sweat glands. The test site is carefully cleaned and dried, then a piece of preweighed filter paper is placed over the test site and covered with parafilm to prevent evaporation. Specialized collection devices may also be used. Sweat is collected for 30 minutes. The filter paper is retrieved and weighed to determine the weight of sweat collected. Several laboratory methods are then used to determine the sodium and chloride concentrations. Before this method of inducing sweat was developed, the method was to place the entire person to be tested in a hemispherical chamber and slowly raise the humidity and temperature of the air inside. Results Reference ranges For infants up to and including 6 months of age, a chloride level of: Equal to or less than 29 mmol/L = CF is very unlikely 30 – 59 mmol/L = intermediate means that CF is possible Greater than or equal to 60 mmol/L = CF is likely to be diagnosed For people older than 6 months of age, a chloride level of: Equal to or less than 39 mmol/L = CF is very unlikely 40 – 59 mmol/L = intermediate means that CF is possible Greater than or equal to 60 mmol/L = CF is likely to be diagnosed Interpretation Two reliable positive results on two separate days is diagnostic for CF. Because of the existence of milder variants, borderline or even near-borderline negative results may be used to diagnose CF. Clinical presentation, family history and patient age must be considered to interpret the results. Highly discordant sodium and chloride values may indicate technical errors. Sources of error Technical errors, insufficient sample, evaporation, contamination, dehydration, mineralocorticoid hormone therapy, and skin rash on the tested area may produce incorrect results. Positive test results may also be caused by malnutrition, adrenal insufficiency, glycogen storage diseases, hypothyroidism, hypoparathyroidism, nephrogenic diabetes insipidus, G6PD deficiency or ectodermal dysplasia. References External links Cystic Fibrosis Foundation Sweat Test Information Page Skin tests Cystic fibrosis fr:Mucoviscidose#Test de la sueur