In a pregnant woman presenting with threatened preterm labor, should fetal fibronectin testing be used rather than insulin‑like growth factor (IGF) measurements to assess the risk of imminent delivery?

Fetal Fibronectin Testing Should Be Used Over Insulin-Like Growth Factor Measurements in Threatened Preterm Labor

Use fetal fibronectin (fFN) testing rather than insulin-like growth factor (IGF) measurements to assess risk of imminent delivery in women presenting with threatened preterm labor. The available evidence supports fFN testing as a validated tool with established clinical utility, while IGF measurements lack sufficient data for this indication.

Evidence-Based Rationale

Guideline Recommendations

ACOG guidelines explicitly address fFN testing but make no mention of IGF measurements as a screening tool for preterm birth 1. The guidelines state that:

• Fetal fibronectin testing may be useful in women with symptoms of preterm labor to identify those with negative values and a reduced risk of preterm birth, thereby avoiding unnecessary intervention 1
• The clinical usefulness of fFN rests primarily with its negative predictive value given the lack of proven treatment options to prevent preterm birth 1
• For high-risk symptomatic women, fFN testing should meet specific criteria: intact amniotic membranes, minimal cervical dilatation, testing between 24+0 and 34+6 weeks gestation, with results available within 24 hours 1

Research Evidence on Comparative Performance

When comparing fFN to phosphorylated insulin-like growth factor binding protein-1 (phIGFBP-1), the research shows:

Negative Predictive Value (The Key Clinical Utility):

• Both tests demonstrate excellent NPV for delivery within 7 days: fFN 98-99% vs phIGFBP-1 97-99% 2, 3, 4
• For delivery within 48 hours, both maintain NPV >98% 3, 4
• A negative fFN test has 100% NPV in some studies, meaning no woman with negative fFN delivered within 7 days 5

Sensitivity and Specificity:

• phIGFBP-1 shows higher sensitivity (81.1% vs 19.4%) but similar specificity (97.1% vs 99.4%) for delivery before 37 weeks 2
• For delivery before 34 weeks, phIGFBP-1 demonstrates superior sensitivity while maintaining high NPV 2, 6
• fFN has higher specificity when used independently 4

Clinical Decision Algorithm

When to Use fFN Testing:

1. Patient presents with threatened preterm labor symptoms (contractions, pelvic pressure)
2. Gestational age 24+0 to 34+6 weeks
3. Intact membranes confirmed
4. Cervical dilation ≤3 cm
5. No sexual intercourse within 24 hours (fFN requirement)

Interpreting Results:

• Negative fFN (<50 ng/mL): Delivery within 7-14 days is highly unlikely (NPV 97-100%). Safely withhold tocolytics and corticosteroids 5
• Positive fFN (≥50 ng/mL): Increased risk but low positive predictive value (10-40%). Consider additional assessment with cervical length ultrasound 1, 7

Important Clinical Caveats

Limitations of fFN Testing:

• Low positive predictive value (10-40%) means most women with positive fFN will NOT deliver preterm 7, 4, 5
• Requires speculum examination and has specific collection requirements 4
• Cannot be used after digital cervical exam or recent intercourse (traditional teaching, though some data suggest blood contamination doesn't affect reliability) 5

Why Not IGF/phIGFBP-1 as First-Line:

While phIGFBP-1 shows promise in research settings 2, 6, 3, it:

• Lacks guideline endorsement from major obstetric societies [1-1]
• Has less established clinical protocols and thresholds
• Is not mentioned in ACOG recommendations for preterm labor assessment

Practical Advantages of phIGFBP-1 (When Available):

• Point-of-care testing with rapid results 4
• Can be used after digital exam or intercourse 2
• Does not require speculum examination 7
• More cost-effective in some settings 4

The Bottom Line for Clinical Practice

Use fFN testing as your primary biomarker because it has guideline support, established protocols, and excellent negative predictive value that allows you to confidently avoid unnecessary interventions 1, 5. The primary clinical utility is ruling out imminent delivery rather than predicting who will deliver preterm.

If phIGFBP-1 testing is available at your institution and you cannot perform fFN (due to recent digital exam or intercourse), it serves as a reasonable alternative with comparable negative predictive value 2, 3, 4. However, standard IGF measurements without the phosphorylated variant have no established role in preterm labor assessment.

Consider combining fFN with transvaginal cervical length measurement for enhanced risk stratification, particularly when fFN is positive 1, 8.