NORCET 6 mains-2024: What is the confirmatory diagnostic test for cystic fibrosis?

NORCET 6 mains-2024

Pathology & Genetics

Easy

Appeared in: NORCET 6 mains-2024

Explanation

The sweat chloride test is the gold standard and primary confirmatory test for diagnosing cystic fibrosis (CF).
It directly measures the function of the CFTR protein by quantifying the chloride concentration in sweat, which is elevated in CF due to a reabsorption defect.
A sweat chloride level greater than 60 mmol/L on two separate occasions is considered diagnostic for CF.
This test is highly reliable and is used to confirm the diagnosis following a positive newborn screening (elevated IRT) or in individuals presenting with symptoms.

Option A: A blood test for immunoreactive trypsinogen (IRT) is a newborn screening tool, not a confirmatory diagnostic test. Elevated levels indicate a higher risk and necessitate further, definitive testing.
Option B: While identifying two CF-causing mutations confirms the diagnosis, there are over 2,000 known mutations. Standard genetic panels can miss rare mutations, leading to inconclusive results, making the functional sweat test the primary diagnostic tool.
Option D: A chest X-ray is used to assess and monitor the progression of lung disease (like bronchiectasis or mucus plugging) caused by CF. It does not diagnose the underlying genetic condition.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Diagnosis of Cystic Fibrosis as background academic context rather than a clinical decision trigger.
Nurses are responsible for educating families about the sweat test procedure (pilocarpine iontophoresis), explaining that it is painless, and ensuring proper collection to prevent inaccurate results.
It is crucial for nurses to differentiate between screening (IRT) and diagnostic (sweat test) tests when counseling families to manage expectations and reduce anxiety after a positive newborn screen.
A key nursing role involves preparing the child and family for the diagnosis and initiating education on the complex, lifelong management of CF, including airway clearance, nutrition, and medication regimens.

How to Approach the Question

First, identify the key terms in the question: "confirmatory diagnostic test" and "cystic fibrosis."
Understand that "confirmatory" implies the gold standard test that definitively establishes a diagnosis, distinguishing it from a preliminary or screening test.
Evaluate each option's role in the context of diagnosing CF.
Recall that a blood test (for IRT) is the initial newborn screen, not a confirmation.
Recognize that a chest X-ray assesses complications (lung damage) but doesn't diagnose the root cause.
Differentiate between genetic testing (identifies the gene mutation) and the sweat test (measures the functional defect). The sweat test is the established primary method for confirmation.

Concept Tested & Keywords

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E6 Nelson Textbook of Pediatrics(2024) — Volume 2 pp. 456-458, 457-459