Why were the results of randomized trials on the clinical utility of fetal fibronectin negative? A systematic review of their study designs

Randomized trials on the clinical utility of fetal fibronectin in women with symptoms of preterm labor have thus far failed to demonstrate benefits. We systematically reviewed the methodology of these trials to assess if these negative results may be related to their study designs. We searched the literature for randomized trials that evaluated fibronectin testing in women with symptoms of preterm labor. Study results were evaluated and five methodological criteria were assessed: (1) randomization of discordant test results, (2) use of a fixed management protocol, (3) description of interventions in relation to the test result, (4) evaluation of a learning curve, and (5) sample size calculation in agreement with the prevalence of the test results. We detected four randomized trials that met inclusion criteria. All trials allocated women to a strategy with or without availability of fibronectin results without using a discordancy design or a fixed management protocol. One study reported the given treatment in relation to the test results. Learning curves were evaluated in one study. Two studies used transport sample size calculations. The negative results of randomized trials on fetal fibronectin may be due to particular choices in their study design.