Incidence and implications of false-negative sweat test reports in patients with cystic fibrosis.

It is well known that false-positive sweat tests occur commonly, but since a positive test should always be repeated, such an error is likely to be discovered. False-negative results, however, are more difficult to detect and are generally assumed to be much less frequent than false-positive results. Diagnostic delay caused by false-negative reports may lead to substantial morbidity. Review of the medical records of 147 unselected patients with cystic fibrosis followed in our center revealed that 17 (12%) initially had one or more sweat tests that were falsely negative (or were so reported or interpreted). In seven patients, there was diagnostic delay with adverse clinical consequences. Many factors may contribute to the incidence of false-negative sweat tests, and negative results must not be accepted as definitively excluding cystic fibrosis if the clinical syndrome is otherwise suggestive.