Classification of tuberculin reaction of Mantoux test

The results of this test must be interpreted carefully.Individual medical risk factors determine which increment of induration (5 mm, 10 mm, or 15 mm) is considered positive.Positive results indicate tuberculosis exposure.

• 5 mm or more is positive

• HIV positive

• People who have recently been in contact with someone with TB

• Persons with chest x-ray showing nodules or fibrotic changes consistent with old cured tuberculosis

• Transplant patients and other immunosuppressed patients

• 10 mm or more is positive

• Recent immigrants (less than five years) from high prevalence countries

• Injecting drug users

• Residents and employees of high-risk congregate settings (e.g., prisons, nursing homes, hospitals, homeless shelters, etc.)

• Mycobacteriology laboratory personnel 

• People with clinical conditions that put them at high risk (eg, diabetes mellitus, long-term corticosteroid therapy, leukemia, end-stage renal disease, chronic malabsorption syndrome, low body weight, etc.)

• Children under the age of 4, or high-risk children and adolescents exposed to adults

• 15mm or more is positive

• People without known risk factors for TB

A negative tuberculin test was defined as an increase of 10 mm or more within two years, regardless of age.Alternative standards include 6, 12, 15 or 18 mm increase.

False positive result

A positive TST (tuberculin skin test) is measured by the size of the induration.The size of an induration that is considered a positive result depends on risk factors.For example, low-risk patients must have larger indurations than high-risk patients to obtain a positive result.High-risk groups include recent contacts, HIV-infected individuals, those with fibrotic changes on chest radiographs, organ transplant recipients, and immunosuppressed individuals.According to the Ohio Department of Health and the U.S.Department of Health, the Bacillus Calmette-Guerin (BCG) vaccine does not prevent tuberculosis infection.However, it does protect 80% of children from tuberculous meningitis and miliary tuberculosis.Therefore, TST/PPD positivity in BCG-vaccinated individuals was interpreted as latent tuberculosis infection (LTBI).Due to the low specificity of the test, most positive reactions in low-risk individuals are false positives.False-positive results may be caused by nontuberculous mycobacteria or previous vaccination with BCG.BCG Vaccination May Give False Positive Results Years After Vaccination.

False positives can also occur when the injected area is touched, causing swelling and itching.If the swelling is less than 5 mm, it may be a medical staff error that caused the area to become inflamed.Another source of false positive results may be allergic reactions or hypersensitivity reactions.Although rare (approximately 0.08 reported reactions per million doses of tuberculin), these reactions can be dangerous and should be prevented by administering epinephrine.

False negative results

The following conditions inhibit the response to the PPD or tuberculin test:

• Recent TB infection (less than 8-10 weeks old)

• Infectious mononuclear cells

• Live virus vaccines - should not be tested within 3 weeks of a live virus vaccine (eg MMR vaccine or Sabin vaccine).

• Sarcoidosis

• Hodgkin's disease

• Corticosteroid therapy/steroid use

• Malnutrition

• Immunocompromised PPD tests are often negative in people on immunosuppressive therapy or in people who are HIV-infected and have low CD4 T-cell counts.This is because the immune system needs to be functional in order to respond to the subcutaneous injection of the protein derivative.False negative results may occur in people who have recently been infected with TB but whose immune system has not yet responded to the bacteria.

• Upper respiratory viral infection

If a second tuberculin test is required, it should be done on the other arm to avoid skin irritation.