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How to Determine if you have had a False Positive Reaction to a TB Skin Test

Mycobacterium Tuberculosis (TB) was once the leading cause of death in the United States. Treatment for this disease has been available since the 1940’s and except for the years between 1985 to 1992, there has been a steady decline in reported diseases in the US. Unfortunately, the disease is one of the leading causes of death worldwide, along with HIV/Aids and Malaria. Ten percent of those infected with TB are also co-infected with HIV/Aids and TB kills 1.7 million people around the world annually.

TB disease is transmitted through the air by actively infected persons and then inhaled by others in close proximity to them. TB usually affects the lungs but it is possible to be infected in other parts of the body as well.

The standard test for TB is the Mantoux Tuberculin Skin Test (TST) which consists of a small injection of a purified protein derivative (PPD) administered just under the skin of the forearm. The skin test should be read by a trained and licensed health care professional between 48 and 72 hours after the test is administered. Many people believe a reaction consisting of redness indicates the test is positive but a true reaction reveals a raised, palpable induration at the site of the original injection. Only the induration is measured using a ruler. The results are reported in millimeters of induration and there are distinct parameters for interpretation.

According to the Centers for Disease Control and Prevention (CDC) A false positive reading of a Mantoux Tuberculin Skin Test means there may be a measurable skin reaction to the serum without actual infection.
Test may occur for various reasons including:

An infection with a nontuberculosis mycobacterium
Improper administration
Incorrect measurement and interpretation
Previous vaccination with BCG, a vaccine not used in the US

A person will not know if they have a false positive reaction to a TB test unless they are evaluated by a physician or TB clinic. Any person who has not had a positive skin test in the past and then converts to a positive reaction, or anyone with a first time positive skin test, needs to be assessed by a physician. When TB presents as an active infection with positive signs and symptoms it is infectious to others and requires prescription TB medications for treatment. It may also present with a positive skin test but no signs or symptoms of disease. This is referred to as latent TB and is not considered infectious to others. Latent TB does not always progress to the active, infectious stage but requires monitoring. The physician will assess the patient’s risk factors for developing active disease and make a decision as to whether or not treatment is indicated. For these reasons it is vitally important for anyone with a positive reaction to the Mantoux Tuberculin Skin Test to follow up immediately with their physician. There is no reason to panic if your test is read as positive. The first consideration is the reliability of the test and it’s administration. The physician will interpret the relevancy of the test, order appropriate lab and radiology studies and then after examining the results, determine which course of treatment is indicated.