Ask an Expert: Size of Induration (Reaction) That Makes a TB Test Positive

Tuberculosis

Size of Induration (Reaction) That Makes a TB Test Positive
11/06/2008

Question:

I want to know the size of induration that makes a TB test positive.

Answer:

If a person is immunocompromised, such as HIV-infected or an organ or marrow transplant recipient, or if the person has an abnormal chest x-ray suggestive of TB, or if the person was recently exposed, >5 mm of induration is a positive PPD skin test.

If a person is a health care worker, or has other medical conditions that would predispose to TB exposure or disease, >10 mm induration is a positive PPD skin test.

For all other people (without risk factors for TB exposure or disease), >15 mm induration is a positive PPD skin test.

In general, the CDC recommends that you should get tested for TB if:

You have spent time with a person known to have active TB disease or suspected to have active TB disease.

You have HIV infection or another condition that puts you at high risk for active TB disease.

You think you might have active TB disease.

You are from a country where active TB disease is very common (most countries in Latin America and the Caribbean, Africa, Asia, Eastern Europe, and Russia).

You live somewhere in the United States that active TB disease is more common such as a homeless shelter, migrant farm camp, prison or jail, and some nursing homes).

You inject illegal drugs.

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Response by:

Lisa A Haglund, MD
Associate Professor of Clinical Medicine
College of Medicine
University of Cincinnati