What testing is recommended to confirm latent TB infection before starting INH?

To confirm latent TB infection before starting isoniazid (INH), the tuberculin skin test (TST) or an Interferon Gamma Release Assay (IGRA) blood test is recommended. These tests specifically assess whether a person has been exposed to the bacteria that cause tuberculosis and if the immune system has responded to that exposure.

The tuberculin skin test involves intradermal injection of a purified protein derivative (PPD) and requires reading the induration after 48 to 72 hours, while the IGRA blood tests, such as QuantiFERON-TB Gold, measure the immune response to specific TB proteins in a blood sample. Both methods are crucial for identifying individuals with latent TB infection, allowing for proper treatment initiation with INH to prevent progression to active TB disease.

Other tests mentioned, such as a complete blood count or urinalysis, do not provide specific information regarding TB infection status. X-ray imaging of the lungs, while useful in determining active TB disease, is not a method for diagnosing latent TB since individuals with latent TB typically have normal chest X-rays.