What are the categories of a non‑stress test (NST) and how are they defined?

Categories of Non-Stress Test (NST)

Primary NST Categories

The non-stress test is categorized as either reactive (normal) or non-reactive (abnormal), based on the presence or absence of fetal heart rate accelerations in response to fetal movement. 1, 2, 3

Reactive NST (Normal)

• Defined as ≥2 fetal heart rate accelerations of 15 beats per minute above baseline, lasting 15 seconds, occurring within a 20-minute observation period 1
• A reactive NST is highly predictive of fetal well-being with a negative predictive value >99.9%, and stillbirth risk within 1 week is only 0.8 per 1,000 cases (0.08%) 1
• The acceleration criterion can be measured from the beginning of the acceleration until return to baseline (short criterion), which identifies more tests as reactive (89%) compared to requiring the full 15 bpm elevation be maintained for the entire 15 seconds (long criterion, 79% reactive) 4

Non-Reactive NST (Abnormal)

• Defined as failure to meet reactive criteria—fewer than 2 accelerations or accelerations that do not meet the amplitude/duration requirements within the observation period 1, 2
• A non-reactive NST requires immediate further evaluation with either a full biophysical profile or modified biophysical profile to determine if delivery is indicated 1
• Non-reactive tests are associated with significantly increased cesarean section rates for fetal distress and higher perinatal mortality compared to reactive tests 2

Unsatisfactory NST

• Represents a small percentage (0.6%) of tests where technical factors prevent adequate interpretation 2
• Requires repeat testing or alternative assessment methods 2

Modified Criteria for Preterm Gestations

• At gestational ages <32 weeks, a 10-beat acceleration (rather than 15-beat) lasting 10 seconds (rather than 15 seconds) may be used as the reactive criterion 5
• However, after adjustment for gestational age and birth weight, there is no appreciable difference between 10-beat and 15-beat criteria in predicting perinatal outcomes in preterm pregnancies 5

Critical Clinical Context

Baseline Heart Rate Parameters

• Normal baseline fetal heart rate ranges from 110-160 bpm, defined as the mean heart rate rounded to increments of 5 bpm during a 10-minute segment 6
• Tachycardia (>160 bpm) or bradycardia (<110 bpm) sustained for ≥10 minutes represents abnormal baseline findings 6

Important Limitations

• No antenatal test, including NST, can predict stillbirth related to acute events such as placental abruption or cord accidents, which account for the majority of false-negative results 1, 7
• False-negative NSTs (fetal death within 7 days of reactive test) occur in approximately 0.026% of cases, most commonly due to cord accidents, abruptio placentae, or congenital anomalies 7
• Fetal sleep cycles lasting 20-40 minutes can cause temporary non-reactivity without indicating fetal compromise 6
• Maternal medications (analgesics, anesthetics, barbiturates, magnesium sulfate) can decrease fetal heart rate variability and reactivity 6

Management Algorithm Based on NST Category

If Reactive NST

• Continue routine surveillance schedule (weekly or twice-weekly in high-risk pregnancies) 1
• No additional testing required unless other clinical concerns arise 1

If Non-Reactive NST

• Proceed immediately to modified biophysical profile (NST + amniotic fluid assessment) or full biophysical profile 1
• If BPP score is 8-10, continue surveillance with increased frequency (twice weekly) for preterm pregnancies 1
• If BPP score is ≤6, deliver immediately regardless of gestational age 1
• At term gestation, a non-reactive NST is an indication for delivery even with reassuring BPP 8

Common Pitfalls to Avoid

• Do not use non-reactive NST alone for delivery decisions; always perform additional testing 1
• Do not ignore the presence of fetal heart rate decelerations during NST, as they may identify fetuses at risk for cord accidents 7
• Do not assume all non-reactive tests indicate fetal compromise—consider fetal sleep cycles and maternal medications before proceeding to intervention 6