OBJECTIVE: To date, a FFN value of >50 ng/ml has been used to define women at high risk for SPTB. We sought to evaluate the relationship between quantitative FFN values including those <50 ng/ml and SPTB at <35 weeks gestation. STUDY DESIGN: Cervical and vaginal specimens for FFN were obtained at 22-24rt weeks gestation (designated as the 24 week visit) in 2923 women as part of the NICHD's Freterm Prediction Study. Quantitative FFN values ranging from 0-2418 rig/ml were calculated using absorhances determined by ELISA. RESULTS: The highest FFN value (cervical or vaginal) for each woman at the 24 week visit was evaluated in relation to SPTB at <35 weeks and is graphically presented below. Examination of an ROC curve confirms 50 ng/ml as the optimal cutoff for a positive test for cervicovaginal FFN at 24 weeks gestation to predict SPTB at <35 weeks. CONCLUSION: As FFN values increase from approximately 20 to 150 ng/ml, the risk of SPTB also increases. FFN values above 150 ng/ml are not associated with a further increase in SPTB. At 24 weeks, 50 ng/ml appears to be the optimal cutoff for a positive test.