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| The Gibson-Cooke Sweat Test Apparatus | ||
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Model IPS-25 Specifications:
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The quantitative pilocarpine iontophoresis. (Gibson-Cooke) sweat test is currently recommended for the diagnosis of cystic fibrosis corroborated by the clinical picture. A genetic disease, cystic fibrosis (CF) almost always leads to an early death. It is present in one out of every 1,500-2,000 Caucasians born alive. About 2,200 children are born with the disease every year. Some die in the neonatal period, some in the first few months of life, and some in early childhood. The average lifespan is 15 years; few of its victims survive to middle age. |
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There is no known cure for Cystic Fibrosis. With early detection, however, a person can hope to slow or prevent the secondary complications, chronic pulmonary disease and pancreatic exocrine deficiencies. Correct diagnosis is essential for proper genetic counseling of the CF patient’s parents. Before the late 1930’s, testing for CF was a matter of clinical examination, with a family history of cystic fibrosis as the single best determiner. Analysis of duodenal fluid came next. Because most CF patients have pancreatic deficiency, analysis of enzymes in the stool and pancreatic fluid and determination of viscosity of the pancreatic fluid became a useful test. The sweat test, devised by di Sant’Agnese, was the result of the observation that in hot weather, the depletion of salt in the body caused some CF patients to go into deep shock. di Sant’Agnese and associates examined the sweat of these patients and found increased sodium and chloride concentrations. The Gibson-Cooke test is a further refinement. While incorporating the iontophoretic procedure as suggested by di Sant’Agnese, this method effectively minimizes the problems associated with spurious increases in ion concentration due to evaporation of the sample and allows for independent determination of sodium and chloride ion concentrations. "Although there are not yet any widely accepted large-scale screening tests, the sweat test is generally regarded as a definitive and effective diagnostic test." At a recent Cystic Fibrosis Foundation GAP conference on the sweat test, it was stated that "Diagnosis of Cystic Fibrosis must include a carefully performed and quantitative pilocarpine iontophoretic sweat test which is interpreted by an experienced clinician. Generally, the test should be done in duplicate, or repeated at least once." Copies of the proceedings of the entire GAP conference are available on request from the Cystic Fibrosis Foundation. By quantitative pilocarpine iontophoretic sweat test, it is meant that sweating is induced by the iontophoresis of pilocarpine; that the sweat produced is collected either upon a gauze square or filter paper, and that its weight is determined by measurement with an analytic balance. We describe a procedure using electrodes covered by 2 in x 2 in gauze squares, a current of 4 milliamperes for 5 minutes, and 2 to 3 ml of 0.4% pilocarpine nitrate. The sweat sample maybe analyzed for sodium, chloride, or both. Sodium values from controls and CF patients overlap slightly more than chloride values, so a chloride analysis may be slightly more preferable. The combined analysis may occasionally prevent errors, but the usual sources of error (contamination and evaporation) generally affect both ions. The sweat glands on a localized area of skin are activated by iontophoretic introduction of pilocarpine. In the process of iontophoresis, an electric potential is established so that pharmacologically active ions carry a current and are thereby introduced into the skin. In this case, a positive electrode moistened with pilocarpine is used. The positively charged pilocarpine radicals move away from the electrodes and into the skin. A negative electrode is applied to the same extremity. After the sweat glands have been stimulated, the electrodes are removed, the skin cleaned, and sweat collected on a weighed piece of gauze place over the exact region where the pilocarpine was iontophoresed and covered with plastic to prevent evaporation. Farrall Instruments produced the first commercial sweat test apparatus in 1958. This model carries the endorsement of the report of the Committee for a Study for Evaluation of Testing Cystic Fibrosis by the National Academy of Sciences. The circuit of the Farrall Instruments IPS-25 Gibson-Cooke Iontophoresis unit contains a transistorized overcurrent and over voltage circuit. The overcurrent protective circuit has several advantages: it makes the iontophoresis current more uniform, it extends battery life, minimizes discomfort, and prevents meter failure if the electrodes are accidentally shorted. The over voltage circuit is a passive breakdown type system which prevents excessive voltage or current in case of circuit failure. Another unique feature is an extremely low leakage current between the electrodes and the metal parts of the apparatus. Following the recommendations of the National Academy of Sciences Evaluation Committee, the IPS-25 is designed to use 16 AA batteries. With moderate use, the IPS-25 will perform about 200 sweat tests on a set of batteries. Each unit is supplied with a step by step manual written by Louis E. Gibson, M.D., which explains how to perform a sweat test and analyze the results. Two methods of analysis are given, by standard flame photometer and for use with a chloridimeter. The IPS-25, built into a travel case for easy portability to bedside in hospital settings, also has a built-in storage compartment for all electrodes and materials needed to conduct a test. References: Gibson, L.E., & Cooke, R.E. A test for concentration of Gibson, L.E., di Sant’Agnese, P.A. , & Shwachman, H. di Sant’Agnese, P.A., Darling, R.C., Perera, G.A, & Shwachman, H. & Antonowicz, I. The sweat test in cystic
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