Tuberculin Skin Test: Positive and Negative Results of Tuberculin Skin Test

 

Definition of Tuberculin Skin Test

Tuberculin skin test is a diagnostic procedure for tuberculosis – the disease caused by rod-shaped gram-positive aerobic bacterium called Mycobacterium tuberculosis. Its other names are tuberculin sensitivity test, Mantoux screening test, Purified Protein Derivative (PPD) test, and Latent tuberculosis infection test.

Tuberculin skin test is used to identify persons who may have been infected by Mycobacterium tuberculosis. This test, however, cannot tell how long an individual has been infected with tuberculosis. It cannot also tell if the infection is inactive (latent) or is active and can be transmitted to others.
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People Who Needs Tuberculin Skin Test

  • People showing TB symptoms including night sweats, continuing cough, and weight loss for no apparent reason
  • People with weak immune system because of having diseases (e.g. AIDS) that make them more susceptible to TB infection
  • People who are confined in schools, homeless shelters, correctional facilities, and nursing homes
  • People who have been exposed to tuberculosis bacteria by close contact with people who have active and contagious TB infection
  • Healthcare professionals and workers who are at high-risk for M. tuberculosis exposure
  • People who migrated from countries with high prevalence of tuberculosis
  • People who are required to take a routine examination (where TB skin test is a part) before starting a new job or attending school

 

Tuberculin Skin Test Procedure

Tuberculin skin test is done by injecting a small amount (standard: 0.1 ml) of purified protein derivative (PPD) tuberculin intracutaneously or just under the top layer of the skin. Tuberculin is a combination of bacterial antigens obtained in a culture of Mycobacterium tuberculosis. Antigens are foreign substances that trigger the immune system to produce antibodies (e.g. immunoglobulin E). The usual location of the injection is on the inside of the forearm, halfway between the elbow and the wrist.

 

Tuberculin Skin Test Result

The result of tuberculin skin test cannot be obtained just after the injection. The patient will be asked to go home and return after 48-72 hours for the test result. The patient is prevented in scratching the injected skin area so as not to affect the result. The formation of greater than 5 mm of wheal or bump in the injected area indicates a positive result; mere redness without a bump indicates a negative result. The bump is measured by a trained professional in millimeters. The erythema or redness is excluded in the measurement. A person who didn’t return within 48-72 hours will be rescheduled for another skin test.

Since a positive tuberculin skin test cannot tell if tuberculosis infection is active or inactive (latent), other procedures like blood test, x-ray, and sputum culture should be done. People who are positive to the skin test but do not show the symptoms of tuberculosis may have latent or inactive TB infection. Active tuberculosis disease is confirmed by positive skin test, M. tuberculosis in sputum culture, abnormal chest x-ray, and mycobacterial antigens in the blood.

 

Interpretation of Results

A bump measuring 5 or more millimeters is considered positive in

  • People with diseases or conditions that weaken their immune system (e.g. AIDS)
  • People who are taking drugs (e.g. prednisone, TNF-a antagonists) that suppress their immune system
  • People with close contact to a person with active TB disease
  • Organ transplant recipients
  • People with abnormal chest X-ray result (e.g. lung fibrosis or scarring)

A bump measuring 10 or more millimeters is considered positive in

  • Recent immigrants (less than 5 years) from countries where TB diseases is prevalent
  • People who are living and working in high-risk confined places such as correctional facilities, nursing homes, homeless shelters, etc.
  • People working in mycobacteriology laboratory
  • Children who are less than 4 years old
  • Healthcare professionals with clinical conditions that place them at high risk
  • Injection drug users

A bump measuring 15 or more millimeters is considered positive in any person, including persons with no risk factors (e.g. abnormal chest X-ray, injection drug users, organ transplant, close contact with TB patients, etc.).

 

False-Positive Results of Tuberculin Skin Test

The reasons for false-positive result of tuberculin skin test may include, but are not limited, to the following:

  • Incorrect method of administering the test
  • Incorrect antigen used
  • Incorrect interpretation of reaction
  • Previous Bacillus Calmette-Guerin (BCG) vaccination
  • Infection with nontuberculosis mycobacteria

 

False-Negative Results of Tuberculin Skin Test

The reasons for false-positive result of tuberculin skin test may include, but are not limited, to the following:

  • Incorrect interpretation of reaction
  • Incorrect method of administering the test
  • Cutaneous anergy or the inability to react to the antigen because of weak immune system
  • Viral illnesses like chicken pox and measles
  • Recent live-virus vaccination for chicken pox, measles, and other viral infections; the vaccination may interfere with tuberculin skin test reaction
  • Very young persons (< 6 months old) who have underdeveloped immune system
  • Recent TB infection; sensitivity to antigen develops after 8-10 weeks

Pictures of Tuberculin Skin Test

mantoux test, Image from Wikimedia Commons

Intradermal injection of PPD tuberculin (Image from Wikimedia Commons)

Mantoux-2, Image from Wikimedia Commons

The size of the bump is measured after 48-72 hours (Image from Wikimedia Commons)

mantoux-3, Image from Wikimedia Commons

PPD Skin test injection site after 50 hours.; the picture shows negative result (Image from Wikimedia Commons)


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References & Further Reading