Is intrapartum fetal blood sampling a gold standard diagnostic tool for fetal distress?

Developed in 1960s, cardiotocography is a screening test and fetal blood sampling (FBS) is an adjunctive, diagnostic technique to detect fetal hypoxia. A fetal blood sample pH value of less than 7.20 has a higher specificity than a pathological CTG to predict low Apgar score at 1 min. Though with a pathological CTG and despite a normal FBS pH value the risk of delivering a hypoxic infant is 30-50%, FBS has assumed considerable importance in purportedly reducing unnecessary obstetric intervention. The evidence for this is weak: the use of FBS with CTG has been shown to reduce operative vaginal deliveries though not Caesarean sections due to fetal distress. There is no difference in the umbilical artery pH at delivery with the use of intermittent FBS with CTG compared to CTG alone. FBS is an invasive procedure: obtaining an adequate blood sample is often difficult and the pH results are affected by handling of the sample, aerobic contamination and processing. Validation of intrapartum FBS requires that the pH and other values obtained are compared to a 'gold standard' technique. Although FBS has been compared to other tests such as scalp lactate, pulse oximetry, fetal ECG waveform analysis, and central haemodynamics in labouring rhesus monkeys, none of these can be considered as 'gold standard'. In the light of the existing evidence, the role of intrapartum FBS as a gold standard diagnostic technique is unproven.