Interferon Gamma Release Assays (IGRA) for Tb Testing

• Two IGRAs currently available, Quantiferon and T-Spot TB Assay
• Basic idea is that you mix patient's blood with TB antigens and measure IFN-gamma release by T-cells (measured by ELISA)
• Results reported as positive, negative, or indeterminate

Advantages of Quantiferon over PPD:

1. Higher specificity - > 95% vs ~70% for PPD.  Similar sensitivity ~80%]
2. No cross reactivity with BCG vaccination
3. One time lab draw, eliminates need for return visit
4. Easier to interpret - eliminates reader variability

Disadvantages vs PPD

1. More expensive.  However, some argue that due to higher specificity, you get less false positives --> less f/u visits, CXRs, and needless prescription of INH (with all of its toxicities)
2. Still does not distinguish active vs latent TB
3. One big argument is that there is no good long-term data that treating patients with positive IGRA leads to decrease in active TB rates, unlike for PPD where it is well known that treating latent TB results in decreased active TB by 60-90%.  Treating quantiferon positive patients probably does decrease TB reactivation rates, but it just hasn't been studied much yet.

(Chanu Rhee MD, 10/8/10)