Tb test how many days reading

Section Navigation. Facebook Twitter LinkedIn Syndicate. Testing for TB Infection. Minus Related Pages. Reading the result of a TB skin test. To receive email updates about this page, enter your email address: Email Address. What's this? When placed correctly, the injection should produce a pale elevation of the skin a wheal 6 to 10 mm in diameter.

The skin test reaction should be read between 48 and 72 hours after administration by a health care worker trained to read TST results. A patient who does not return within 72 hours will need to be rescheduled for another skin test. The reaction should be measured in millimeters of the induration firm swelling. The reader should not measure erythema redness. The diameter of the indurated area should be measured across the forearm perpendicular to the long axis. Some persons may react to the TST even though they are not infected with M.

The causes of these false-positive reactions may include, but are not limited to, the following:. A TB blood test is the preferred method of testing for people who have received the BCG vaccine in order to prevent false-positive reactions.

Some persons may not react to the TST even though they are infected with M. The reasons for these false-negative reactions may include, but are not limited to, the following:. Most persons can receive a TST. TST is the recommended method of testing for children younger than 5 years of age. TST is contraindicated only for persons who have had a severe reaction e. It is not contraindicated for any other persons, including infants, children, pregnant women, or persons living with HIV.

In general, there is no risk associated with repeated tuberculin skin test placements. If a person does not return within hours for a tuberculin skin test reading, a second test can be placed as soon as possible.

A boosted reaction occurs mainly in previously infected, older adults whose ability to react to tuberculin has decreased over time. When given a TST years after infection, these persons may have an initial negative reaction.

However, the TST may stimulate the immune system, causing a positive or boosted reaction to subsequent tests. Vaccination with live viruses, including measles, mumps, rubella, oral polio, varicella, yellow fever, BCG, and oral typhoid, may interfere with TST reactions. For persons scheduled to receive a TST, testing should be done as follows:. There are two kinds of tests that are used to determine if a person has been infected with TB bacteria: the TB blood test and the TB skin test. The tests measure the response of TB proteins when they are mixed with a small amount of blood.

Only one visit is required to draw blood for this test. Latent TB is usually treated as well to prevent future active disease. There are two types of tests used to diagnose TB : a blood test and a skin test.

The test is usually well-tolerated, and people rarely have negative reactions to it. Tuberculin is a sterile extract purified protein derivative PPD made from the bacteria that cause TB. The second phase of the test takes place 48 to 72 hours later. At that time, your doctor will look at your skin to see how it reacted to the tuberculin. The size of the induration, not the redness, is used to determine your results. The induration should be measured across the forearm, perpendicular to the axis between your hand and elbow.

Several factors affect how the test is interpreted. An induration of less than 5 millimeters mm is considered a negative test result. The same is true if you live in a high-risk environment such as a nursing home or work in a high-risk setting such as a hospital or medical laboratory.

A 10 mm induration may also be considered positive in children under the age of 4 or people who use injected drugs. The TB skin test is less accurate than the blood test, so you could have a positive skin test and a negative blood test. Again, incorrect administration of the test or interpretation of the result could lead to a false-negative test result.

Certain immune system conditions, especially an organ transplant , may also cause a false-negative skin test. If you were exposed to TB in the past few weeks, you may not yet test positive for TB. Infants, even if they have TB, may not always have a positive skin test. A blood test can also be done any time. Having only the TB infection will not produce any noticeable symptoms. You may also cough up blood. Other symptoms include:.

Even a negative test is helpful because it can rule out TB and help your doctor find other causes for your symptoms. A positive skin test will usually be followed by a chest X-ray. This can help determine the difference between active TB disease and latent TB infection.