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Tuberculosis

How Accurate is the Quantiferon Test for TB?

11/16/2009

Question:

I read your reply to a question regarding someone with a positive TB skin test who had had BCG while young and so I wanted to follow up on this, since my case is similar. I also received BCG while young growing up in Asia. I have had borderline positive TB skin tests since and have had to check with chest x-rays which have always been negative. A few weeks ago as part of any employee health (I am a health care worker), I had the Quantiferon test done, which came back indeterminant. A repeat a couple of week later was also indeterminant. (one done two years prior was negative). So how reliable is the Quantiferon test? I had the PPD skin test done and it was 1cm. The chest X-ray performed was negative. So given all of this I wanted to find out if it is advisable to do the INH treatment since I am already on blood pressure and Cholesterol medications which also have potential to effect my liver. So would my doing the INH treament put my liver more at risk? Thanks!

Answer:

The purpose of TB screening in healthcare workers is to diagnose previously undetected (and presumably occupational) TB exposures. As you have discovered, when Interferon Gamma Release Assays (IGRA) are used serially in healthcare workers, a small number of people's results are indeterminate. Studies are underway to better describe the reproducibility of the IGRA's in a person over time, and also to try to define "conversions," "reversions," and the clinical significance in terms of whether "converters" should be offered Isoniazid TLTBI.

You should discuss your results with your employee health physician, and also ask if the Quantiferon Gold results are being reported as positive/negative/indeterminate or if the result is expressed quantitatively as INF-gamma in pg/mL or IU/mL, to see if your result was near the cut-off point, or if there was some other problem with the assay (such as unusual readings from the control wells) that would cause the ideterminate readings.

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Response by:

Lisa A Haglund, MD Lisa A Haglund, MD
Associate Professor of Clinical Medicine
College of Medicine
University of Cincinnati