Introduction The majority of women symptomatic of preterm labour (PTL) do not deliver within 1 week of presentation. Thus, interventions administered on the basis of symptoms will over treat a high proportion. Quantitative fetal fibronectin (qfFN) has predictive power to assess risk of spontaneous preterm birth.

Methods A predefined analysis of a prospective observational study of qfFN results obtained from women symptomatic of PTL (23⁺⁰–34⁺⁶ weeks’ gestation). The impact of varying qfFN thresholds to define administration of antenatal corticosteroids (n = 306) was modelled. The outcome measure was spontaneous delivery within 7 days of fFN testing.

Results Without consideration of qfFN concentration, the number needed to treat (NNT) to successfully administer steroids to one woman who delivered within 1 week of presentation was 77 (1540 to prevent one case of respiratory distress syndrome (RDS)), and 3 women would have delivered before receiving a full course. Utilising fFN thresholds of 10, 50, 200 and 500ng/ml to guide steroid administration reduced the NNT to 58, 28, 9 and 5 respectively; only one extra ‘case’ would have been missed at a threshold of ≥500 ng/ml. At ≥200 ng/ml, the NNT to prevent one RDS case reduces to 180.

Conclusion The implementation of qfFN testing into clinical practice would facilitate more appropriate use of antenatal interventions such as steroids, tocolysis, hospital admission and in utero transfer. It enables targeted management of women symptomatic of PTL, reducing the potential harm caused by unnecessary intervention and requirements for repeated steroid dosing, and has cost saving implications for the NHS.