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TB Blood Test, Forget TB Skin Tests

by Dr. Randy

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Excerpt from Child Health Guide, North Atlantic Books, Spring 2005

Tuberculosis skin tests have existed for 100 years to diagnose possible infections. The test is presented as a benign, protective necessity. However, TB tests may not be as benign as purported. A TB skin test, the Mantoux (tuberculin) test or the Tine test, involves injecting a small amount of tuberculosis antigen into the skin. A clinician will observe the skin reaction to this test. If there is a positive reaction of swelling, redness, or blistering then the test is pronounced positive. A positive result means that the person has been exposed to tuberculosis bacteria at some time in the past and developed antibodies. However, false positive and false negative test results are common. If someone had a TB vaccination at any time in the past, or if they were exposed to TB, then their antibodies to the vaccine will cause a positive TB skin test. Ninety percent of people exposed to TB will not contract the disease. The skin test fails to detect half of people with latent TB, and false-negatives are common in individuals with weakened immune systems. Usually anyone with a positive skin test for TB is advised to complete at least six months of treatment with the antibiotic isoniazid (INH). It is estimated that 50 percent of people prescribed antibiotics on the basis of a positive skin reaction do not have TB infection at all.

The injection of a small quantity of antigen is not supposed to cause any systemic reaction, but it is possible that even this small amount of TB antigen could trigger the body’s immune system to react abnormally. I have seen autoimmune disease processes begin soon after TB tests, and others have also reported this occurrence. This observation does not prove that a TB test can cause this reaction, but it should give us pause before allowing the test to be performed on children.

A much safer, more accurate, quicker, and convenient blood test exists for the detection of tuberculosis infection. The blood test is performed on a plasma sample with results available in 24 hours. In Europe the blood test was approved for use in July 2004 and will replace the tuberculin skin test. This test is approved to detect both active and latent TB in infants, children, and adults. The test detects T cell response to infection which can exist in the body for years without symptoms. The population of latently infected people acts as a reservoir of TB with undetected individuals capable of spreading infection. A similar blood test was approved by the FDA in 2001 for detection of latent TB in the US and recommended for individuals at high risk or for screening purposes. Some labs do the test, but not all, so it may take some searching to find it. The name of the test is the Quantiferon TB gold blood test.

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