The tuberculosis skin test is also known as the Mantoux tuberculin test. This test measures the response of your immune system to the bacteria that causes tuberculosis. The results will be assessed by the doctor a few days after the test is performed. If you're curious about how to read a tuberculosis skin test, this article will guide you through the process, but remember: the results of the test "must" be read by a doctor. You may interpret the test, but the results need to be documented by a medical professional to ensure proper treatment and/or follow-up.
Step
Part 1 of 2: Reading the Test
Step 1. Visit a doctor for a tuberculosis skin test
The doctor will give an injection containing a purified protein derivative (tuberculosis diagnostic agent) into the inner forearm. This injection will create a small 0.5-1 cm bump on the skin that will go away on its own in a few hours.
Step 2. Leave your arms open
Do not apply plaster to the injection site for 48 to 72 hours. You can wash and dry your arms carefully.
You should also not scratch or rub the area. This can cause redness or swelling which can lead to incorrect readings of the test results. You can put a cold, wet cloth on your arm if it itches
Step 3. See the doctor again
This test should be read in 48-72 hours. If you do not return within 72 hours, the test is judged invalid and must be repeated.
Step 4. Locate and mark the induration area
Use your fingertips to look for indurations, which are hard, thick, raised skin formations with clear boundaries. If there is a firm, firm bulge, use a ballpoint pen to mark the widest side of this induration on your forearm. The single most important part of your test results are these hard bumps. Areas of redness or mild swelling do not count as a measure of induration.
Induration is not always visible to the naked eye. You have to search for it with your fingertips
Step 5. Measure the induration
The area of the skin where the test was performed may be red, but this does not mean you have TB. You have to measure induration. Induration is measured along the forearm in millimeters. Use a millimeter ruler. Place the side of the ruler with a value of "0" to the left of the bump you marked with the pen. Look at the value on the ruler at the mark made to the right of the bump.
If the mark is between two values on the ruler, use the smaller value
Part 2 of 2: Interpreting the Test
Step 1. Determine if a person belongs to a high risk group
Induration measuring 5 mm or more was categorized as positive in individuals in the high-risk group. People who fall into this group include people who:
- have HIV
- receive an organ transplant
- experiencing immunosuppression (weakened immune response) for various reasons
- have recently been in contact with a TB positive person
- perform chest X-rays consistently and have old healed TB
- have end-stage kidney disease
Step 2. Determine if a person belongs to the medium risk group
Induration measuring 10 mm or more was categorized as positive in persons in the medium risk group. These include people who:
- recently immigrated from a country with a high incidence of TB
- use injectable drugs
- work in the area of health services, prisons, nursing homes (orphanages), or the like
- have a clinical condition that puts the individual at risk, for example diabetes, leukemia, underweight
- children under 4 years old
- children and adolescents exposed to or in environments with high-risk adults
Step 3. Check for large induration in other individuals
For people who are not in the medium or high risk group, an induration measuring 15 mm or more is categorized as positive. All individuals are included, regardless of other possible risk factors. This test is also considered positive if a blister is present, even if there is only a small swelling.
Step 4. View negative results
If there are no hard, firm lumps, the result is negative. If there is mild (soft) swelling or redness, but no hardened lump can be felt in the test area, the result is negative.
Even if you believe that your skin test is negative, you should see your doctor again to have the test read professionally
Tips
Perform additional tests as the doctor may ask if the test result is positive or if the test result is considered to be on the threshold of positive
Warning
- Error reading the result as positive or negative can occur in this test. If you have any questions about your tuberculosis test results, consult your doctor as soon as possible.
- A TB test should always be assessed by a doctor within 72 hours of the test being performed. They have undergone training and practice to measure test results correctly.
