables towards the end which give you a lot of information about the sensitivities and specificities of different IGRAs.

Hope this helps

Dear colleague

Because IGRAs do not cross-react with BGC, an IGRA is the preferred test by many experts for the diagnosis of LTBI in a BCG-immunized child older than 4 years of age. The sensitivity of these blood IGRA tests is similar to that of TSTs for detecting infection in adults and children who have untreated culture confirmed tuberculosis. The specificity of IGRAs is higher than that for TSTs, because the antigens used are not found in BCG or most pathogenic nontuberculous mycobacteria.