How soon is "promptly"? storage and handling of samples for the analysis of sweat chloride

Background: Analysis of sweat chloride concentration remains the most common test for the diagnosis of cystic fibrosis. Proper collection and analysis of chloride in sweat samples collected onto filter paper requires the samples to be weighed promptly after collection to prevent evaporation from affecting the final sweat chloride result. This study was undertaken to determine the effects of storage variables and to define the acceptable delay between the end of collection and the weighing of the sample. Methods: The study was perfomed with deionized water or chloride control material pipetted onto filter paper which was stored under various conditions. Filter paper was weighed immediately after addition of the liquid and then at timed intervals. Results: This study demonstrated that samples maintained properly at room temperature and weighed within 8 hours after collection were not significantly affected by evaporation. Conclusion: Once weighed, the sample could be stored for longer periods as long as the initial weight was used to calculate the final sweat chloride result.