PPD Test for Tuberculosis

Active TB Disease and Latent TB Infection (LTBI)

• Active TB Infection
  Active TB infection occurs when the bacteria Mycobacterium tuberculosis multiply and cause symptomatic disease. Individuals with active TB infection typically experience constitutional symptoms, such as unexplained weight loss, fever, fatigue, loss of appetite, and night sweats. The infection primarily affects the lungs, resulting in symptoms like persistent cough, chest pain, and coughing up blood. However, active TB can also affect other organs, including the kidneys, bones, and brain.
  Active TB is highly contagious and can be transmitted through respiratory droplets when an infected individual coughs, sneezes, or speaks. It is important to note that not everyone with active TB infection is immediately contagious. The contagiousness depends on various factors, such as the extent of lung involvement and the presence of coughing.
• Latent TB Infection (LTBI)
  Latent TB infection occurs when individuals are infected with Mycobacterium tuberculosis, but the bacteria remain inactive within their bodies. People with LTBI do not exhibit symptoms, and they are not contagious. LTBI is diagnosed through positive skin or blood tests, indicating exposure to TB bacteria.
  While individuals with LTBI do not feel sick or spread the infection, they carry the risk of developing active TB in the future. The risk of LTBI progressing to active disease varies depending on factors such as age, immune status, and other underlying medical conditions. Individuals with weakened immune systems, such as those with HIV/AIDS or receiving immunosuppressive medications, are at higher risk of LTBI reactivation.

Interfering Factors for PPD Test

PPD test is a simple and cost-effective diagnostic tool that doesn't require specialized laboratories. This is particularly valuable in resource-limited countries with high tuberculosis burdens, where access to quality sputum microscopy may be challenging. TST has been widely used in these settings for epidemiological surveys, especially among healthy children. However, difficulties in test administration and interpretation often lead to false results. Several factors can interfere with the accuracy and interpretation of the PPD test.

• PPD Dose, Application Method, and Interpretation Criteria
  Weaker doses of PPD increase the risk of false-negative results, while stronger doses increase the risk of false-positive results. Different tests like the Heaf test and Mantoux test are available, but the Heaf test has been discontinued in the UK due to safety concerns. Various cutoffs for positivity of TST exist, with recommendations ranging from 5 to 15 mm of skin reaction based on the clinical situation. Operator variability during test administration and reading, as well as digit preference and interpretation bias, can influence TST results.
• Bacille Calmette-Guerin (BCG) Vaccination
  BCG vaccination, which is commonly administered in many countries, can lead to a positive PPD test result. The live attenuated strain of Mycobacterium bovis used in BCG shares antigens with M. tuberculosis, leading to cross-reactivity. The effect of BCG vaccination on the PPD test can persist for several years, making it difficult to differentiate between true TB infection and the immune response induced by the vaccine.
• Non-Tuberculous Mycobacteria (NTM)
  Exposure to non-tuberculous mycobacteria can also result in a positive PPD test due to shared antigens between these mycobacteria and M. tuberculosis. NTM are commonly found in the environment and can cause infections in humans. In areas with a high prevalence of NTM, interpreting a positive PPD test can be challenging and may require additional tests or clinical evaluation.
• Prior Tuberculosis Infection
  Individuals who have previously been infected with M. tuberculosis and have successfully cleared the infection may still show a positive PPD test result, even if they are not actively infected. This is because the immune response generated during the initial infection can persist and be detected by the PPD test.
• Immunosuppression
  Immunosuppressed individuals, such as those with HIV infection, certain medical conditions, or taking immunosuppressive medications, may have a diminished immune response. This can result in false-negative PPD test results, where a person infected with M. tuberculosis does not show a significant reaction to the test due to the compromised immune system.
• Recent Tuberculosis Exposure
  The PPD test may not immediately detect an immune response in individuals recently exposed to M. tuberculosis. It can take several weeks for the immune system to mount a detectable reaction to the PPD test after exposure to the bacteria. Testing too soon after exposure may lead to false-negative results.