['To determine the cost effectiveness of implementing fetal fibronectin testing in women with threatened preterm labor.', "We developed a cost analysis model based upon our institution's experience with threatened preterm labor. Model estimates related to fetal fibronectin were obtained from the literature. The model considered hospital admission and assay cost.", "Approximately 2000 women deliver annually at our tertiary care facility. In the prior 11 months, 340 (19%) presented for threatened preterm labor with 45 (13%) admissions. In a group of individuals with threatened preterm labor and < 3 cm cervical dilatation, approximately 25% can be expected to have a positive fetal fibronectin test. If fetal fibronectin testing were used to influence the decision of admission, 93 (25%) would have a positive test over a 12-month period, potentially increasing admissions by 94%. At a cost of $225 per test, our institution's antepartum admission cost of $1919, a prevalence of threatened preterm labor of 19% and admission rate of 13%, fetal fibronectin testing applied prior to the decision to admit would result in a total cost of $262 583 with 373 assays performed, and 93 admissions. If only those who would have been admitted based on traditional criteria are considered, > 25% should have a positive fetal fibronectin test. If we assume a positive rate of 70%, fetal fibronectin testing employed after the decision to admit would result in a total cost of $75 963, with 48 assays performed and 34 admissions. Without using the assay, total costs are $92 950 for 48 admissions."] Does cost effectiveness of bedside fetal fibronectin testing vary according to treatment algorithm?