Positive QuantiFERON-TB Gold Plus Test
A positive QuantiFERON-TB Gold Plus test indicates Mycobacterium tuberculosis infection (either latent TB infection or active TB disease), and you must immediately rule out active TB disease through clinical evaluation, chest radiograph, and symptom assessment before considering treatment for latent TB infection. 1
What the Test Indicates
A positive QFT-Plus result means the patient has been infected with M. tuberculosis at some point, but the test cannot distinguish between latent TB infection (LTBI) and active TB disease 1. The test detects interferon-gamma release from sensitized T-cells when exposed to M. tuberculosis-specific antigens (ESAT-6 and CFP-10), which are absent from BCG vaccine strains and most non-tuberculous mycobacteria 1.
Immediate Next Steps
1. Rule Out Active TB Disease First
You must exclude active TB before diagnosing LTBI. This requires:
- Symptom assessment: Ask specifically about chronic cough (>2-3 weeks), fever, night sweats, weight loss, hemoptysis
- Chest radiograph: Look for infiltrates, cavitation, pleural effusion, or lymphadenopathy
- Sputum examination: If symptoms present or chest X-ray abnormal, obtain 3 sputum samples for acid-fast bacilli smear and culture
- HIV testing: Essential as HIV significantly increases progression risk 1
If any findings suggest active TB, refer immediately for full TB evaluation and do not treat as LTBI alone.
2. Assess Risk Factors for Progression
Once active TB is excluded, evaluate risk for LTBI progression to active disease:
- Recent infection: Contact with active TB case within 2 years (highest risk)
- Immunosuppression: HIV/AIDS, TNF-α inhibitors, high-dose corticosteroids, organ transplant recipients, chemotherapy
- Medical conditions: Diabetes, chronic renal failure, silicosis, head/neck/lung cancer, hematologic malignancies
- Recent immigrants: From high TB prevalence countries (within 5 years)
- Age: Young children (<5 years) have higher progression risk
3. Treatment Decision for LTBI
If active TB is excluded and risk factors are present, treat for LTBI to prevent progression to active disease. The positive test result, combined with risk factors, justifies treatment even though the test's ability to predict future TB disease progression has not been definitively established 1.
Important Caveats and Pitfalls
Test Limitations to Consider:
- Sensitivity is ~80-86% in culture-confirmed TB cases 1, 2, meaning negative results don't completely exclude infection, especially in immunocompromised patients
- Cannot be used alone to exclude TB in symptomatic patients—clinical suspicion overrides negative results 1
- Indeterminate results occur in ~1.7-3.2% of cases 3, 4, particularly in severely immunocompromised patients; these require repeat testing or alternative evaluation
- Performance in immunocompromised patients is not well-established, so interpret negative results cautiously in HIV, transplant candidates, or those on immunosuppressive therapy 1
Common Pitfalls:
- Treating without ruling out active TB: Always exclude active disease first—treating LTBI when active TB is present leads to inadequate therapy and drug resistance
- Ignoring clinical context: The predictive value depends on TB prevalence in the tested population; interpret results alongside epidemiologic and clinical findings 1
- Over-reliance on negative results in high-risk patients: In immunocompromised or symptomatic patients, negative QFT-Plus doesn't rule out TB infection 1
Advantages Over TST
The QFT-Plus test offers several benefits:
- No false positives from BCG vaccination 5
- No cross-reactivity with most non-tuberculous mycobacteria (except M. kansasii, M. szulgai, M. marinum) 1
- Single visit required (no return for reading) 4
- Objective measurement (no reader variability)
The CDC recommends QFT-Plus may be used in all circumstances where TST is currently used, including contact investigations, immigrant screening, and healthcare worker surveillance 1.