What is the confirmatory diagnostic test for cystic fibrosis?

NORCET 6 mains-2024

Medical Surgical Nursing

Easy

Appeared in: NORCET 6 mains-2024

Explanation

The sweat chloride test is the gold standard and most reliable method for diagnosing cystic fibrosis (CF).
It directly measures the function of the CFTR protein by assessing the concentration of chloride in sweat.
A defective CFTR protein, the underlying cause of CF, leads to elevated chloride levels in the sweat.
A sweat chloride concentration of 60 mmol/L or greater is diagnostic for CF.

Option A: A blood test is used for newborn screening, not for confirmation. It measures immunoreactive trypsinogen (IRT), which, if elevated, indicates a high likelihood of CF and necessitates further, confirmatory testing.
Option B: While identifying two disease-causing CFTR mutations confirms the diagnosis, genetic testing is not the primary confirmatory test because standard panels can miss rare mutations. The sweat test provides a functional assessment of CFTR protein activity, making it the gold standard, especially when genetic results are ambiguous.
Option D: A chest X-ray is used to assess the extent of lung damage and monitor the progression of pulmonary disease in patients already diagnosed with CF. It shows complications like bronchiectasis and mucus plugging but does not diagnose the root cause of the disease.

Visual 1: Flowchart - A diagnostic algorithm for Cystic Fibrosis, starting with newborn screening (IRT), leading to a sweat chloride test, and showing the role of genetic testing for intermediate results.
Visual 2: Diagram - A comparison of a normal sweat duct and a CF sweat duct, illustrating how the faulty CFTR channel prevents chloride reabsorption, leading to salty sweat.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Confirmatory Diagnosis of Cystic Fibrosis as background academic context rather than a clinical decision trigger.
Early and accurate diagnosis via the sweat test is crucial for initiating timely interventions, such as pancreatic enzyme replacement and airway clearance techniques, which significantly improve long-term outcomes and quality of life.
Nurses play a key role in educating parents about the sweat test procedure, ensuring it is performed correctly at an accredited center, and providing support while awaiting results.
What if? - If a sweat test result is in the intermediate range (30-59 mmol/L), the next step is typically extensive CFTR gene sequencing to look for less common mutations that might not be on standard panels. This highlights the complementary roles of functional and genetic testing.

How to Approach the Question

First, identify the key term in the question: 'confirmatory diagnostic test'. This asks for the definitive test that establishes a diagnosis, not a screening or monitoring tool.
Next, evaluate each option based on this keyword.
Consider 'Blood test'. Recall that newborn blood spots are used for screening (IRT), which identifies at-risk infants but doesn't confirm the disease.
Consider 'Genetic testing'. While it can be confirmatory, it has limitations (missing rare mutations). Is there a more standard, functional test?
Consider 'Sweat chloride test'. Recognize this as the classic, 'gold standard' test that measures the direct physiological effect of the disease.
Consider 'Chest X-ray'. This is an imaging modality that shows the consequences of the disease on the lungs, not the disease itself. It's for monitoring, not diagnosis.

Concept Tested & Keywords

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