A false positive occurs when?

• A. Infectious agent is present but test fails to detect it
• B. Infectious agent is not present but test detects it
• C. The test gives a borderline result
• D. The test detects the agent only after dilution

Answer: (B)

In diagnostic testing, a false positive is when the test indicates the agent is present even though it isn’t actually there. The scenario described as the infectious agent not being present but the test detecting it matches that idea exactly, which is why it’s the correct choice. If the agent is present but the test fails to detect it, that’s a false negative, not a false positive. A borderline result is uncertain or inconclusive, not a definite positive. Detecting the agent only after dilution relates to how the test performs at low concentrations (its detection limit), not to producing a positive result when nothing is there.