Which test is used to determine a tuberculosis infection?

Which test is used to determine a tuberculosis infection?

• A. Chest x-ray
• B. Positive PPD test (Purified Protein Derivative Test)
• C. Sputum culture
• D. Complete blood count

Answer: (B)

The main idea is that an infection with tuberculosis is detected by a test that shows immune sensitization to TB antigens, not by imaging or general blood work. The Purified Protein Derivative test, a tuberculin skin test, measures this immune response: tuberculin is injected under the skin and the size of the induration read after 48–72 hours indicates whether the person has been infected with TB and mounted a delayed-type hypersensitivity reaction. A positive result suggests infection or exposure, though it does not by itself prove active disease; further evaluation is needed to distinguish latent infection from active TB. In contrast, chest x-ray can reveal lung abnormalities that might be seen with active TB but cannot confirm infection on its own. Sputum culture detects Mycobacterium tuberculosis when bacteria are present in the sputum, which confirms active disease rather than mere infection. A complete blood count is non-specific and not used to determine TB infection. So, the test best for determining TB infection is the PPD skin test.

The main idea is that an infection with tuberculosis is detected by a test that shows immune sensitization to TB antigens, not by imaging or general blood work. The Purified Protein Derivative test, a tuberculin skin test, measures this immune response: tuberculin is injected under the skin and the size of the induration read after 48–72 hours indicates whether the person has been infected with TB and mounted a delayed-type hypersensitivity reaction. A positive result suggests infection or exposure, though it does not by itself prove active disease; further evaluation is needed to distinguish latent infection from active TB.

In contrast, chest x-ray can reveal lung abnormalities that might be seen with active TB but cannot confirm infection on its own. Sputum culture detects Mycobacterium tuberculosis when bacteria are present in the sputum, which confirms active disease rather than mere infection. A complete blood count is non-specific and not used to determine TB infection.

So, the test best for determining TB infection is the PPD skin test.