TB Skin Test

The degree of the skin’s response will determine if the results are positive or negative. If your skin test is positive, it means you are infected with the TB bacteria but further testing will be needed to see whether it is a latent or active infection.1

The test requires two visits to your healthcare professional. The test itself will be administered on the first visit, with the results read on the second.

Purpose of Test

Your healthcare professional will arrange a TB test if it is suspected that you might have been infected with TB or if you are at high risk of being infected. They will recommend the best test for you based on the reason for testing, test availability, and cost.

Those at a higher risk of being infected with TB bacteria include:3

• People who have spent time with someone who is infected
• People from a country where TB disease is common, including most countries in Latin America, the Caribbean, Africa, Asia, Eastern Europe, and Russia
• People who live or work in high-risk settings, such as hospitals, prisons, and homeless shelters
• Healthcare workers who care for patients at increased risk for TB
• Infants, children, and adolescents exposed to adults who are at increased risk for latent tuberculosis infection or TB

A TB skin test is the most common type of test used to diagnose suspected active TB. However, it is not suitable for everyone. Blood tests are also used, though not as frequently.

The skin test is the preferred test for children under the age of 5. Blood tests are the preferred test for those who have received the TB vaccine bacille Calmette-Guerin (BCG) and those who would have difficulty returning for a second appointment.1

It is generally not advised to get both a skin and blood test. There is some evidence that TB blood tests, called interferon-gamma release assays (IGRAs), are slightly more accurate than the skin test.4

Skin tests are also more likely to result in false negatives and false positives than blood tests. This is often due to the test being administered improperly or due to the results being misinterpreted.
However, skin tests can be repeated if necessary.

False Positive TB Skin Tests

A false positive is a test result that wrongly indicates that you do have TB when you don’t. Factors that might cause a false positive are:5

• Previous TB vaccination with the BCG vaccine
• Infection with nontuberculosis mycobacteria, which can also cause lung infections
• Incorrect measurement or interpretation of reaction
• Not using the correct antigen for the test

False Negative TB Skin Tests

A false negative is a test result that wrongly indicates that you do not have TB when you do. Factors that might cause a false-negative are:5

• Anergy, the absence of the normal immune response to a particular antigen or allergen
• Recent infection, within the past eight to 10 weeks
• Children that are younger than 6 months
• Recent live virus measles or smallpox vaccination
• Incorrect administration of the test
• Incorrect measuring or interpretation of the reaction to the test

Risks and Contraindications

Most people can get a TB skin test and for the majority of people it’s completely safe and doesn’t hold any risks. However, the test is contraindicated (meaning it should not be used) for those who have had a previous bad reaction to the test.

Unlike many other tests, it is not contraindicated for infants, children, people who are pregnant, or people who have HIV.5

The test uses a very small amount of a solution that contains certain components of the TB bacterium. However, there is no risk at all of contracting TB from the skin test.

Before the Test

Before the skin test, a healthcare professional will take your medical history and determine whether you can take the test. This will most likely involve asking whether the test is contradicted for you, such as if you had a negative reaction to it in the past.

Timing

The test itself is very quick, and the solution can be injected very easily. However, you should plan ahead as you will need to set aside time to come back to have the results read. This needs to be done between 48 and 72 hours after the test is administered. If you don’t return within that time, you will need to schedule another test.

Location

TB skin tests can be done in your doctor’s offices as well as in outpatient locations. The medical healthcare professional will have a small vial of the solution, a small single-dose disposable syringe, and a measurement tool. There will also be alcohol swabs and gauze pads or cotton balls.6

What to Wear

As the healthcare professional will need to inject your arm during the test, make sure you wear something where you can roll up the sleeves. That is the only thing you need to consider and you won’t need to undress or bring a change of clothing along to the test.

Costs and Health Insurance

There may be a cost or copay associated with this test. Check with your insurer or your healthcare provider to see what your costs may be. If you are uninsured, there may be a free tuberculosis clinic available to you that will do the test at no cost. Check with your state and county health division.

Other Considerations

There are no food or drink restrictions before or after the test. Be sure to bring your health insurance card, identification, and any needed paperwork with you.

During the Test

You do not need to do anything to prepare for this test. You will not need to drink anything or change out of your clothing.

Throughout the Test

During the test, 0.1 milliliters (mL) of tuberculin purified protein derivative (PPD), also referred to as tuberculin, will be injected just under the skin of your forearm.5 The injection will be done with a small needle, so you should only feel a slight pinch and it should be over very quickly.

Immediately following the injection, the healthcare professional should check that the skin test was administered properly. They will do this by using the millimeter (mm) ruler to immediately measure the wheal, the pale elevation of the skin caused by the injection. If the wheal is less than 6 mm then it will need to be administered again.6

Post-Test

Remember to book your follow-up appointment to get your test interpreted. This needs to be done within 48-72 hours of the test being administered. If you do not come back for your follow-up within this time, you will need to get the test redone because they will not be able to interpret the test.

After the Test

You can eat, drink and shower as normal, even immediately after the test. However, you will be advised to keep the test site uncovered and not scratch or rub the area.7

Managing Side Effects

The majority of people will not experience any side effects from the TB skin test, however a small number of people may experience a negative reaction. Symptoms of a negative reaction at the site of the injection include:

• Mild itching
• Swelling
• Irritation

These kinds of reactions should disappear in one to two weeks.

Rarely, more severe symptoms may occur, such as:8

• Severe allergic reactions, such as anaphylaxis and angioedema
• Blistering
• Ulceration
• Shortness of breath
• Fever
• Fainting or feeling lightheaded

If you experience these more severe symptoms you should contact your doctor immediately. If someone is having a serious allergic reaction, call 911.

Interpreting Results

Your doctor will read the results of the test by measuring the diameter of the swelling caused by the injection.

If the result is negative, the skin where the test was administered will not be swollen, or the swelling will be very small. If the result is positive, there will be swelling, however the size differs depending on the patient.

In people with no known risks of TB, 15 mm or more of firm swelling at the site indicates a positive reaction.9

For the following, 5 mm of firm swelling at the site is considered to be positive:6

• Those who have HIV/AIDS
• Those who have received an organ transplant
• Those who have a suppressed immune system or are taking steroid therapy
• Those who have been in close contact with a person who has active TB
• Those who have changes on a chest X-ray that look like past TB

For the following, a reaction that is larger than or equal to 10 mm is considered positive:6

• People with diabetes, kidney failure, or other conditions that increase their chance of getting active TB
• Laboratory professionals who work in the mycobacteriology lab
• Injection drug users
• Immigrants who have moved from a country with a high TB rate in the past five years
• Children under the age of 4
• Infants, children, or adolescents who are exposed to high-risk adults
• Students and employees of certain group living settings, such as prisons, nursing homes, and homeless shelters

Follow-Up

If your results are not read within 48 and 72 hours of the test then you will need to have another test. The skin test can be repeated as soon as possible unless there has been a severe reaction to the test in the past.

If your test is positive then your doctor will conduct further tests to see if there is active disease. These tests include chest X-rays and a culture of a sputum sample, which is the thick fluid produced in the lungs as a result of illness. Computed tomography (CT) scans and magnetic resonance imaging (MRI) may be used as well.

TB can be successfully treated. However, it will take at least six months, and may take up to a year, to kill the bacteria.1

Other Considerations

If you have ever tested positive for TB, then you will always get a positive skin test result, even after getting treatment for the condition. Your doctor will add a note to their medical files indicating you have received treatment for TB. This will help doctors who may conduct future TB skin tests interpret the results.

Your healthcare professional can address any concerns or questions you may have so it is important to have an open discussion with them.