Authors' reply: Fetal scalp blood sampling during labour: is it a useful diagnostic test or a historical test that no longer has a place in modern clinical obstetrics?

Intrapartum continuous electronic fetal heart rate monitoring using a cardiotocograph (CTG) was introduced in the 1960s to identify and respond to intrapartum fetal hypoxia promptly. Unfortunately, CTG was found to have a high false-positive rate of 60% or more. In addition, no significant decrease in the rates of cerebral palsy or perinatal deaths was reported over a 30-year period despite an increase in caesarean section rate. Fetal scalp blood sampling (FSBS) was advocated as an ‘additional test’ of fetal wellbeing to reduce the false-positive rate of CTG. The aim was to identify the presence of acidosis (low pH) in a sample of blood taken from the fetal scalp so as to differentiate at least 60% of fetuses who were not hypoxic from 40% of fetuses who were experiencing intrapartum hypoxia when the CTG was classified as ‘pathological’. The aim was to avoid unnecessary operative interventions due to the false-positive rate of CTG. However, FSBS itself has been shown to have a poor positive predictive value for intrapartum hypoxia and recent systematic reviews have reported no evidence of benefit in reducing the operative interventions. Hence, practising clinicians need to critically examine whether FSBS is a useful test or just a historical ‘outdated’ test perpetuated by an obstetric culture, without sufficient scientific and physiological basis to support its use in modern obstetric practice. Contrary to the popular belief in the UK, FSBS did not develop as an additional test of fetal wellbeing to reduce the false-positive rate of CTG. FSBS developed as a test of fetal wellbeing in its own right, used by Saling in Berlin, Germany in 1962, before commercial production of CTG machines in 1968. He took scalp blood samples from babies during labour to detect acidosis and published his series. This test, which was then developed as an alternative to CTG, was subsequently introduced in the UK, when CTG was found to have a high false-positive rate, to reduce unnecessary operative interventions. The recommended ‘normal’ scalp blood pH values were obtained from the original study, which included <80 babies during labour. He recommended that ‘if fetal scalp pH is pre-pathological or even pathological (<7.19), fetal blood sampling should be repeated again and if there is a further fall in pH, then delivery should be instituted’. Hence, there was very limited scientific evidence to support the current National Institute for Health and Care Excellence (NICE) recommendation that stipulates an immediate delivery when the FSBS (scalp pH) value is <7.20. In the UK, an observational study in 1968 erroneously assumed that a ‘normal’ fetal pH during labour should be between 7.30 and 7.36. Based on this assumption, this author postulated that a pH of <7.25 was suggestive of ‘asphyxia’. A closer scrutiny of this study reveals that ‘clinical evidence of fetal distress’ was made based on the presence of ‘meconium’ in the amniotic fluid and the study had only 37 babies on the ‘fetal distress’ arm. A subsequent large study that included over 15 000 babies, however, concluded that ‘normal’ fetal pH during labour is skewed to the left, indicating that babies may have a pH of less than 7.20 at birth, even though there is no ‘asphyxia’. Therefore, current normal and abnormal values for fetal scalp pH that were recommended by the NICE Guideline Development Group on Fetal Monitoring were derived from two small studies that were performed in 1962 and 1968, without sound scientific basis.