Blood Tests for Tuberculosis
The primary blood tests for TB are interferon-gamma release assays (IGRAs), specifically QuantiFERON-TB Gold In-Tube and T-SPOT.TB, which detect TB infection by measuring immune response to TB-specific antigens. These tests are FDA-approved and recommended by CDC guidelines for diagnosing both latent TB infection (LTBI) and active TB disease 1.
Available Blood Tests
Interferon-Gamma Release Assays (IGRAs)
Two commercial IGRAs are currently endorsed:
QuantiFERON-TB Gold In-Tube (QFT-GIT): Measures interferon-gamma concentration in blood plasma after exposure to TB antigens (ESAT-6, CFP-10, and TB7.7). Results available within 8-30 hours 2.
T-SPOT.TB: Enumerates individual cells producing interferon-gamma using an ELISPOT technique. Requires blood processing within 8 hours (extendable to 32 hours with T-cell Xtend additive) 2.
Both tests use antigens specific to Mycobacterium tuberculosis that are absent from BCG vaccine and most non-tuberculous mycobacteria, making them more specific than the tuberculin skin test 1.
Key Advantages of Blood Tests
IGRAs offer several clinical benefits:
- Higher specificity (92-97%) compared to tuberculin skin test (56-95%), particularly in BCG-vaccinated populations 3
- No cross-reactivity with BCG vaccination or most atypical mycobacteria (except M. kansasii, M. szulgai, M. marinum) 1
- Single visit required - no need for return appointment to read results 1
- No reader bias - laboratory-based assay eliminates placement and reading errors 1
When to Use Blood Tests
CDC guidelines recommend IGRAs may be used in all circumstances where tuberculin skin testing is currently used 1, including:
- Contact investigations
- Evaluation of recent immigrants
- Sequential-testing surveillance programs (e.g., healthcare workers)
- Screening before immunosuppressive therapy (TNF-α inhibitors, biologics) 4, 5
For immunosuppressed patients or those at highest risk, consider dual testing strategy (both tuberculin skin test and IGRA) to maximize sensitivity, particularly in countries with medium or high TB prevalence 5.
Important Limitations
Be aware of these critical caveats:
- Indeterminate results occur more frequently with QuantiFERON (11%) than T-SPOT.TB (3%), particularly in immunosuppressed patients and children under 5 years 6
- Advanced immunosuppression may cause false-negative results in all immunologically-based tests 3
- Low positive predictive value - a positive test does not predict who will develop active TB disease 7
- Cannot distinguish between latent infection and active disease - must rule out active TB with clinical evaluation, chest X-ray, and sputum testing before treating as LTBI 8, 5
Baseline Laboratory Tests for Active TB
When diagnosing or treating active TB, obtain these blood tests 9:
- HIV testing - all TB patients should be tested
- CD4 count - if HIV-positive
- Hepatitis B and C serology - for patients with risk factors (injection drug use, foreign birth in Asia/Africa, HIV infection)
- Liver function tests: AST, ALT, bilirubin, alkaline phosphatase
- Serum creatinine
- Platelet count
These baseline measurements guide treatment decisions and monitor for medication toxicity during TB therapy 9.
Emerging Technologies
Newer blood-based tests are under development, including host response assays that measure mRNA expression patterns and cell-free DNA detection methods, but these are not yet widely available or endorsed for routine clinical use 10, 11.