Table.

Cervicovaginal fetal fibronectin testing among symptomatic women and number of women needed to be treated (NNT) at 31 weeks' gestation with antenatal steroids to prevent one case of neonatal respiratory distress syndrome (RDS) associated with spontaneous preterm birth within 7-10 days of testing

Test result	Probability of spontaneous preterm birth within 7-10 days of testing (%)	Risk of RDS at 32 weeks' gestation56 57	Rate of RDS* at 32 weeks' gestation (%)	NNT†
No testing	4.5‡	0.53	2.0	109
Test positive	20.6§	0.53	11.0	17
Test negative	1.0§	0.53	0.4	509

^*Calculated as probability of spontaneous preterm birth for positive test result at 32 weeks (31 weeks+7-10 days)=20.6%. Risk of RDS at this gestation=0.53^56;57, therefore, probability of RDS in neonate of woman with positive result=20.6 x 0.53=11% (similar calculation may be carried out for negative result). 

^† For example, rate of RDS at 32 weeks′ gestation=11%, converted to odds of RDS without treatment=11/(100−11)=0.12. Odds of treatment benefit=0.12×0.53=0.064 (where 0.53 is odds ratio for treatment benefit of antenatal steroids, obtained from Cochrane review,⁶ which coincidentally, is the same figure as the risk for RDS at 32 weeks' gestation), converted to rate of RDS after antenatal steroid treatment=0.064/(1+0.064)=0.059. Rate difference of RDS between treatment and without antenatal steroid treatment=0.12−0.059=0.061 and number need to treat is 1/0.061=17. This means that with positive test results, 17 symptomatic women who presented at 31 weeks′ gestation need to be treated with antenatal steroids to prevent one case of RDS (similar calculation may be carried out for negative test result). 

^‡Pretest probability of spontaneous preterm birth within 7-10 days of testing for symptomatic women presenting at 31 weeks′ gestation.^{21;22;46;50;51;58-65} (see webextra table). 

^§Calculation of probabilities with likelihood ratios shown in figure 6: pretest probability (4.5%) converted to pretest odds=4.5/(100−4.5)=0.047; post-test odds for spontaneous preterm birth among women with a positive test=pretest odds×LR+=0.047×5.45= 0.26 (LR+ indicates likelihood ratio for positive result). This is then converted to post-test probability=0.26/(0.26+1)=0.206=20.6% (a similar calculation may be carried out for negative test result using LR- found in figure 6).