The sweat test is performed to diagnose which condition?

• A. Cystic fibrosis
• B. Lung cancer
• C. Tuberculosis
• D. Asthma

Answer: (A)

Measuring chloride in sweat is used to diagnose cystic fibrosis. In CF, defective CFTR channels prevent the sweat ducts from reabsorbing chloride properly, so sweat contains unusually high levels of chloride. The test stimulates sweating (often with pilocarpine) and collects sweat to measure chloride concentration. A chloride level typically above about 60 mmol/L supports a CF diagnosis, while lower values make CF unlikely (with borderline ranges that can vary by age). This test isn’t used for diagnosing lung cancer, tuberculosis, or asthma, which don’t cause the characteristic elevated sweat chloride.