PPD Skin Test

Excerpt

Tuberculosis (TB) is a potentially fatal bacterial infection caused by the bacterium Mycobacterium tuberculosis (M. tuberculosis). It is a highly contagious droplet infection that primarily affects the lungs. However, it can affect numerous organ systems. Treatment for TB is available and effective, yet it remains a significant public health concern and a leading cause of morbidity and mortality worldwide, especially in developing countries. It is estimated that more than 1.7 billion people are infected with M. tuberculosis. Although the overall incidence and prevalence of tuberculosis have declined, the incidence of multidrug-resistant tuberculosis remains significant.

Active and Latent TB Infection (LTBI)

A person with an active infection usually presents with constitutional symptoms, including unexplained weight loss, fever, fatigue, loss of appetite, and night sweats. LTBI is asymptomatic and non-infectious.

Early diagnosis of active TB is crucial to managing and preventing its spread. LTBI has a worldwide prevalence of 33%. Those with immunocompromised states are at an increased risk of reactivation and progression to active TB disease, which is symptomatic and highly contagious. The risk of LTBI progressing to active disease is greatest within the first two years of exposure. Therefore, detection and treatment of latent TB are critical to controlling the spread of TB and reducing the disease burden.

The risk of LTBI progression to active disease declines with age, secondary to increased immunity. The risk of progression in infants is 50%, and this decreases to 1% to 2% by age ten.

TB Screening Tests

Two screening tests are available to detect TB infection in the United States:

1. Tuberculin skin (TST) or purified-protein derivative (PPD) skin test via the Mantoux technique:

2. Interferon-gamma release assay blood test (IGRA)

Both tests measure delayed-type hypersensitivity reaction or type IV cell-mediated immunity involving T-lymphocytes, which are activated after exposure to mycobacteria. Notably, a positive test does not distinguish between latent and active TB. Therefore, symptom assessment and further testing with chest radiography, sputum test for acid-fast bacilli, or chest computed tomography scans are essential to diagnosing active infection.

There is no definitive test to diagnose LTBI. Instead, it is a clinical diagnosis based on a history of prior TB infection and ruling out active disease.

PPD Skin Test (TST)

Robert Koch described the tuberculin reaction in 1890, and Felix Mandel developed the test in 1908. Charles Mantoux was credited with creating the technique of injecting material intradermally on the inner surface of the forearm. In 1934, Florence Seibert published her method of obtaining the purified-protein derivative, effectively creating the PPD test.

The tuberculin protein used in the test is extracted from Mycobacterium tuberculosis cultures and is used as a purified-protein derivative. A standardized PPD-S utilizes a tuberculous mycobacterium. Non-tuberculous mycobacteria are identified by a letter other than S. The test results are interpreted by measuring the delayed-type hypersensitivity reaction. The peak of the induration reaction occurs 24 hours after the test and is secondary to cell infiltration.

It takes approximately six to eight weeks after exposure to bacteria for the PPD test to become positive. The PPD test should be read between 48 and 72 hours after administration.