A skin test for tuberculosis is also known as the Mantoux test. This test measures your immune system's response to the bacteria that causes tuberculosis. The results will be interpreted by your doctor a few days after the test. If you're wondering how to read a skin test for tuberculosis, this article can help you understand, but remember, the test should be read by a trained professional. You can interpret the test yourself, but the results should be recorded by a doctor to ensure proper treatment and follow-up treatment is given.
Part 1 of 2: Reading the test
Step 1. See your doctor for the tuberculosis skin test
You get an injection of a purified protein derivative into the inside of your forearm. This will result in a 6-10mm swelling that will disappear within a few hours.
Step 2. Keep your arm uncovered
Do not bandage the injection site until the required 48 to 72 hour period has elapsed. You can gently wash and dry your arm.
You should also not scratch or rub the injection site. This can make the area redder or swell, which in turn can cause the results to be misinterpreted. You can put a cold washcloth on your arm if it itches
Step 3. Go back to your doctor
The test should be read within 48-72 hours. If you do not return to the doctor within 72 hours, the test is considered invalid and will have to be retaken.
Step 4. Look for the pavement and mark it
Use your fingertips to discover the hardening. It is a hard, compact, raised structure with clear edges. If a hard bump is present, use a pen to mark the outer limits of the pavement. The only thing that matters to your test results is the hard bump. The red-colored area, or a slight swelling, does not count against the size of the induration.
You can't always see the hardening. You will have to discover the hardening with your fingertips
Step 5. Measure the pavement
The site of the test may be red, but this does not mean you have TB. You have to measure the hardening. This hardening is measured, in millimeters, across the width of your forearm. Use a ruler that indicates millimeters. Place one end of the ruler (with the "0") near the left edge of the bump, where you marked it with a pen. See at which point on the ruler the mark you made on the right side of the bump falls.
If the mark is between two lines, use the smaller measurement
Part 2 of 2: Interpreting the test
Step 1. Check whether the individual belongs to the high-risk group
A 5 mm induration is seen as positive for individuals in the high-risk group. This group includes people:
- who have HIV
- who have received a donor organ
- in whom immune system responses are suppressed for certain reasons
- who have recently come into contact with a TB-positive person
- whose chest X-ray shows old, healed TB
- who are going through the final stages of a nephrological disorder (kidney disease)
Step 2. See if the individual is in the medium risk group
A pavement of 10 mm or more is considered positive for people in the medium risk group. This includes people who:
- have recently emigrated from a country where TB is widespread
- taking medicines that you have to inject
- work in health facilities, prisons, nursing homes and other similar places
- have certain medical conditions, such as diabetes, leukemia, or being underweight, that make them more likely to get TB
- be under 4 years old
- children and adolescents exposed to adults at high risk for TB
Step 3. Look for significant hardening in individuals who do not fall into one of these categories
In those who are not in the high and medium risk groups for TB, a hardening of 15 mm or more is considered a positive result. This applies to everyone, regardless of the possible contributing risk-increasing factors. The test is also positive if blisters are present, even if there is only a slight swelling.
Step 4. Look for things that indicate a negative test result
If no solid bulge is present, the result is negative. If there is a slight swelling or redness, but you can't feel a noticeable hardened bump in the affected area, the result is negative.
Even if you suspect your skin test is negative, you will still need to return to the doctor to have a test read by him
Participate in any further tests your doctor deems necessary if the test is positive, or if the test is considered near positive
- False positive and false negative results can occur with this type of test. If questions arise regarding the results of your tuberculosis test, it is important to consult your doctor as soon as possible.
- A TB test should always be interpreted by a health professional within the specified 72 hour period. These professionals receive training and practice to be able to perform the various measurements correctly.