The sweat chloride test is used to test for CF in symptomatic patients. It is also used to help confirm or rule out a diagnosis of CF in patients who have tested positive or indeterminate with other tests. It is not used to monitor CF since elevated levels do not correlate with the severity of the disease or its symptoms.

The sweat chloride test is ordered when a patient has symptoms of CF, and/or has a close relative who has been diagnosed with CF. Symptoms of CF include: noticeably salty sweat; frequent respiratory infections and chronic cough; gastrointestinal symptoms – distal intestinal obstruction (meconium ileus in infants), bulky offensive greasy stools, malnutrition, and male infertility (obstructive azoospermia).

The sweat chloride test may also be ordered to help confirm a diagnosis in patients who have tested positive for CF. So, for example, a doctor might request a sweat chloride for patients based on other tests including a screening sweat test, the immunoreactive trypsin test (IRT), the trypsin test, and the nasal (transepithelial) potential difference (NPD) test.

It may also be ordered in the case of patients who have tested negative with a standard CF gene mutation panel but who are clinically still suspected of having CF. The CF gene mutation panel generally tests for the 25 most common CF gene mutations (of almost 1000 possible mutations). If the panel is negative, then the patient may not have CF or they may have a more rare mutation. Sweat chloride testing can detect CF even if the genetic mutations causing it cannot be identified.

A positive sweat chloride test indicates that there is a good chance that the patient has CF. Positive sweat chloride tests should be repeated for verification and confirmed, wherever possible, by CF gene mutation panel testing.

A few people with CF will have a normal or inconclusive sweat chloride test and will have to be evaluated using other tests, or the diagnosis may be made on clinical grounds alone.

The sweat test may not always be useful in newborns. They may not produce enough sweat for a reliable diagnosis, so the test may have to wait until the baby is several weeks old. The sweat test may not need to be done if the patient has an elevated IRT and a positive CF gene mutation test.

The sweat test will not pick up carriers of cystic fibrosis. It takes two mutated copies of the CFTR gene to cause cystic fibrosis. Those who are only carriers do not have CF and will not have sweat chloride abnormalities.

There are other conditions besides CF that can cause positive sweat tests. These include anorexia nervosa, Addison’s disease, nephrogenic diabetes insipidus, and hypothyroidism. Edema can result in false negatives.

The sweat chloride test should only be performed at specialized centers with experience with this kind of testing. Otherwise, problems in accuracy, including false negatives due to poor collection technique, can arise.