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

Appeared in: KPSC Staff Nurse - 2017

Explanation

The tuberculin skin test (TST) must be read 48 to 72 hours after the intradermal injection.
This specific time frame is critical because it corresponds to the peak of the delayed-type (Type IV) cell-mediated hypersensitivity reaction, which is the immunological basis for the test.
Reading the test within this window ensures the most accurate measurement of the induration (hardening) caused by the immune response.
An accurate reading is essential for correctly identifying individuals who may have been infected with the tuberculosis bacteria.

Option A: Reading the test immediately after injection is incorrect. The immune response is a delayed reaction and requires time to develop. An immediate reading would show no reaction, even in an infected person, leading to a false negative.
Option B: Evaluating the test within 24 hours is too early. The cell-mediated immune response is still developing and has not reached its peak, which can lead to an underestimation of the induration size and a false-negative result.
Option D: Reading the test after 5 days (120 hours) is too late. The immune reaction begins to subside after 72 hours, and the induration will decrease in size or disappear. This would lead to a false-negative result.

Clinical Relevance

Nursing practice connection: Safe nursing care depends on performing Timing and Interpretation of Tuberculin Skin Test (Mantoux) in the correct sequence, documenting the action clearly, and monitoring for the expected response.
Accurate timing for reading a TST is a critical nursing responsibility. A misread test can lead to a missed diagnosis and delayed treatment, posing a risk to both the patient and public health.
Patient education is paramount. The nurse must clearly instruct the patient on the importance of returning for the reading within the 48-72 hour window and advise them to avoid scratching or covering the site.
A positive TST does not confirm active TB disease; it only indicates that the person has been infected with TB bacteria at some point. Further evaluation, such as a chest X-ray and sputum analysis, is required.

How to Approach the Question

First, identify the key subject of the question, which is the 'tuberculosis intradermal skin test' (or Mantoux test).
Recognize that the question is asking for a specific time frame, which is a test of factual recall about a standard clinical procedure.
Recall the underlying principle of the test: it measures a delayed-type hypersensitivity reaction.
The word 'delayed' is a crucial clue. It tells you that an immediate or very early reading is incorrect, which helps eliminate the first two options.
Access your knowledge of standard nursing procedures to retrieve the specific time window for this delayed reaction's peak.
Evaluate the options against the standard of care: 48-72 hours is the universally accepted time frame for reading a TST. This makes it the only correct choice.

Concept Tested & Keywords

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

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

Practise the full KPSC Staff Nurse - 2017

Attempt every question from this paper in a timed mock, then review the full solution for each one.