Common test for Cystic Fibrosis, whose symptoms can be very similar to asthma. When a child begins to present asthma symptoms a sweat test is typically done to rule out CF as a cause. The child's forearm or shin is electrically stimulated to produce perspiration, which is then measured for salt (chloride) content using a special digital machine. Too much salt in the sweat indicates CF. A borderline reading will usually result in retesting. It is not painful and usually takes about an hour or less from start to finish. Sometimes two locations are tested (left and right forearms) for quality control. For more information on sweat testing, visit www.cff.org.