The fetal fibronectin (fFN) test alone had a sensitivity of 66.7% and a specificity of 97.2% in predicting spontaneous preterm birth (SPTB) in symptomatic twin pregnancies, according to a small pilot study of 40 women in Scientific Reports.
In addition, the test showed a positive predictive value (PPV) of 66.7% and a negative predictive value (NPV) of 97.2% for spontaneous delivery within 7 and 14 days of testing, and SPTB at less than 34 and 37 weeks’ gestation.
“Thus, 66.1% of patients with a positive fetal fibronectin test would deliver within 7 days versus 2.4% if negative testing; starting with a pre-test probability of 7.5%,” wrote the French authors.
In contrast, cervical length did not significantly predict preterm birth.
Also, combining cervical length and the fetal fibronectin test did not increase sufficiently the positive likelihood ratio (24.0 with test alone) or decrease significantly the negative likelihood ratio (0.3 with test alone).
The authors calculated that a sample size of between 32 and 50 patients would be sufficient to detect a difference in sensitivity and specificity for fFN and cervical length less than 25 mm.
All patients in the double-blind study had uncomplicated diamniotic twin pregnancies, with symptoms of preterm labor and intact membranes at 24 to 33 weeks’ + 6 days gestation.
Spontaneous twin pregnancy occurred in 65% of the women and 35% from assisted reproductive treatment (two cases of ovulation induction and 12 cases of in vitro fertilization).
Mean gestational age at delivery was 34.5 weeks, with SPTB occurring at less than 32 weeks in 25% of patients, less than 34 weeks in 32.5% and less than 37 weeks in 67.5%.
Median days from testing to delivery were 28.7.
Overall, 67.5% of women delivered vaginally without a cesarean for the second twin.
The study results reflect the literature in achieving excellent negative predictive value of fFN to predict delivery within 7 days of testing.
For instance, one previous study found in a secondary analysis of a randomized trial of 170HPC that a positive fFN was stronger than a cervical length of 25 mm or less in predicting early preterm birth in asymptomatic twins.
Another study found that presence of fetal fibronectin in maternal vaginal secretions was predictive of delivery within 7 days, with a sensitivity of 71%, a specificity of 64%, a PPV of 26% and a NPV of 93%.
But the same study found that the test was ineffective in predicting PTB within 14 days.
Authors of the current study are even more encouraged by their results with fFN’s ability to classify a twin pregnancy as a very high-risk pregnancy, compared with outcomes in previous studies.
The study also found that fFN was predictive of delivery within 14 days.
However, only three patients had a positive test and all delivered within 2 weeks of inclusion, thus limiting statistical analyses. Therefore, a large cohort would be required to provide more precise prediction of PTB within 14 days.
Despite being a pilot study, all important outcomes were evaluated—both delayed outcomes and gestational age cutoffs—which was not the case in previously published studies, according to the authors.
“In such a high-risk group, studying delayed outcomes seems more appropriate than gestational cutoffs because the main goal of bedside tests is to give quick results and rapid short-term prognosis, rather than long-term prediction,” the authors wrote.
The authors pointed out, though, that their overall results need to be confirmed in a large prospective study of symptomatic twin pregnancies.