Abstract
Introduction: The QFN result is a composite of a nil control, a mitogen positive control and a test sample (TB Ag) utilising ESAT-6, CFP-10 and TB 7.7 antigens. The result is defined as TB Ag minus nil control (positive >0.35 IU/ml). A strongly positive nil control (>8 IU/ml) or failure of the positive control are defined as indeterminate results. We were concerned that high values obtained after antigen stimulation might be significant.
Method: All patients tested by QFT-GIT between 31/3/10 and 28/1/11 (n=651) had the raw data from their test reviewed. Clinical diagnosis was examined in all those with a TB Ag result >0.35 IU/ml with a nil control high enough to bring the net result to <0.35 IU/ml or an indeterminate result.
Results: There were 39 (6%) instances of a net negative result despite a response to antigen, including four cases of active TB (4.7% of TB cases). Nil control results ranged from 0.1-3.07 IU/ml, with TB Ag levels up to 1.52 IU/ml.
Eight with a negative mitogen response included two cases of active TB, one of whom also had HIV. No cases were defined as indeterminate by virtue of a strongly positive negative control.
Conclusion: Reporting of a net result without a breakdown leaves clinicians vulnerable to missing cases of active TB. Sarcoidosis, Crohn's disease and bronchiectasis, all diseases associated with polyclonal B cell activation, also appear to show spontaneous interferon-gamma release.
- © 2011 ERS