1 in 3 people worldwide carry TB. It's hard to know who does and who doesnt.

There's a simple test to find out who likely carries TB and who likely doesn't.

Most who carry TB are not infectious. They have latent TB - just a small amount of TB "sleeping" in their lungs. These people are at risk for having active TB - which is dangerous to their health and to others (who can catch TB from active cases).

It's important to identify who does and who does not carry TB. We can give medications to reduce the risk of these people developing TB. Some people take a pill every day for 9 months. Some take a dose of meds once a week for 12 weeks.

The test to find out who might have TB is called a PPD test or Tuberculin Skin Test (TST) or even a Mantoux test.

How is a PPD performed?

You put your forearm facing up. Into the inner surface of your forearm, a very small amount of tuberculin purified protein derivative (PPD) is injected (0.1 ml) just under (or within) your skin. This is done with a very small needle. It should create a wheal where your skin puffs up. This is then read in 48 and 72 hours. If it is after 72 hours, it will need to be repeated.

Reading a PPD involves feeling the forearm for whether it has become hard where the injection was placed. This is not a visual test; it can only felt. The size of the area is measured (across the width of the arm) and is usually, if anything, only a few millimeters.


What if it is red? 

Color does not matter.

What if it puffs up as soon as I get the test?

The initial response doesn't matter. The puffiness usually goes away in a few hours. Any initial reactions do not matter. It only matters at 48 to 72 hours.

What do the numbers mean?

What your "reading" means depends on what your risk factors are.

A positive read will be at 5mm or more for those at highest risk (HIV positive, persons with organ transplants, recent contacts of TB patients, those with X-Rays consistent with TB, and those on immunosuppresive drugs, such as over 15 mg of prednisone a day for a month).

Those with medium risk are read as positive if at 10 mm or more (such as immigrants from high risk areas, children less than 4 years old, children exposed to those at high risk for TB, health care workers, those who live and work in high risk areas (especially prisons, nursing homes, homeless shelters), TB lab personnel, those who use IV drugs, had a negative test within last 2 years, or have diabetes, kidney failure, or other medical conditions that place them at higher risk for developing active TB.

Any reading of 15mm or more is considered positive, regardless of risk factors.

Can it be read as positive but not be?

Yes. It may have been read wrong. You may have had BCG vaccination (the TB vaccine that isn't used in the US).

Can I still have TB if my PPD is read as negative?

Yes. There are many reasons why a test may be read as negative when it is not.Those who are very young or very old may have negative results when they have latent TB. Those who are very sick with TB (overwhelming infections) may have a negative PPD. Those who have a similar bacterial infection that does not cause TB (nontuberculous mycobacteria) may also have a positive reaction. Some people have a weakened immune response that does not respond to the test. Others have recent TB infections (6-10 weeks after exposure). The test may also be incorrectly performed or read.

Are there any other tests for TB?

People who are coughing are given sputum tests that look for TB (either under a microscope, through a PCR machine, or by growing TB in a culture). There is also a blood test called Quantiferon that can be used to identify those with latent TB, but is not available to many in high TB risk areas (where public health resources are more limited).

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