What does a negative Mantoux test look like?

The test is “negative” if there is no bump (or only a very small bump) at the spot where the fluid was injected. A negative TB skin test usually means that you don’t have TB.

What happens if Mantoux test is negative?

A negative Mantoux test may lead to misdiagnosis and delayed treatment. Doctors should become more familiar with the manifestations of systemic tuberculosis and use advanced diagnostic tools in cases of clinical suspicion.

What is considered positive TB skin test?

In a healthy person whose immune system is normal, induration greater than or equal to 15 mm is considered a positive skin test. If blisters are present (vesiculation), the test is also considered positive. In some groups of people, the test is considered positive if induration less than 15 mm is present.

Can a negative TB test be red?

Results. Redness alone at the skin test site usually means you haven’t been infected with TB bacteria. A firm red bump may mean you have been infected with TB bacteria at some time. The size of the firm bump (not the red area) is measured 2 to 3 days after the test to find out the result.

When do you read Mantoux test?

The skin test should be read between 48 and 72 hours after administration. A patient who does not return within 72 hours will probably need to be rescheduled for another skin test.

How do you know if a TB skin test is positive or negative?

A skin test is the only way to tell if you have TB infection. The test is “positive” if a bump about the size of a pencil eraser or bigger appears on your arm. This bump means you probably have TB infection. Other tests can show if you have TB disease.

What causes a false positive TB blood test?

The causes of these false-positive reactions may include, but are not limited to, the following: Previous TB vaccination with the bacille Calmette-Guérin (BCG) vaccine. Infection with nontuberculosis mycobacteria (mycobacteria other than M. tuberculosis)

Can a TB skin test be wrong?

People with recent TB infections and very old TB infections can also show false negative test results. If the test is done incorrectly, a false negative might occur.