The test measures chloride levels in sweat. High levels indicate a possible diagnosis of CF.

Chloride is an electrolyte essential for maintaining fluid balance and crucial for regulating salt and water movement in cells. People with cystic fibrosis (CF) have higher chloride levels due to a genetic defect that disrupts this process.

The sweat test is usually recommended for newborns with abnormal screening results or for individuals showing symptoms of CF.

In babies and children, chronic coughing, frequent lung infections, salty-tasting skin, diarrhea, and trouble gaining weight could indicate CF.

In adults, chronic sinus infections, pancreatitis, persistent lung infections, or infertility can sometimes be linked to undiagnosed CF.

During a sweat test, a small area of skin — usually on the forearm or thigh for infants — is stimulated to produce sweat. This is done by applying a colorless chemical called pilocarpine along with a mild electric current, which may cause a slight tingling sensation but shouldn’t be painful.

Once the sweat begins to form, a small coil or pad collects it for testing. The entire process takes about 30 to 60 minutes.

Afterward, the sweat is analyzed in a lab to measure its chloride levels. The results help determine whether someone has cystic fibrosis, with high chloride levels suggesting the condition.

Preparing for a sweat test is relatively straightforward. Before the test, it’s important for you to ensure that you’re well-hydrated, as good hydration helps produce an adequate amount of sweat.

Drink plenty of fluids, especially water, in the days leading up to the test. It’s also advisable to avoid using lotions or creams on the skin where the test will be performed, as these can interfere with the results.

The sweat chloride test can yield three main types of results, each indicating a different likelihood of CF:

Chloride level ≤ 29 millimoles per liter (mmol/L)

  • Interpretation: CF is unlikely.
  • Next steps: No immediate action is typically required, but follow-up with a healthcare professional for routine monitoring is advised.

Chloride level 30–59 mmol/L

  • Interpretation: CF is possible; further testing is needed.
  • Next steps: The sweat test is usually repeated to confirm results. If the results remain in this range, consultation with a CF specialist is recommended, and additional genetic testing may be conducted.

Chloride level ≥ 60 mmol/L

  • Interpretation: CF is likely.
  • Next steps: A CF diagnosis is typically confirmed, and the patient will be referred to a CF care center. Here, a multidisciplinary team will provide a comprehensive care plan, which may include treatment options and monitoring for complications associated with CF.

How accurate is sweat testing for CF?

Sweat testing is a reliable method for diagnosing CF. The Macroduct system is the most widely used, with a sensitivity level of 99%. The sensitivity indicates how well the test can correctly identify individuals with CF, meaning a higher sensitivity means fewer missed cases.

The Nanoduct system is a newer method for measuring sweat conductivity. It’s particularly effective for newborns, as it requires less sweat and has a higher success rate on the first try. However, it has a slightly lower sensitivity of 98%.

The test measures chloride levels in sweat to confirm or rule out a diagnosis of CF. It’s regarded as the gold standard for diagnosing the condition.

Understanding the results — whether typical, intermediate, or high — guides the next steps in diagnosis and care.