False negative or false positive results may be due to inadequate sweat stimulation, incorrect collection and sample handling or inaccurate specimen analysis [8].

A chloride sweat test helps diagnose cystic fibrosis (CF), an inherited disorder that makes kids sick by disrupting the normal function of epithelial cells. These cells make up the sweat glands in the skin and also line passageways inside the lungs, liver, pancreas, and digestive and reproductive systems.

A chloride level greater than 60 confirms a diagnosis. A borderline sweat test result—when the chloride level is between 30 and 60—means your child may have a metabolic syndrome related to an abnormal cystic fibrosis transmembrane conductance (CFTR) gene.

Normal results Characteristically, sweat chloride concentrations less than 40 mmol/L are normal (does not have CF); values between 40 to 60 mmol/L are indeterminate (borderline), and sweat chloride concentrations greater than 60 mmol/L are consistent with the diagnosis of CF.

Symptoms of cystic fibrosis

• recurring chest infections.
• wheezing, coughing, shortness of breath and damage to the airways (bronchiectasis)
• difficulty putting on weight and growing.
• yellowing of the skin and the whites of the eyes (jaundice)
• diarrhoea, constipation, or large, smelly poo.

The sweat test is a reliable test for the diagnosis of CF in approximately 98% of patients with CF. However patients have been reported with characteristic clinical manifestations of CF yet normal or borderline sweat electrolyte values. This is an uncommon problem presenting in only 1 – 2% of cases.

A sweat chloride test result of less than 30 mmol/L in all populations means cystic fibrosis is less likely. A result between 30 to 59 mmol/L does not give a clear diagnosis. Further testing is needed. If the result is 60 mmol/L or greater, cystic fibrosis is present.

A positive sweat chloride test indicates that it is likely that the infant or person tested has cystic fibrosis (CF). Positive sweat chloride tests are typically repeated for verification. The results may also be confirmed by CF gene mutation panel testing.

False-Positive Results and Specificity. When a person is not infected with HIV but receives a positive test result, that result is considered a false positive. Generally, HIV tests have high specificity, meaning that there are few false-positive results and most uninfected individuals are classified as uninfected by the test.

Once the window period has elapsed (two months after possible exposure to HIV), if you test twice, using a different test device on each occasion, and both tests give a negative result, you can be confident that the result is accurate. For more information about HIV test accuracy, read our pages on sensitivity and specificity and on window periods.

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 had positive or equivocal (uncertain) results from a blood spot in the Newborn Screening programme. When is it requested?