From the Guidelines
The next step after a positive Purified Protein Derivative (PPD) test is to perform a chest radiography to distinguish between latent TB and active TB, as these are managed differently 1.
Key Considerations
- A positive PPD test indicates that a person has been infected with Mycobacterium tuberculosis, but it does not differentiate between latent and active TB 1.
- The decision to initiate treatment should be based on epidemiologic information, clinical and radiographic findings, and the results of diagnostic tests, including the PPD test and chest radiography 1.
- For individuals with a positive PPD test, treatment for latent TB infection (LTBI) may be recommended, which typically involves a course of isoniazid (INH) for 9 months, or alternative regimens such as rifampin (RIF) with or without INH for 4 months, or RIF and pyrazinamide (PZA) for 2 months 1.
Interpretation of PPD Test Results
- The interpretation of PPD test results is risk-stratified, with a reaction of 5 mm or greater considered positive for close contacts of TB cases, immunosuppressed persons, and individuals with clinical or radiographic evidence of current or prior TB 1.
- For other persons at increased risk of LTBI, a reaction of ≥10 mm is considered positive, while a reaction of 15 mm or greater is considered positive for all other persons 1.
From the Research
Next Steps After a Positive PPD Test
- A clinical evaluation and chest x-ray are recommended for asymptomatic patients with a positive PPD test result to exclude the possibility of active tuberculosis (TB) 2
- Patients with radiographic evidence of old (healed) TB infection should also undergo sputum testing 2
- Treatment with isoniazid (INH) monotherapy is recommended to reduce the progression of latent tuberculosis to active disease 2
- A 3-month course of combined rifampin and INH is equivalent in efficacy to INH monotherapy and is associated with similar rates of toxicity 2
Diagnostic Considerations
- The PPD test is currently the only available skin test reagent used worldwide for the diagnosis of tuberculosis (TB) 3
- Recombinant purified protein derivative (PPD) antigens can be used as a specific skin test for tuberculosis, reducing false positive results due to Bacillus Calmette-Guerin (BCG) vaccination 3
- A chest radiograph (CXR) is valuable in high-burden settings, but its usefulness is less certain in low-prevalence contexts 4