A tuberculosis intradermal skin test to detect tuberculosis is given to a high-risk patient. How long after administration should the result be evaluated?

1. A

   Immediately

2. B

   Within 24 hr

3. C

   48-72 hr

4. D

   After 5 days

Appeared in: NHM MP Staff Nurse - 2015

Explanation

• The Mantoux test relies on a delayed-type (Type IV) hypersensitivity reaction, which is an immune response that takes time to develop.
• The reaction to the tuberculin antigen begins 5-6 hours after injection and reaches its peak induration (hardening) at 48 to 72 hours.
• This specific window is critical for accurate interpretation, as reading too early or too late can lead to false-negative results.
• The evaluation involves measuring the diameter of the induration (the palpable, raised, hardened area), not just the redness.

Why Other Options Were Wrong

• Option A: Reading the test immediately is incorrect because the delayed-type hypersensitivity reaction has not had time to occur. There would be no visible or palpable reaction at this point.
• Option B: Evaluating the test within 24 hours is too early. The immune response is still developing, and the induration will not have reached its peak size, likely resulting in a false-negative reading.
• Option D: Waiting until after 5 days (120 hours) is too late. The immune reaction will have started to subside, and the induration will decrease in size, leading to an inaccurate or falsely negative result.

Related Visual

Clinical Relevance

• Nursing practice connection: Safe nursing care depends on performing Timing for Tuberculin Skin Test (TST/Mantoux) evaluation in the correct sequence, documenting the action clearly, and monitoring for the expected response.
• Patient education is a key nursing responsibility. The nurse must instruct the patient to return for the reading within the 48-72 hour window and to avoid scratching or covering the site, as this can alter the result.
• A positive TST does not diagnose active disease; it indicates exposure and infection. Nurses must be prepared to explain the need for follow-up tests like chest X-rays and sputum cultures to rule out active TB.
• Accurate documentation is crucial. The nurse must record the date and time of administration, the site, and then the exact measurement of the induration in millimeters (mm) at the time of reading.

How to Approach the Question

• First, identify the core of the question: it's asking for the specific time frame to read a TB skin test.
• Recall the pathophysiology of the test. The Mantoux test is a classic example of a delayed-type (Type IV) hypersensitivity reaction.
• Understand that 'delayed' means the reaction takes time to peak. This immediately rules out 'Immediately' and 'Within 24 hr' as too short.
• Remember the standard window for reading the test. Clinical guidelines universally state 48 to 72 hours for optimal accuracy.
• Eliminate the 'After 5 days' option because the immune reaction would have started to fade, making the reading unreliable.
• Select the option that matches the standard peak reaction time for a delayed hypersensitivity test.

Concept Tested & Keywords

• Concept Tested: Timing for Tuberculin Skin Test (TST/Mantoux) evaluation
• Stem keywords: tuberculosis, intradermal skin test, high-risk patient, evaluated
• Lead-in keywords: How long after
• Negative lead-in flag: false

Question ID

Ql8R_lyfIWLqs7UVgrx8g0

Copy ID

Reference Book

E6 Nursing Fundamentals Potter Perry 12e Part 3 p. 203-205

E6 Ghai Essential Pediatrics(pp 26-904 of 913) p. 257-259