Accuracy of intrapartum fetal blood gas analysis by scalp sampling A retrospective cohort study

Fetal blood gas analysis (FBGA) using scalp blood is commonly used to identify serious fetal distress. However, there is a lack of data regarding its accuracy and reliability. The aim of this studywas to determine the positive predictive value (PPV) and negative predictive value (NPV) of FBGA for predicting postpartum acidosis in case of nonreassuring fetal heart rate tracings (NRFHRT). To this end, we conducted a retrospective cohort study of singleton term deliveries with NRFHRT according to Federation Internationale de Gynecologie et d'Obstetrique and Fisher cardiotocography scores undergoing FBGA in a university hospital. The PPV and NPV of FBGA regarding neonatal acidosis (defined as a pH value <= 7.15 in arterial or venous umbilical cord blood) and Apgar scores indicating neonatal depression (defined as a 5-min Apgar score <= 5) were evaluated. Multivariate analysis was used to determine the influence of cardiotocography variations and the time delay between FBGAand delivery on the accuracy of FBGA. We analyzed 343 deliveries with NRFHRT. In 32 (9%) of these cases, fetal acidosis was confirmed by a postpartum umbilical cord blood pHvalue <7.15. In 308/343 (90%) cases, FBGA identified NRFHRT as false positive (as confirmed by nonacidotic postpartum pH values) and thus avoided unnecessary interventions such as operative delivery. The overall test accuracy ofFBGAwas 91%. FBGAaccurately predicted postpartum cord bloodpH values with amargin of +/- 0.2 in 319/343 (93%) cases. On the other hand, the false negative rate of FBGA was 8% (29/343). The PPV and NPV of FBGA for predicting postpartum acidosis were 50% and 91%, respectively. The sensitivity was 9% and the specificity was 99%. In a multivariate logistic regression analysis, maternal body mass index (odds ratio [OR] 1.1; 95% confidence interval [CI] 1.01-1.17; P=. 029) and cardiotocography variations (OR0.80; 95% CI 0.66-0.98; P=. 029) independently affected the predictive value of FBGA. ThePPV of FBGA regarding neonatal depression according to Apgar scores was low with only 17%. Weconclude that FBGA may be used in clinical practice to rule out, but not to rule in, neonatal acidosis in parturients with NRFHRT. It can avoid unnecessary interventions such as cesarean section or operative vaginal delivery in up to 90% of cases, but cannot reliably detect fetal acidosis.