Cystic Fibrosis Sweat Testing

Cystic fibrosis (CF) is a debilitating, often life-shortening disease that can affect the whole body, but targets mainly the lungs and pancreas. Mucus builds up and clogs organs in the body. However, thanks to sweat testing in early childhood, CF can be detected and the management of the disease can begin before it can be too late.

According to the Cystic Fibrosis Foundation website, sweat testing has been the “gold standard” for diagnosing CF for more than 50 years. The sweat test measures the amount of chloride in the sweat. People with CF have more chloride in their sweat than those who do not.


There are two parts to the sweat test. In the first part, a colorless, odorless chemical that causes sweating is placed on a small area on the arm or leg. An electrode is then put over that spot. A technician applies a weak electrical current to the area to cause sweating. In the second part of the test, the area is cleaned and sweat is collected on a piece of filter paper, gauze or in a plastic coil. The collected sweat is then sent to a hospital laboratory to measure the chloride in the sweat. The test itself usually takes about an hour.

People with CF will be positive shortly after birth. The sweat test will be done around 2 to 4 weeks of age for babies who had a positive newborn screen. However, a baby has to sweat enough to be able to do the test.


Vicky LeGrys, DrA, a professor in the Division of Clinical Laboratory Science at the University of North Carolina at Chapel Hill School of Medicine, is a resource on the subject of sweat testing. She advises that there are several critical issues that can make the evaluation of a sweat test challenging.

Dr. LeGrys describes what is referred to as either the qualitative or quantitative analysis of the sweat from a test. The difference here is that a qualitative sweat test is a screening for CF. A person with a positive or borderline result needs to have a quantitative sweat test. In this type of sweat test, the amount of sweat is measured and the amount of chloride or sodium is quantified.

A value of less than 40 mmol/L is negative. A range of 40 to 60 mmol/L means that that patient is borderline. However, a reading of 60 mmol/l is consistent with CF.

Margin for Error

Of course, there is always the chance for error in the sweat testing process and Dr. LeGrys cites these potential issues. Problems have been reported with some of the methods used for sweat testing. She pointed out that methods that do not quantitate sweat collected or do not have an established minimum sample volume or weight are subject to false-negative results because an adequate sweat rate cannot be ensured.

There can also be technical errors with sweat testing. Dr. LeGrys advises that there could be technical errors of evaporation and contamination as well as in dilution, instrument calibration and result reporting. These errors occur more often in institutions that perform relatively fewer tests.

Standard Procedures

There are standard procedures that must be followed for proper sweat testing, according to Dr. LeGrys. The National Committee for Clinical Laboratory Standards published document C34-A, entitled Sweat Testing: Sample Collection and Quantitative Analysis-Approved Guideline. It describes the appropriate methods of sweat collection and analysis, quality control and the evaluation and reporting of the test results.

Labs should document competency testing of all personnel performing sweat test collection or analysis. Analytically, said Dr. LeGrys, labs should be running a positive and a negative control with every patient sample, using procedures described in C34-A.

Steve Eichmann is a freelance writer based in Glen Mills, PA.

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