Reading a PPD (Purified Protein Derivative) skin test, also known as a Mantoux test, involves checking for a raised, hardened area (induration) at the injection site 48 to 72 hours after administration. Redness without induration doesn't indicate a positive result.
Steps for Reading a PPD Skin Test:
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Locate the Injection Site: Find the area where the PPD was injected. It should be on the inner forearm.
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Observe the Reaction: Look for any reaction at the injection site. Pay close attention to:
- Redness (Erythema): Note the presence of redness. Redness alone is usually not significant.
- Swelling: Note the presence of swelling.
- Induration: This is the most important factor. Induration is a raised, hardened area that indicates a possible TB infection. It's often easier to feel than to see.
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Measure the Induration: If induration is present, use a ruler to measure its diameter across the forearm (perpendicular to the long axis of the arm). Only measure the hardened area, not the redness. Measure in millimeters (mm).
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Interpret the Results: The interpretation of the test depends on the size of the induration in millimeters and the person's risk factors. Cutoff points for a positive test vary. Common cutoff points include:
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≥ 5 mm: Considered positive in:
- People with HIV
- Recent contacts of people with active TB disease
- People with fibrotic changes on a chest X-ray consistent with prior TB disease
- Organ transplant recipients
- People who are immunosuppressed for other reasons (e.g., taking TNF-alpha inhibitors or high-dose corticosteroids).
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≥ 10 mm: Considered positive in:
- Recent immigrants (within the last 5 years) from high-prevalence countries
- Injection drug users
- Residents and employees of high-risk congregate settings (e.g., prisons, nursing homes, homeless shelters, hospitals)
- Mycobacteriology lab personnel
- People with certain medical conditions that increase the risk of progressing to active TB disease (e.g., diabetes, end-stage renal disease, silicosis, leukemia, lymphoma, head or neck cancer, weight loss of more than 10% of ideal body weight, gastrectomy, jejunoileal bypass)
- Children younger than 4 years old
- Infants, children, and adolescents exposed to adults in high-risk categories
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≥ 15 mm: Considered positive in:
- People with no known risk factors for TB
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Record the Results: Document the size of the induration in millimeters and whether the test is considered positive or negative based on the above criteria. Note any blistering or other significant reactions.
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Further Evaluation: A positive PPD test does not necessarily mean that the person has active TB disease. It indicates that the person has been infected with Mycobacterium tuberculosis. Further evaluation, including a chest X-ray and sputum samples (if indicated), is necessary to determine if active TB disease is present. A negative PPD test does not completely rule out TB infection, especially in individuals with weakened immune systems.
Example Scenarios:
- A person with HIV has an induration of 6 mm. The test is considered positive.
- A healthcare worker from the US has an induration of 8 mm. The test is considered negative.
- A recent immigrant from a high-prevalence country has an induration of 12 mm. The test is considered positive.
- A person with no risk factors has an induration of 16 mm. The test is considered positive.
Reading a PPD skin test requires careful observation and measurement, and the interpretation should always be done in the context of the individual's risk factors. Always consult with a healthcare professional for accurate interpretation and follow-up.
