Which of the following statements about fetal fibronectin (FFN) testing is correct: it is a better predictor of preterm labor than cervical length measured by ultrasound (US), the results are valid for 3 days, it has a high negative predictive value, or it may only be done in tertiary care centers?

Fetal Fibronectin Testing in Preterm Labor Assessment

Fetal fibronectin testing has a high negative predictive value for preterm delivery, which is its most clinically valuable characteristic. 1, 2

Test Characteristics and Clinical Utility

• Negative Predictive Value: The strongest feature of fetal fibronectin (fFN) testing is its excellent negative predictive value:

  • 99.7% for birth within 7 days 2
  • 93% for delivery before 37 weeks 2
  • 97.4% likelihood of delivering more than seven days after testing 3
  • 100% negative predictive value for delivery within 7 days in recent studies 4

• Positive Predictive Value: Much lower than its negative predictive value:

  • Ranges from 43-79% in symptomatic patients 2
  • As low as 10% in some populations 4

• Validity Period: A negative fFN test result is useful for ruling out preterm delivery within two weeks, not just 3 days 1, 5

• Comparison with Cervical Length: The ACOG guideline does not state that fFN is a better predictor than cervical length measured by ultrasound. In fact, both tests may be complementary:

  • Transvaginal cervical ultrasonography is described as "a reliable and reproducible way to assess the length of the cervix" 1
  • The combination of both tests may be more useful than either alone 1

Clinical Application

• Testing Criteria: For high-risk women, fFN testing should meet these criteria:

  • Intact amniotic membranes
  • Minimal cervical dilatation
  • Testing between 24 weeks, 0 days and 34 weeks, 6 days of gestation
  • Results available within 24 hours for timely decision-making 1, 6

• Setting: There is no guideline recommendation that fFN testing may only be done in tertiary care centers. The test has been successfully implemented in community hospitals 7

• Clinical Impact: Knowledge of a negative fFN result can:

  • Help avoid unnecessary interventions 5
  • Shorten hospital stays by 40% in patients observed for >6 hours 7
  • Allow physicians to safely withhold tocolytics and corticosteroids in women with symptoms of preterm labor 4

Important Considerations

• Low-Risk vs. High-Risk Populations: The test has limited usefulness in low-risk women but may be valuable in high-risk populations 1, 6

• False Positives: The test carries a high percentage of false-positive results, which can add significantly to the cost of prenatal care if used in low-risk populations 1

• Blood Contamination: Blood present on the fFN sample does not affect the reliability of the test; the negative predictive value remains high 4

The primary clinical value of fFN testing lies in its ability to rule out imminent preterm delivery when the test is negative, allowing clinicians to avoid unnecessary interventions and hospitalizations while maintaining patient safety.