PPD Does Not Affect QuantiFERON-TB Gold Results
A recent PPD skin test does not interfere with QuantiFERON-TB Gold blood draw results, and the blood test can be performed immediately after PPD placement without any waiting period. 1, 2
Why There Is No Interference
The QuantiFERON-TB Gold test measures interferon-gamma release from blood cells when exposed to specific TB antigens (ESAT-6, CFP-10, and TB7.7) in a laboratory setting, and this measurement is not affected by prior skin testing with PPD. 1, 3 The test uses fresh heparinized whole blood to detect cellular immune responses, which remains independent of any cutaneous delayed-type hypersensitivity reaction occurring at the PPD injection site. 2
Important Caveat: The Booster Phenomenon
However, there is one clinically significant exception to be aware of: performing PPD before QuantiFERON-TB Gold may cause a "booster phenomenon" that can convert a negative QuantiFERON result to positive. 4, 5
In a study of 97 medical students, 15% (5 of 33) who had negative initial QuantiFERON-TB Gold tests converted to positive when retested one month after PPD injection, demonstrating that prior intradermal PPD can boost the interferon-gamma response measured by QuantiFERON. 4
Another study confirmed that IFN-γ response to mycobacterial antigens significantly increased after tuberculin application, though it typically did not reach the positive threshold in most patients. 5
Clinical Implications for Your Patient
For your specific scenario where the patient just received the PPD and you need to draw QuantiFERON-TB Gold blood:
You can proceed with the blood draw immediately without concern that the PPD will invalidate the QuantiFERON results. 1, 2
The blood must be collected in heparinized vacutainer tubes and incubated with test antigens within 12 hours of collection for accurate results. 2
Be aware that if this QuantiFERON is negative but you need to repeat it in the future (such as for serial monitoring), the prior PPD exposure may cause a positive result on subsequent testing due to the booster phenomenon. 4
Practical Recommendation for Future Testing Sequences
When both tests are needed, the optimal sequence is to perform QuantiFERON-TB Gold first, then PPD afterward if still needed. 4, 5 This avoids the potential boosting effect of PPD on subsequent QuantiFERON testing and maintains the superior specificity of QuantiFERON, particularly in BCG-vaccinated populations where it demonstrates far less cross-reactivity than PPD. 1, 6