Explain the negative TST (tuberculin skin test) or PPD (purified protein

derivative)

Mantoux tuberculin skin test Normal (negative results):• No firm bump forms at the test site, or a bump

forms that is smaller than 5mm.Abnormal (positive results): • A firm bump that is 5mm in size shows a TB

infection in people who are in a high-risk group.• A firm bump that is 10mm in size shows a TB

infection in people who are in a moderate-risk group.

• A firm bump that is 15mm in size shows a TB infection in people who are in a low-risk group.

• 5 mm or more is positive in – HIV-positive person– Recent contacts of TB case– Persons with nodular or fibrotic changes on chest x-ray consistent with old

healed TB– Patients with organ transplants and other immunosuppressed patients

• 10 mm or more is positive in – Recent arrivals (less than 5 years) from high-prevalence countries– Injection drug users– Residents and employees of high-risk congregate settings (e.g., prisons,

nursing homes, hospitals, homeless shelters, etc.)– Mycobacteriology lab personnel– Persons with clinical conditions that place them at high risk (e.g., diabetes,

prolonged corticosteroid therapy, leukemia, end-stage renal disease, chronic malabsorption syndromes, low body weight, etc)

– Children less than 4 years of age, or children and adolescents exposed to adults in high-risk categories

• 15 mm or more is positive in – Persons with no known risk factors for TB– (Note: Targeted skin testing programs should only be conducted among

high-risk groups)

False positive• Infections of non-

tuberculosis mycobacterium

• BCG vaccination

False negative• Immunosuppressed

patients• Overwhelming tuberculosis

infection

Negative TST/PPD

• In people who have not been exposed to TB, there will be little or no swelling at the test site after 48-72 hours. This is a negative test result. Negative test results can be interpreted to mean that the person has not been infected with the tuberculosis bacteria or that the person has been infected recently and not enough time has elapsed for the body to react to the skin test.