What additional test should be ordered for a pediatric patient with uncontrolled asthma, failure to thrive, and foul-smelling stools to rule out underlying conditions such as cystic fibrosis?

Sweat Chloride Test for Suspected Cystic Fibrosis

Yes, order a sweat chloride test immediately—this patient's clinical triad of uncontrolled asthma, failure to thrive, and foul-smelling stools (steatorrhea) is highly suggestive of cystic fibrosis (CF), and the sweat chloride test is the gold standard diagnostic test for CF. 1, 2

Clinical Reasoning

This presentation represents a classic CF phenotype that requires urgent diagnostic evaluation:

• Failure to thrive is present in 90% of CF patients at diagnosis 2
• Steatorrhea (foul-smelling, greasy stools indicating pancreatic insufficiency) occurs in 69% of CF patients at diagnosis 2
• Recurrent/uncontrolled respiratory symptoms mimicking asthma are found in 71% of CF patients at diagnosis 2

The combination of these three findings should immediately trigger CF evaluation, as CF commonly masquerades as difficult-to-control asthma in children. 2

Diagnostic Test Specifications

Sweat Chloride Test Details

• Diagnostic threshold: Sweat chloride ≥60 mmol/L confirms CF diagnosis 1, 3
• Normal values: <60 mmol/L rules out CF 1
• Test method: Quantitative pilocarpine iontophoresis (Gibson and Cooke technique) is the gold standard 1, 3
• Confirmation requirement: Two documented positive sweat chloride tests are needed for definitive diagnosis 2

Test Performance Characteristics

The sweat chloride test demonstrates excellent diagnostic accuracy:

• Sensitivity: 83.3% when using ≥90 mmol/L cutoff 1
• Specificity: 99.7% 1
• Positive predictive value: 90.9% 1
• Negative predictive value: 99.4% 1

Important Clinical Caveats

Borderline Results Require Caution

• Borderline sweat chloride values (40-60 mmol/L) can occur in other conditions, including severe combined immunodeficiency (SCID) with hypogammaglobulinemia 4
• If sweat chloride is borderline elevated, consider immune deficiency workup alongside CF genetic testing 4
• In adults, sweat chloride >70 mmol/L always discriminates CF from non-CF conditions, whereas sodium values alone may be falsely elevated 3

Additional Diagnostic Considerations

• CF genetic mutation analysis should be performed if sweat chloride is positive or borderline to identify specific CFTR mutations 4, 5
• Sweat conductivity (if available) can serve as an alternative with good agreement to sweat chloride (kappa: 0.934), using cutoff of ≥90 mmol/L for diagnosis 1

Why This Test Takes Priority

CF is a life-threatening condition requiring immediate diagnosis and intervention:

• Early diagnosis dramatically improves outcomes 2
• Delayed diagnosis leads to progressive, irreversible lung damage 5
• CF presents with multi-organ dysfunction requiring mutation-specific therapies that target the basic CFTR defect 5
• The sweat chloride test provides definitive proof of CFTR dysfunction, which is required to establish CF diagnosis 5

Clinical Pitfall to Avoid

Do not dismiss this as simply "uncontrolled asthma" requiring escalation of asthma therapy. 6 The presence of failure to thrive and steatorrhea indicates systemic disease beyond isolated respiratory pathology. In regions without newborn screening, clinical suspicion and awareness are crucial for early CF diagnosis. 2