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Toward implementation of lactate compared with pH in fetal scalp blood sampling during labor: A clinical comparative
Suzanne J van Woudenberg1,2, Ivonne Mijnheer2, Marcel M G J van Borren3
1Department of Obstetrics and Gynaecology, Utrecht, The Netherlands.
Introduction:
Since 60 years, pH measurement in fetal scalp blood samples (FBS) is used to diagnose fetal hypoxia during labor, although the last three decades lactate measurement in FBS has been increasingly used as a safe alternative with a lower chance of failure. However, studies demonstrating the direct relation between lactate and pH in FBS with the currently used measurement systems are lacking. Therefore, clinical acceptance and implementation is impeded. This study investigated the relation between lactate and pH in FBS and derived an appropriate lactate cutoff value.
Material And Methods:
This observational prospective single-center study included 263 FBS in the period of 8 months. After collection of FBS, pH was assessed first (iSTAT®), after which lactate was assessed (StatStrip® Lactate 2). Clinical decisions were based on pH results only. Our primary objective was to assess the correlation between lactate and pH in FBS and to establish a lactate cutoff value for ruling out a low Apgar score at 5 minutes and neonatal acidosis, with corresponding sensitivity and specificity as secondary outcomes. Diagnostic performance of lactate was assessed by using the area under the curve (AUC) of the receiver operation characteristic curve (ROC curve), from which cutoff values were derived.
Results:
In FBS, lactate and pH showed a moderate inverse linear correlation (r = -0.53). The AUC of the ROC curve was 0.88 (0.83-0.93), indicating a good diagnostic performance of lactate to identify an abnormal pH (pH <7.20 in FBS). A cutoff value for lactate of ≥5.6 mmol/L seemed to have the most optimal balance between sensitivity (78.1% [60.0-90.7%]) and specificity (79.2% [73.4-84.3%]) for a pH <7.20 in FBS.
Conclusions:
This study confirmed that the StatStrip® Lactate 2 is a suitable alternative analyzer for measurement of lactate compared with measurement of pH with the iSTAT in FBS. Despite only a moderate correlation between lactate and pH, a lactate cutoff value of ≥5.6 mmol/L could be proposed for clinical decision-making. This knowledge increased confidence of our gynecologists in this relatively new method and paved the way for successful and universal implementation of lactate in FBS, eventually without measuring pH.
