The nonstress test: how long is enough?

Longer observation intervals might increase the positive predictive value of a nonreactive nonstress test (NST). One thousand one hundred and one NSTs were performed on 343 fetuses in 333 high-risk hospitalized patients. Recordings were terminated when five fetal heart rate (FHR) accelerations with movement occurred in any 20-minute period or after 120 minutes. The conclusion drawn was that the positive predictive value for morbidity and mortality of a nonreactive NST was 85.7% if conducted over 120 minutes, and that the negative predictive value for a reactive NST was 98.5%. A reactive NST was predictive of good fetal health regardless of the length of observation time necessary to demonstrate reactivity up to 120 minutes.