A sweat chloride result of 55 mEq/L and a sweat sodium of 52 mEq/L were obtained on a patient with a history of respiratory problems. The best interpretation is:

• A. Normal
• B. Normal sodium and an abnormal chloride test should be repeated
• C. Abnormal results
• D. Borderline results, the test should be repeated

Answer: (D)

Borderline sweat chloride results require confirmation with a repeat test because the value sits between clearly normal and clearly abnormal ranges. A chloride of 55 mEq/L is not enough on its own to diagnose or exclude a condition like cystic fibrosis—the standard interpretation is that values above about 60 mEq/L are strongly positive, while those below 40 mEq/L are normal. When results fall in the 40–60 range, repeating the test (often on a separate occasion) helps ensure the result isn’t due to technical factors or individual variation in sweating. Given the patient’s respiratory history, repeating the test is especially important to clarify the likelihood of CF. The sweat sodium value is less definitive for diagnosis, so the primary detail to consider is the borderline chloride result and the plan to repeat.