QFT Testing Before Latent TB Treatment
A QuantiFERON-TB Gold (QFT) test is not required to confirm TB before initiating latent TB treatment if you already have a positive tuberculin skin test (TST) result—either test alone is sufficient to diagnose latent TB infection and proceed with treatment, provided active TB disease has been ruled out. 1
Key Decision Points
When QFT Confirmation Is Recommended
For low-risk populations: If a patient has a positive QFT result but is at low probability for latent TB infection, CDC guidelines recommend confirming with TST before starting treatment. 1 LTBI therapy should not be initiated in low-risk persons who are QFT-positive but TST-negative. 1
When QFT Confirmation Is Optional
For high-risk populations: TST can be used to confirm a positive QFT in persons at increased risk for LTBI (recent immigrants, injection drug users, prison populations, healthcare workers), but the decision to treat when QFT is positive and TST is negative should be based on clinical judgment and perceived risk. 1
When Either Test Alone Suffices
If you have a positive TST (≥5 mm induration in HIV-infected persons, ≥10 mm in moderate-risk groups, ≥15 mm in low-risk groups): You can proceed directly to ruling out active TB disease and then initiating LTBI treatment without requiring QFT confirmation. 1, 2
If you have a positive QFT in a high-risk patient: Treatment can be initiated based on clinical judgment even without TST confirmation. 1
Critical Exclusions Before Treatment
Rule Out Active TB Disease First
Before any LTBI treatment, you must exclude active tuberculosis through:
- Chest radiography 1
- Clinical evaluation for TB symptoms 3, 2
- Sputum examination if indicated, even with negative chest X-rays in HIV-positive patients 2
QFT should never be used to diagnose or rule out active TB disease—it is contraindicated for this purpose because active TB suppresses interferon-gamma responses, leading to false-negative results. 1
Special Populations
HIV-Infected Patients
- All HIV-infected patients should be tested for M. tuberculosis infection by TST upon initiation of care, with ≥5 mm induration considered positive. 1
- IGRAs (including QFT) have not been fully validated in HIV-infected populations, though ongoing studies suggest better specificity than TST. 1
- Advanced immunosuppression may cause false-negative results in both TST and QFT. 1
Patients Starting Anti-TNF Therapy
- Both TST and IGRA (such as QFT or T-SPOT) should be considered together to maximize detection of LTBI before anti-TNF therapy, given the high risk of TB reactivation. 3, 4
- IGRAs are preferred over TST in these patients due to higher specificity and less interference from immunosuppressants. 5
Common Pitfalls to Avoid
Do not use QFT to monitor treatment response or confirm cure—the CDC explicitly recommends against this due to test limitations in this context. 6
Do not perform TST first if you plan to use QFT later—injection of PPD for TST might affect subsequent QFT results, though QFT does not affect subsequent TST results. 1
Do not skip LTBI treatment in TST-negative patients—evidence shows benefits of LTBI therapy only occur in individuals who are TST-positive, even among HIV-infected persons. 7