From the Guidelines
The most appropriate diagnostic test for this infant is a sweat chloride test, which is the gold standard for diagnosing cystic fibrosis (CF). This 9-month-old's presentation strongly suggests CF with classic symptoms including respiratory issues (wheezing, labored breathing, productive cough), gastrointestinal problems (malodorous greasy stools indicating malabsorption), failure to thrive, and the significant history of meconium ileus at birth, which occurs in approximately 15-20% of newborns with CF, as noted in studies such as 1. The sweat test measures chloride concentration in sweat; values >60 mmol/L are diagnostic of CF in infants. If the sweat test is positive, genetic testing for CFTR mutations should follow to confirm the diagnosis and identify specific mutations, which may guide therapy options.
Early diagnosis is crucial as prompt initiation of pancreatic enzyme replacement therapy, fat-soluble vitamin supplementation, airway clearance techniques, and potentially CFTR modulator therapy can significantly improve outcomes, lung function, nutritional status, and quality of life for this infant, as highlighted by the importance of early detection and intervention in CF management 1. The clinical validity and sensitivity of newborn screening for CF, including the use of immunoreactive trypsinogen (IRT) and DNA testing, support the use of sweat chloride testing as a diagnostic tool in cases where CF is suspected clinically, especially given the potential for false-negatives in screening tests 1.
Given the potential for delayed diagnosis and its adverse effects on morbidity, mortality, and quality of life, a sweat chloride test is the most direct and relevant diagnostic approach for this infant, considering the clinical presentation and history provided. This approach aligns with recommendations for vigilance in detecting CF, particularly in cases with suggestive symptoms or history, such as meconium ileus at birth 1.
From the Research
Diagnostic Considerations
The patient's symptoms, including wheezing, labored breathing, productive cough, malodorous and greasy stools, and failure to gain weight, along with a history of meconium ileus at birth, suggest a possible diagnosis of cystic fibrosis (CF) 2, 3.
Appropriate Diagnostic Test
Given the clinical presentation, the most appropriate diagnostic test to determine the diagnosis would be the sweat chloride test 3, 4, 5, 6. This test measures the concentration of chloride in sweat and is considered the gold standard for diagnosing CF 3.
Rationale for Sweat Chloride Test
- The sweat chloride test is widely available and reliable 4.
- It can be used to diagnose CF in patients with a clear clinical picture and in those with milder or limited phenotypes 3.
- The test can differentiate between patients with CF and healthy subjects 4, 5.
- Sweat chloride concentrations can be measured using various methods, including pilocarpine iontophoresis and chloride-selective electrodes 5.
Other Diagnostic Options
While other diagnostic tests, such as alpha-1 antitrypsin test, arterial blood gas analysis, chest x-ray study, and CT scan of the thorax, may provide useful information, they are not as specific or sensitive for diagnosing CF as the sweat chloride test 2, 3.
Key Considerations
- A sweat chloride concentration above 60 mmol/L is indicative of CF 3.
- Intermediate sweat chloride values may require further testing, such as nasal potential difference measurement or intestinal current measurement, to confirm the diagnosis 3.
- The diagnosis of CF should be made by a specialist in a CF reference center, especially in cases with inconclusive diagnostic test results 3.