What does positive reading on a Mantoux test mean?


My 6-year-old just had a positive reading on her Mantoux test. What does this mean?


Doctors perform a Mantoux test to see if someone may have been exposed to bacteria that cause tuberculosis (TB). TB is a bacterial infection that typically infects the lungs. Fortunately, in most people exposed to the bacteria, TB never develops.

TB bacteria spread from person to person through the air. The bacteria come out of someone’s mouth or nose when that person coughs or sneezes. When infection occurs, bacteria are inhaled into the deepest portion of the lung. There, the bacteria multiply. They also can spread to other parts of the body. Here is an illustration of TB:


Health AZ Tuberculosis imageWhen the TB bacteria first enter a person’s lung, the immune system can usually keep them from multiplying, but it usually cannot destroy them completely. As a result, the disease often remains inactive for life. People with inactive TB do not have any symptoms. And if the bacteria are not multiplying, and the person is not coughing, a person cannot spread the infection to others.

But it still can be important to determine if someone has inactive TB. That’s because an inactive infection can slowly and quietly become active. The bacteria can start multiplying again, causing symptoms and often making the person contagious. Treatment can eradicate the inactive infection, and thereby prevent a future active infection.

Testing for inactive TB is most important in people who are at high risk. That includes people who live with other people who have had active TB of the lung, and people who have been in casual contact with those who have highly contagious types of TB. Health workers often fall into the latter category.

The only way to tell if a person has been infected is with a Mantoux skin test. I assume the test was done on your daughter because the doctor believed she was at extra risk. Perhaps there was another child in her school who was found to have an active infection that could have been spread to other kids.

Here’s how the skin test works: A small amount of protein taken from dead TB bacteria is placed just under the skin. (It’s completely safe.) The site is checked two to three days later. If TB bacteria have been in the person’s body before, there will be a slightly hard, sometimes red swelling. Whether or not it is “positive” depends on the amount of swelling. Once a Mantoux is positive, it tends to stays positive, even after treatment.

If your daughter’s Mantoux test is positive, it doesn’t necessarily mean she has active TB disease. It just means that the bacteria have entered her body. Your daughter’s doctor will check her for any signs of TB. The next step is a chest X-ray to check for TB infection in the lungs. If there is any indication your daughter has an active infection, she will be treated with antibiotics.

Even if the checkup and X-ray are normal, your daughter’s doctor might still recommend medicine to eradicate the bacteria and prevent a future active infection.