Is the T‑Spot (T‑SPOT.TB) the appropriate test to order for tuberculosis infection?

T-SPOT.TB Testing for Tuberculosis Infection

Yes, T-SPOT.TB is an appropriate and highly accurate test to order for tuberculosis infection, with superior sensitivity (97.2%) compared to traditional tuberculin skin testing and excellent specificity in BCG-vaccinated populations. 1

Test Overview and Mechanism

T-SPOT.TB is an interferon-gamma release assay (IGRA) that detects M. tuberculosis-specific T cells by measuring interferon-gamma production in response to two specific antigens: ESAT-6 and CFP-10. 2, 1 These antigens are not present in BCG vaccine strains or most non-tuberculous mycobacteria, making the test highly specific for true M. tuberculosis infection. 3

Performance Characteristics

Sensitivity and Specificity

• Sensitivity: 97.2% (95% CI: 90.3-99.7%) for detecting active tuberculosis, significantly outperforming TST which showed only 89% sensitivity in head-to-head comparisons 1
• Specificity: 92% for ruling out active TB disease in suspected cases 1
• The test demonstrates 88.3% sensitivity across broader populations with suspected infection, compared to 76.7% for TST 4

Advantages Over Tuberculin Skin Test

• No cross-reactivity with BCG vaccination, eliminating false positives in vaccinated populations 3, 5
• Single patient visit required (no need for return visit to read results) 3
• Lower indeterminate rate (only 1.5% indeterminate results) 5
• Better performance in immunocompromised patients compared to TST 3

Interpretation Criteria

Positive Result

The test is interpreted as positive when a test well (with either ESAT-6 or CFP-10) contains ≥6 spots more than the negative control well AND has at least twice the spots as the negative control well, with the negative control well having ≤10 spots. 2

Indeterminate Result

Results are indeterminate if:

• The mitogen control well has <20 spots (indicating inadequate immune response) 2, 6
• The negative control well has >10 spots (indicating excessive background) 2

Quality Control via Mitogen Response

• The mitogen component verifies functional T-cell response capability 6
• Mitogen wells should show ≥20 spots for valid test interpretation 6
• Low mitogen response may indicate immunosuppression, requiring repeat testing or consideration of alternative diagnostic approaches 6

Clinical Applications

When to Order T-SPOT.TB

• Screening for latent TB infection in high-risk populations 3, 5
• Evaluation of suspected active TB when rapid diagnosis is needed 7
• BCG-vaccinated individuals where TST would have poor specificity 3, 5
• Immunocompromised patients (HIV, immunosuppressive medications) where TST performance is suboptimal 3
• Contact investigations requiring accurate identification of infected individuals 5

Diagnostic Accuracy in Clinical Practice

In real-world clinical settings, T-SPOT.TB achieved an AUC of 0.896 in ROC analysis, superior to TB-DNA from sputum (0.772) and sputum smears (0.698). 7 This demonstrates excellent discriminatory ability for tuberculosis infection.

Important Clinical Caveats

Cannot Distinguish Active from Latent Infection

• T-SPOT.TB cannot differentiate between active TB disease and latent TB infection 3
• Clinical correlation, imaging, and microbiological confirmation remain essential for diagnosing active disease 3

Immunosuppressed Populations

• While T-SPOT.TB performs better than TST in immunocompromised patients, indeterminate results may still occur due to inadequate T-cell function 6, 3
• If mitogen response is inadequate on repeat testing, consider TST as an alternative diagnostic approach 6

Correlation with TST Results

• 79% concordance observed between T-SPOT.TB and TST (using 10mm cutoff) 5
• Strong correlation exists between positive T-SPOT.TB results and TST induration ≥15mm 5
• Discordant results (positive TST/negative T-SPOT.TB) often represent false-positive TST from BCG vaccination 5

Practical Implementation

Order T-SPOT.TB when:

• Rapid, accurate TB infection diagnosis is needed 7
• Patient has BCG vaccination history 3, 5
• Patient is immunocompromised 3
• Two-step TST visit is impractical 3
• High specificity is required to avoid unnecessary treatment 1

The test requires a single blood draw with results typically available within 24-48 hours, making it highly practical for clinical decision-making. 7