Proposed cutoff for fetal scalp blood lactate in intrapartum fetal surveillance based on neonatal outcomes: a large prospective observational study

Objective: Determination of lactate in fetal scalp blood (FBS) during labour has been recognised since the 1970s. The internationally accepted cutoff of >4.8 mmol/l indicating fetal acidosis is exclusive for the point-of-care device (POC) LactatePro™, which is no longer in production. The aim of this study was to establish a new cutoff for scalp lactate based on neonatal outcomes with the use of the StatstripLactate^®/StatstripXpress^® Lactate system, the only POC designed for hospital use. Design: Observational study. Setting: January 2016 to March 2020 labouring women with indication for FBS were prospectively included from seven Swedish and one Australian delivery unit. Population: Inclusion criteria: singleton pregnancy, vertex presentation, ≥35⁺⁰ weeks of gestation. Method: Based on the optimal correlation between FBS lactate and cord pH/lactate, only cases with ≤25 minutes from FBS to delivery were included in the final calculations. Main outcome measures: Metabolic acidosis in cord blood defined as pH <7.05 plus BD_ecf >10 mmol/l and/or lactate >10 mmol/l. Results: A total of 3334 women were enrolled of whom 799 were delivered within 25 minutes. The areas under the receiver operating characteristics curves (AUC) and corresponding optimal cutoff values were as follows; metabolic acidosis AUC 0.87 (95% CI 0.77–0.97), cutoff 5.7 mmol/l; pH <7.0 AUC 0.83 (95% CI 0.68–0.97), cutoff 4.6 mmol/l; pH <7.05 plus BD_ecf ≥12 mmol/l AUC 0.97 (95% CI 0.92–1), cutoff 5.8 mmol/l; Apgar score <7 at 5 minutes AUC 0.74 (95% CI 0.63–0.86), cutoff 5.2 mmol/l; and pH <7.10 plus composite neonatal outcome AUC 0.76 (95% CI 0.67–0.85), cutoff 4.8 mmol/l. Conclusion: A scalp lactate level <5.2 mmol/l using the StatstripLactate^®/StatstripXpress^® system will safely rule out fetal metabolic acidosis. Tweetable abstract: Scalp blood lactate <5.2 mmol/l using the StatstripLactate^®/StatstripXpress system has an excellent ability to rule out fetal acidosis.