The sweat test, also known as the sweat chloride test or the Gibson-Cooke Quantitative Pilocarpine Iontophoresis Sweat Test, is the gold standard for diagnosis of cystic fibrosis. Although there are known genetic markers for cystic fibrosis, genetic testing is not 100% accurate. People with cystic fibrosis produce higher than normal amounts of chloride in their sweat, and this is what the test measures.
The sweat test is performed on a small patch of skin on the flexor surface of the forearm, or on the thigh in infants. The skin is carefully cleaned and dried. A trained technician tapes a small gauze pad saturated with a pilocarpine solution to the skin and attaches an electrode. The pilocarpine solution induces sweating, and the slight electric current helps the skin absorb the solution. The technician lets the current run for approximately five minutes.
Next, the skin is cleaned and dried again, and another clean gauze pad is taped to the same area of skin. This pad will collect the sweat to be measured for chloride content. After anywhere from 30 minutes to an hour, the pad is removed and weighed to make sure that enough sweat has been collected for the test. If too little sweat is produced, as is often the case with infants, the test must be repeated at a later date. The gauze is then soaked in water to elute the sweat electrolytes, and this solution is analyzed by a chloridometer.
The results of the chloridometer reading are converted to mmol/L using the following equation:
(chloridometer reading) (5/2004) [ (1000y + sweat weight) / sweat weight ] = sweat Cl (mmol/L)
where sweat weight is measured in mg and y=mL of H2O used to elute the sweat electrolytes.
A sweat Cl result of 40 mmol/L or less is considered normal. Results of 40 - 60 mmol/L are borderline, and any reading greater than 60 mmol/L results in a diagnosis of cystic fibrosis. When performed correctly, the test is 100% accurate. People who test positive as infants test positive for their entire lives, and people who test negative will always test negative. Borderline results happen in a very small number of cases; these people must undergo the sweat test again and perhaps other tests as well.
Other tools used to diagnose cystic fibrosis include genetic testing and chest x-rays.
The Cystic Fibrosis Foundation: http://www.cff.org/living_with_cf/sweat_testing.cfm
The National Library of Medicine: http://www.nlm.nih.gov/medlineplus/ency/article/003630.htm
Michael Rock, M.D.: http://www.pediatrics.wisc.edu/childrenshosp/sweat/protocol.html
Respiratory technician at Children's Hospital, Boston, MA