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Tuberculosis

Gold Standard Test: False Positive?

07/27/2009

Question:

I recently showed positive with the Gold Standard blood test for TB. As I am having a difficult time with another recent diagnosis of Addison`s Disease, I am very hesitant to begin treatment for TB without being extremely sure I need the medications. I have questions about the Gold Standard blood test used.

I`ve read that it is 80% accurate. Doesn`t this leave enough lee-way to want to re-do the test, just to be sure? I`ve also read that it needs to be processed within 12 hours of draw. If this is not done, how accurate is the test?

Finally, the person drawing my blood drew some into the vial, then the blood stopped drawing, so he withdrew the TB test tube, used another tube to get the blood going again, re-inserted the TB test tube, and finished filling it. Will this affect the accuracy of the test?

My doctor is an infectious disease doctor. His specialty is TB. I know I should trust him on this, but I am having a very hard time on the steroids for Addison`s (I am weaning and having adrenal episodes on and off, all days and nights), and even the B6 I started last week to go with the Isoniazid is causing me reactions. My pharmacist thinks it has to do with the B6 trying to stimulate my ill adrenal glands. I just want to be sure that, if I`m to go on TB medication, it is warranted. I would feel better being re-tested, but my doctor has lost all patience with me and won`t do it.

Could the gold standard test be wrong?

Thank you.

Answer:

The test you are referring to is the Quantiferon TB Gold blood test. This test measures the immune system's response to tuberculosis. When the test is positive then it strongly suggests that the immune system recognizes tuberculosis. When you have a positive test it is about 97% correct in predicting that you have been exposed to tuberculosis and have tuberculosis infection in the body. The 80% figure that you refer to tells us how often this test will be positive in a person who is known to have active tuberculosis. So the problems with the quantiferon TB gold blood test can be false negative reactions. A false positive reaction is much less likely.

The blood drawing episode you refer to should not affect the accuracy of your test.

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Response by:

Catherine A Curley, MD, MS Catherine A Curley, MD, MS
Assistant Professor of Medicine
School of Medicine
Case Western Reserve University