Sweat can be collected onto pads or into Macroduct® coiled capillary tubing. The collecting technologist must be very careful to avoid contaminating the sweat sample or allowing evaporation to occur.
Sweat is collected for no longer than 30 minutes. During this time period, at least 15 µL of sweat in Macroduct® coils (shown on the right) or 75 mg of sweat on 2x2 inch gauze or filter paper must be collected to reflect an appropriate sweat rate needed to yield an accurate diagnosis. This is a physiological requirement rather than a specimen volume requirement for the analyzer. The area of stimulation and collection for the pad method is larger than that of the Macroduct®, hence the difference in volume. The minimum acceptable volume or weight depends on the size of the electrode and stimulation area, the type and size of the collecting devices and the length of time the sweat is collected. If the collection time exceeds 30 minutes, the amount of sweat collected will need to be increased. Extending the collection time beyond 30 minutes does not significantly increase the sweat yield and may provide additional time for evaporation. Samples with sweat volumes of less than 75 mg or 15 µL must not be analyzed and are referred to as “quantity not sufficient” or QNS. Insufficient samples from multiple sites may not be combined to yield an adequate volume.
Sweat collected in Macroduct® coils can be directly analyzed while gauze and filter paper pads must undergo an elution step prior to measurement of chloride. To quantitate the amount of sweat collected, generally the volume collected in the Macroduct® is determined using micropipets and with the pad method, the weight of the filter paper or gauze is measured before and after the addition of the sweat sample.