When Is The TB Test Read After The Injection? How Many Days?


3 Answers

Charlie Proofer Profile
Charlie Proofer answered
The best way to test for Tuberculosis (TB) it to perform a tuberculin skin test (TST) which is performed by injecting Tuberculin into the forearm of the patient. This will leave a 6mm or 7mm welt in the arm which shows that the tuberculin has been properly injected. You will then have to wait around 48 hours before you go back to the Doctor’s for the results.

Once you have taken the test, return to your Doctor’s where you got the test within 48 to 72 hours otherwise you will have to retake the test. Your Doctor will measure the diameter of the reaction at the site where the Tuberculin was injected. It is not a measure of how red it is. The interpretation also depends on your own risk factor. If you have been recently exposed to someone with TB, then a diameter of 5mm or wider is considered to be positive. Whereas if you are a recent immigrant, then a diameter of 10mm or wider is positive. Despite these difference, anyone with a diameter of at least 15mm is considered to be positive for TB.

A blood test for TB is not very widely available as it is only a recent introduction. They have a number of advantages over the traditional skin test, the first being that only one trip to the Doctor’s is needed. Another advantage is that the blood test is less open to interpretation. Blood test results usually come back within 24 hours of the test being performed.

As of Spring 2009, a new test has been available in Europe called Xpert MTB/RIF which detects the present of TB within two hours and does this much more accurately that both the skin test and the blood test. Not only will this test for TB in a person’s blood, it will tell if there is a resistance to rifampicin (RIF) which the most common drug used for treating TB. This test is under investigation and testing by the FDA before it is approved for tests in the United States.
R Maye Profile
R Maye answered
With regards to the normal skin test, reaction begins 5-6 hours after injection and peaks at 48-72 hours. You should have your test read at about the 3rd day. Some places will require you to retest if you wait longer than 3 days to have it read.
thanked the writer.
Anonymous commented
I thought that you had to have it read IN 48-72 hours or else the test is void, in the sense that they can't sign you off as not having tb after the 72 hours
Genesis Blanco Profile
Genesis Blanco answered
After 72 hours the test site should be checked by the doctor.  He/she will be able to sign off on your form after he checks the site.

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