The Purified Protein Derivative (PPD) test screens for exposure to the bacteria that cause tuberculosis (TB). This test primarily detects a silent, or latent, TB infection by identifying the body’s immune response to specific TB proteins. The PPD test suggests exposure to the bacteria but does not confirm active TB disease.
Understanding Tuberculosis
Tuberculosis is an infectious disease caused by the bacterium Mycobacterium tuberculosis, typically affecting the lungs. This bacteria spreads through the air when an infected person coughs, sneezes, or speaks, releasing tiny droplets containing the microbes. Once inhaled, the bacteria can settle in the lungs and potentially spread to other parts of the body.
There are two main forms of TB infection: latent TB infection (LTBI) and active TB disease. In latent TB, the bacteria are present in the body but remain inactive, causing no symptoms and not being contagious. The immune system usually contains the bacteria, preventing them from multiplying. Conversely, active TB disease occurs when the bacteria multiply, causing symptoms such as fever, weight loss, coughing, and night sweats, making the individual contagious.
What the PPD Test Is
The PPD skin test, also known as the Mantoux tuberculin skin test, involves injecting a small amount of PPD solution, typically 0.1 ml, just beneath the skin on the inner surface of the forearm. This solution contains proteins extracted from TB bacteria, and the test assesses the body’s immune system reaction to them.
The injection creates a small, pale bump on the skin, called a wheal, typically 6 to 10 mm in diameter. Healthcare professionals observe this site after a specific period to determine if an immune response has occurred. A reaction indicates prior exposure to TB bacteria but does not confirm active infection.
Interpreting PPD Test Results
After the PPD solution is injected, the test site is inspected by a healthcare professional between 48 and 72 hours later. The interpretation focuses on measuring the size of any firm, raised area, known as induration, at the injection site. It is the diameter of this induration in millimeters that is measured, not any redness that might appear.
A positive result is indicated by a firm bump, and the specific size considered positive varies based on individual risk factors. For instance, an induration of 5 millimeters or more may be considered positive in individuals with HIV/AIDS, those who have received an organ transplant, or people with a suppressed immune system. For others, a larger induration, such as 10 mm or 15 mm, might be required for a positive reading, depending on their exposure history and other health conditions. A negative result means there is no or minimal reaction, suggesting the individual has likely not been infected with TB bacteria. Factors like a previous BCG vaccine or infection with non-tuberculosis mycobacteria can sometimes lead to false-positive results, while a weakened immune system can cause false negatives.
Next Steps and Further Evaluation
A positive PPD test result typically prompts further medical evaluation to differentiate between latent TB infection and active TB disease. This subsequent assessment often includes a chest X-ray to look for signs of active disease in the lungs. In some cases, sputum cultures may also be collected and analyzed to detect the presence of Mycobacterium tuberculosis bacteria.
Blood tests, known as Interferon-Gamma Release Assays (IGRAs), such as Quantiferon-TB Gold or T-Spot, offer an alternative method for detecting TB infection. These tests measure the immune response to specific TB antigens and can be used to confirm a PPD result or as a primary screening tool, particularly in individuals who have received the BCG vaccine, which can cause false-positive PPD results. If latent TB infection is confirmed, treatment usually involves a course of antibiotics to prevent the infection from progressing to active disease. Active TB disease, on the other hand, requires a more extensive treatment regimen involving multiple antibiotics over a longer period.