False Negative Sweat Tests with Apparently Adequate Sweat Collections

False negative sweat tests with apparently adequate sweat collections. Sweat tests collected on two or more occasions from normal children and children with cystic fibrosis have been analysed according to the weight of sweat collected. In five cases collections of 50–99 mg of sweat gave false negative results. Although 50–100 mg of sweat is considered adequate by some authorities our results do not support this. The study confirms the view that at least 100 mg of sweat must be collected to give reliable results.