Tuberculosis Skin Test (PPD) – Procedure, Interpretation, and Limitations
Tuberculosis Skin Test
Also Known As:
- PPD Test
- Mantoux Test
- Anergy Testing
- Histoplasmin Skin Test
What the Test Includes
This test involves the intradermal injection of 0.1 mL of a purified protein derivative (PPD) solution. The injection is typically made on the forearm after cleaning the area with an alcohol swab.
Turnaround Time
Results should be read and interpreted between 48–72 hours after administration.
Reference Range & Interpretation
- Negative: < 5 mm induration
- Intermediate: 5–9 mm (positive in HIV patients or immunocompromised individuals)
- Positive: ≥10 mm induration in general population
Note: The reaction is evaluated by measuring the diameter of induration, not erythema. Wheal-and-flare or inflammatory edema may appear but should not be considered positive.
Use of the Test
This test helps identify individuals with a delayed-type hypersensitivity reaction to Mycobacterium tuberculosis, which may indicate latent or active tuberculosis (TB).
Limitations
- False-negatives can occur in immunocompromised individuals, including patients with HIV or advanced TB.
- False-positives may occur in individuals who received the BCG vaccine.
- Not definitive for active disease – clinical correlation and further testing (e.g., chest X-ray, culture) are required.
Contraindications
- Retesting known positives without medical consultation is not advised.
- Anergic patients may not mount a response even if infected.
Special Considerations for HIV Patients
- 5 mm of induration is considered positive in HIV-positive patients with suspected TB.
- Up to 50% of AIDS patients may show no reaction due to immune suppression.
Additional Information
Skin testing is sometimes combined with multiple antigen tests (e.g., tetanus, diphtheria, Candida) to evaluate immune responsiveness. The Histoplasmin Skin Test is rarely used today due to potential cross-reactivity with serologic tests and limited geographic relevance.
Procedure Summary
- Clean the forearm skin with an alcohol swab.
- Inject 0.1 mL of PPD intradermally to form a wheal.
- Advise the patient to return in 48–72 hours.
- Measure only the induration (firm swelling), not redness.
References
- Chaisson RE & Slutkin G, J Infect Dis, 1989
- Johnson MP et al., J Infect Dis, 1992
- Graham NM et al., JAMA, 1992
- Delafuente JC et al., J Am Geriatr Soc, 1988
- Jacobs, Demott, Finley et al., Lab Test Handbook, Lexi-Comp Inc, 1994
