Tuberculin skin testing (Mantoux): In standard practice for adults with no special risk factors, which induration size is interpreted as a positive Mantoux test?

Introduction / Context:
The Mantoux test (tuberculin skin test) measures delayed-type hypersensitivity to purified protein derivative injected intradermally. Reading is based on the transverse diameter of induration, not erythema. Thresholds for a positive result depend on risk category, but a commonly taught cut-off for people without special risk factors is 10 mm or more of induration.

Given Data / Assumptions:

• Population considered: general adults without known high-risk conditions.
• Measurement is at 48–72 hours after intradermal PPD.
• Interpretation is by size of induration, not redness.

Concept / Approach:
Cell-mediated immunity from prior Mycobacterium tuberculosis exposure or BCG vaccination can produce induration. Interpretation uses tiered cut-offs: 5 mm for very high-risk groups (e.g., HIV infection, recent contacts, immunosuppressed), 10 mm for intermediate risk (e.g., recent immigrants, healthcare workers, certain medical conditions), and 15 mm for persons with no known risk. In routine exam questions, 10 mm is the standard positive threshold unless a specific high-risk scenario is stated.

Step-by-Step Solution:
Identify the group as general risk (no special modifiers given). Recall standard interpretation thresholds for induration. Match the general-risk threshold to 10 mm or more. Select the option that states 10 mm or more in diameter.

Verification / Alternative check:
Clinical guidelines demonstrate tiered cut-offs. Absent explicit high-risk factors in the stem, examination convention defaults to 10 mm as positive for most adults.

Why Other Options Are Wrong:

• 1–4 mm or 4–9 mm: Typically negative in general-risk individuals.
• 5 mm for everyone: Applies only to high-risk categories.
• None of these: Incorrect because 10 mm is a valid positive threshold for many adults.

Common Pitfalls:
Measuring erythema instead of induration; ignoring patient risk category; reading before 48 hours or after 72 hours reduces accuracy.

Final Answer:
10 mm or more in diameter.