Which laboratory result is critical in evaluating a patient for cystic fibrosis?

• A. Tensilon challenge
• B. Sweat chloride level
• C. Acid-fast sputum
• D. Elisa test

Answer: (B)

The sweat chloride level is pivotal in the evaluation of a patient for cystic fibrosis because it helps to confirm the diagnosis of this genetic disorder. Cystic fibrosis is caused by mutations in the CFTR gene, which affects the body's ability to regulate salt and water in epithelial cells. As a result, individuals with cystic fibrosis typically have higher concentrations of chloride in their sweat.

Testing the sweat chloride level involves stimulating sweat production and measuring the chloride concentration in the collected sweat. A sweat chloride level of greater than 60 millimoles per liter is considered indicative of cystic fibrosis and is a critical diagnostic criterion.

Other testing options may be relevant in different clinical scenarios but do not specifically diagnose cystic fibrosis. For instance, the Tensilon challenge is primarily used in diagnosing myasthenia gravis, while an acid-fast sputum test is used to identify tuberculosis. The Elisa test is often used for various immunological assessments and screening for infections but is not specific to cystic fibrosis. Thus, the sweat chloride level remains the key laboratory result in the context of evaluating cystic fibrosis.