What are the false negative rates of a Biophysical Profile (BPP) and a modified Biophysical Profile (BPP)?

False Negative Rates of BPP and Modified BPP

The false negative rate of the biophysical profile (BPP) is approximately 0.7 per 1000, while the modified biophysical profile (mBPP) has a false negative rate of approximately 0.8 per 1000. Both tests demonstrate excellent negative predictive values exceeding 99.9% for predicting fetal demise within one week of testing 1, 2, 3.

Understanding the BPP and Modified BPP

Biophysical Profile (BPP)

• Consists of four ultrasound-based assessments:
  • Fetal breathing movements (≥1 episode lasting 30 seconds within 30 minutes)
  • Fetal body/limb movements (≥3 discrete movements)
  • Fetal tone (≥1 episode of active extension with return to flexion)
  • Amniotic fluid volume (≥1 pocket measuring 2 cm)
• Often includes a non-stress test (NST) as a fifth component
• Each component receives a score of 0 or 2, for a total score out of 8 (or 10 with NST)
• Score interpretation: 8-10 normal, 6 equivocal, ≤4 abnormal 1

Modified BPP (mBPP)

• Combines NST with amniotic fluid assessment
• Simpler and less time-consuming than full BPP
• Considered normal when NST is reactive and amniotic fluid volume is adequate 1, 2

Performance Metrics

False Negative Rates

• BPP: 0.7 per 1000 tested pregnancies 3
• mBPP: 0.8 per 1000 tested pregnancies 2
• Both tests have extremely high negative predictive values (>99.9%) 1, 2

False Positive Rates

• mBPP: Approximately 60% of deliveries prompted by abnormal mBPP showed no evidence of fetal compromise 2
• Iatrogenic prematurity due to false positive results occurred in 1.5% of women tested before 37 weeks 2

Clinical Implications

Strengths

• Both tests are highly reliable when results are normal
• A normal result for either test is strongly reassuring, with stillbirth occurring within one week of a normal test result being extremely uncommon 1
• The mBPP reduces testing time compared to the complete BPP while maintaining similar performance 2, 4

Limitations and Pitfalls

• Neither test can predict acute events such as placental abruption or cord accidents 1
• False positive results can lead to unnecessary interventions and iatrogenic prematurity 1, 2
• Rare cases exist where a reassuring BPP (score 8/8) occurred despite significant fetal compromise, highlighting that no single test is perfect 5
• BPP may show late manifestations of placental disease, becoming abnormal 48-72 hours after ductus venosus Doppler abnormalities in 90% of cases 1

Testing Recommendations

• Testing is typically initiated at 32-34 weeks' gestation in high-risk pregnancies
• Weekly or twice-weekly testing is standard clinical practice, though this frequency is not based on rigorous scientific evidence 1
• For fetuses at highest risk, testing may occur twice weekly or even daily 1
• Testing should be individualized based on indication, gestational age, and likelihood of neonatal survival 1

Conclusion

Both the BPP and mBPP demonstrate excellent performance as tests of fetal well-being with very low false negative rates. The mBPP offers comparable performance to the complete BPP while requiring less time to perform. However, clinicians should be aware that no antepartum test can predict acute events leading to fetal compromise, and false positive results can lead to unnecessary interventions.