PPD skin test

Answer 1

Definition

The PPD skin test is a method used to diagnose tuberculosis (TB). PPD stands for purified protein derivative.

Alternative Names

Purified protein derivative standard; TB skin test; Tuberculin skin test; Mantoux test

How the test is performed

The test site (usually the forearm) is cleansed with rubbing alcohol. The PPD extract is then injected under the top layer of skin, causing a welt to form on the skin. This welt usually goes away in a few hours.

The reaction will take 48 - 72 hours to develop. You must return to your health care provider within that time to have the area checked. This test will determine whether you have had a significant reaction to the PPD test. The reaction is measured in millimeters of firm swelling (induration), not redness, at the site.

How to prepare for the test

There is no special preparation for this test.

Tell your health care provider if you have ever had a positive PPD skin test. If so, you should not have a repeat PPD test, except under unusual circumstances.

Tell your doctor if you have a medical condition or if you take certain drugs, such as steroids, that can affect your immune system. These situations may lead to inaccurate test results.

How the test will feel

You will feel a brief sting as a needle is inserted just below the skin surface.

Why the test is performed

The PPD test is done to find out if you have been infected with Mycobacterium tuberculosis, the bacteria that causes TB.

Normal Values

A negative reaction (no induration) or a size of hard swelling that falls below the cutoff for each risk group may mean that you have not been infected with the bacteria that cause TB. There are different cutoffs for children, people with HIV, and other risk groups.

Unfortunately, this is not a perfect test. Up to 20% of people infected with the bacteria that cause TB may not have a reaction on the PPD skin test. In addition, certain conditions that affect the immune system (cancer, recent chemotherapy, late-stage AIDS) may cause a false-negative test result.

What abnormal results mean

The results of the test depend on the size of the skin reaction and on the person being tested.

A small reaction (5 mm of firm swelling at the site) is considered to be positive in people:

• Who have HIV
• Who are taking steroid therapy
• Who have been in close contact with a person who has active TB

Larger reactions (greater than or equal to 10 mm) are considered positive in:

• People with Diabetes or kidney failure
• Health care workers
• Injection drug users

In people with no known risks for TB, a positive reaction means there is 15 mm or more of firm swelling at the site.

What the risks are

There is a very small risk of severe redness and swelling of the arm in people who have had a previous positive PPD test and who have the test again. There also have been a few cases of this reaction in people who have not been tested before.

Special considerations

A positive skin test does not necessarily mean that a person has active tuberculosis. More tests must be done to check whether there is active disease.

Many people who were born outside the United States may have had a vaccine called "BCG," which can lead to a false-positive test result. However, many experts believe that a past BCG vaccination should not change the interpretation of the PPD result when the test is done 15 years or more after the BCG vaccination.

References

Iseman MD. Tuberculosis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 345.

Answer 2

Definition

The PPD skin test is a method used to diagnose tuberculosis (TB). PPD stands for purified protein derivative.

Alternative Names

Purified protein derivative standard; TB skin test; Tuberculin skin test; Mantoux test

How the test is performed

You will need two visits to your doctor's office for this test.

At the first visit, the health care provider will clean an area of your skin.

You will get a small shot that contains PPD. The needle is gently placed under the top layer of skin, causing a bump (welt) to form. This usually goes away in a few hours.

After 48-72 hours, you must return to your doctor's office. The doctor or nurse will check the area to see if you have had a significant reaction to the PPD test.

How to prepare for the test

There is no special preparation for this test.

Tell your health care provider if you have ever had a positive PPD skin test. If so, you should not have a repeat PPD test, except under unusual circumstances.

Tell your doctor if you have a medical condition or if you take certain drugs, such as steroids, that can affect your immune system. These situations may lead to inaccurate test results.

How the test will feel

You will feel a brief sting as a needle is inserted just below the skin surface.

Why the test is performed

This test is done to find out if you have ever come in contact with the bacteria that causes TB.

TB, or tuberculosis, is an easily spread (contagious) disease. It most often affects the lungs. The bacteria can remain inactive (dormant) in the lungs for many years. This is called latent TB.

Most people in the United States who are infected with the bacteria do not have signs or symptoms of active TB.

You are most likely to need this test if you:

• May have been around someone with TB
• Work in health care
• Have a weakened immune system, due to certain medicines or disease (such as cancer or HIV and AIDS)

Normal Values

A negative reaction usually means you have never been infected with the bacteria that cause TB.

A negative reaction means the skin where you received the PPD test is not swollen, or the swelling is very small. This measurement is different for children, people with HIV, and other high risk groups.

This is not a perfect test. Up to 1 in 5 people infected with the bacteria that cause TB may not have a reaction. Also, diseases or medicines that weaken the immune system may cause a false-negative result.

What abnormal results mean

An abnormal (positive) result means you have been infected with the bacteria that cause TB. You may need treatment.

It is important to note that test results depend on the person being tested.

A small reaction (5 mm of firm swelling at the site) is considered to be positive in people:

• Who have HIV
• Who have received an organ transplant
• Who have a suppressed immune system or are taking steroid therapy (about 15 mg of prednisone per day for 1 month)
• Who have been in close contact with a person who has active TB
• Who have changes on a chest x-ray that look like past TB

Larger reactions (greater than or equal to 10 mm) are considered positive in:

• People with a known negative test in the past 2 years
• People with diabetes, kidney failure, or other conditions that increase their chance of getting active TB
• Health care workers
• Injection drug users
• Immigrants who have moved from a country with a high TB rate in the past 5 years
• Children under age 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

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

What the risks are

There is a very small risk of severe redness and swelling of the arm in people who have had a previous positive PPD test and who have the test again. There also have been a few cases of this reaction in people who have not been tested before.

Special considerations

A positive skin test does not necessarily mean that a person has active TB. More tests must be done to check whether there is active disease.

Many people who were born outside the United States may have had a vaccine called "BCG," which can lead to a false-positive test result. However, most experts say that a past BCG vaccination should not change the PPD result when the test is done in people with an increased risk of TB infection or disease.

References

Fitzgerald DW, Sterling TR, Haas DW. Mycobacterium tuberculosis. In: Mandell GL, Bennett JE, Dolan R, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 7th ed. Orlando, FL: Saunders Elsevier;2009:chap 250.

Iseman MD. Tuberculosis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 345.

Ellner JJ. Tuberculosis. In: Goldman L, Schafer AI, eds.Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 332.

Reviewed By

Review Date: 12/06/2011

David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Jatin M. Vyas, PhD, MD, Instructor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.