How to Interpret the PPD Skin Test Result
Nov 09, 2020
Source: Cainiu Health
Dr. Li Ruizhi
Introduction
How to Interpret the PPD Skin Test Result: Criteria for Interpreting the Tuberculosis Skin Test Result
- If there is no erythema or pruritus at the injection site, the result is negative.
- Erythema measuring ≥0.5 cm but <2 cm at the injection site indicates a positive reaction.
- A positive reaction signifies that the body has developed a hypersensitivity response to *Mycobacterium tuberculosis*, indicating prior exposure to tuberculosis; however, it does not necessarily indicate active disease.
The tuberculin skin test—also known as the PPD (purified protein derivative) test—is an intradermal test that involves injecting tuberculin into the skin and evaluating the resulting cutaneous reaction at the injection site to diagnose a Type IV hypersensitivity reaction caused by Mycobacterium tuberculosis infection. Tuberculin consists of components derived from the M. tuberculosis bacillus, including purified protein derivative (PPD) and old tuberculin (OT). This test serves as a useful reference for diagnosing tuberculosis and assessing non-specific cellular immune function.
How to Interpret PPD Skin Test Results
Criteria for interpreting the tuberculin skin test (for pulmonary tuberculosis): - A negative result is indicated by absence of erythema or pruritus at the injection site. - A positive reaction is defined as induration (not just erythema) measuring ≥5 mm but <20 mm in diameter: • Induration <10 mm: weakly positive (+) • Induration 10–19 mm: positive (++) • Induration ≥20 mm, or ≥15 mm with accompanying induration/nodule: strongly positive (+++) • Induration ≥20 mm with vesicles or necrosis: very strongly positive (++++)
The tuberculin skin test (also called the PPD test) involves intradermal injection of tuberculin, followed by assessment of the local cutaneous reaction to determine prior exposure to M. tuberculosis. A positive reaction indicates a delayed-type hypersensitivity response to tuberculin antigens, signifying previous infection with M. tuberculosis; however, it does not necessarily indicate active disease, as individuals vaccinated with Bacille Calmette-Guérin (BCG) may also yield positive results. A strongly positive reaction suggests possible active tuberculosis infection and warrants further diagnostic evaluation.
Precautions for the PPD Skin Test
The PPD tuberculin test requires observation of the injection site at exactly 48–72 hours post-injection. During this period, the injection site must remain dry to prevent secondary infection; swimming is strictly prohibited, as moisture may interfere with accurate interpretation of results. Patients should avoid spicy, greasy foods and alcohol consumption during this time.
The PPD skin test is a tuberculin test used as an early auxiliary diagnostic tool for tuberculosis. If you are experiencing any discomfort or illness, it is advisable to postpone the test until you have fully recovered.
The above provides an overview of how to interpret PPD skin test results. We hope this information is helpful to you.
