QFT Blood Test Results Explained

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Up until 2001, the only way to definitively determine if someone had tuberculosis was to conduct a skin test. A small injection would be placed under the skin and then examined later to determine if it was interaction. With the invention of the QFT blood test, it became possible to use a simple blood test to determine if an active tuberculosis infection is present.

Called the QuantiFERON-TB blood test, it is manufactured be Cellestis Limited in Carnegie, Victoria, Australia and can be performed on a single patient visit. The advantage of having this blood test is clear: instead of having a skin test be subject to the interpretation of the medical examiner who reads the results, the QFT blood test will accurately determine if there is the presence of TB.

A skin test for TB may require up to 4 different visits to a medical provider. The QFT blood test only requires the one visit to a medical provider. This means the QFT blood test as a screening tool is more accurate, cheaper for patients and health providers, and it improves overall specificity.

When Will the QFT Blood Test Be Ordered?

The QFT blood test is used exclusively as a screening process.

This test is also used to replace the skin screening test that may be required in certain professions when there is continual contact with others. Healthcare workers, prison workers, prisoners, people who use injected drugs regularly, and immigrants to Europe, Australia, or North America in the last 5 years may also be screened by the QFT blood test. Military workers and those who travel internationally frequently may also be tested.

It is also used for those who have recently received vaccines or are currently going through cancer-treatment therapies.

Even though it is considered a more reliable test then the TB skin test, both are typically ordered for screening purposes. A positive result on the QFT blood test and the skin test is the best indication of there being the presence of an active TB infection. A positive skin test, but a negative QFT blood test, may be an indication that there is simply a reaction to the skin test or that there was active TB present. A positive blood test, but a negative skin test, may still be considered an overall positive for the presence of TB.

This blood test is typically used in conjunction with other testing methods to confirm a diagnosis if there is a medical problem. If it is being used solely for testing purposes, however, the results received can be considered valid for an employer.

When Will the QFT Blood Test Not Be Ordered?

If a doctor suspects that there is an active and current TB infection, then the QFT blood test is not often ordered. People who have active tuberculosis are associated with having a suppressed response that will provide a negative QFT blood test result. There is a direct correlation with the severity of the disease and the chances of a negative QFT result occurring. The duration of tuberculosis therapies may also aid in providing a negative blood tests result.

If someone has been in known contact with another that has active tuberculosis, then this blood test may not be effective either. Researchers have not compared results of those exposed to infectious tuberculosis compared to those who are just being screened for the presence of TB.

Those who are HIV positive, have a confirmed AIDS diagnosis, or are receiving immunosuppressive drugs for some reason are also not viable candidates to receive this blood test.

Certain demographics of people are also not viable candidates for the QFT blood test. Children under the age of 17 are almost never screened by using this test. Women who are pregnant are also excluded from this test. The skin test to determine if there is an active infection in these populations is still considered the gold standard of tuberculosis discovery.

If the skin test has been given first and analyzed to be potentially positive, the QFT blood test will also typically provide a positive result because of the injection materials that are absorbed into the body. If a negative test result occurs, however, the QFT blood test may be used for proactive monitoring for up to 12 months.

The QFT blood test may not be available at all healthcare providers right now. If you need to be screened for TB, however, be sure to ask your doctor about whether or not this particular blood test might be right for you.