|Good news! No bump.|
My daughter doesn't have TB!
Now let's look at the injection test that my child just had. In the U.S., we have a standard purified protein derivative (PPD) test that is given to both kids and adults. A small shot of PPD is administered just under the top layer of skin, which will cause a small bump to form. After 2 days, a medical professional needs to check to see that the bump disappeared to indicate the patient doesn't have TB. There are a few problems with this.
- The test takes 2 days for a result, so that is obviously inconvenient.
- Many kids in developing countries get a BCG vaccine, which helps prevent some forms of childhood tuberculosis and would test positive. That would be a false positive, however, since it would not indicate TB infection.
Hold up right there. A TB vaccine? Why doesn't everyone use that? Because the BCG vaccine isn't a reliable way to prevent kids from being infected with TB or developing TB disease. It does protect younger children against certain complicated and lethal forms of TB, but the efficacy varies and it does not protect against TB disease in the lungs, which is the most common form. So, it's better than nothing in countries with a high TB burden, but it screws up PPD test results.
IGRA Blood Test
The best alternative to the PPD skin test is a blood test for infection called IGRA. This is a rapid test and will not be affected by BCG, but it is more expensive than PPD and so it is not available everywhere.