A false negative result occurs when a condition is present, but the test designed to detect the condition is negative. When designing new tests, the goal is to keep the rate of false negatives as low as possible without leading to a high rate of false positives (when the test says the condition is present but it isn’t). No test in healthcare is perfect, so there is always a small rate of false positive and negative results. This is true of the CTG as well.
Determining just how often this happens is tricky when we are looking at CTGs. The first challenge is to decide what the condition is that we are testing for. CTGs are meant to detect increased risk for several conditions, linked to low oxygen levels, including stillbirth, neonatal death, brain injury presenting as low Apgar scores, hypoxic-ischaemic encephalopathy, or cerebral palsy. Recent research set out to examine the frequency of different fetal heart rate patterns seen on CTG monitoring during labour for babies who were later diagnosed with cerebral palsy, where the cause of the cerebral palsy was determined to be related to complications arising during labour (Hasegawa, et al., 2022). This research offers insights into the false negative rate of intrapartum CTG monitoring with regards to cerebral palsy arising from intrapartum complications.
The research was conducted in Japan, and examined births between 2009 and 2014 of babies with cerebral palsy. Only singleton infants born at or after 33 weeks of gestation, weighing at least 2000 grams, where a congenital or postnatal cause of cerebral palsy had been excluded, and an interpretable CTG trace was available for review were included. Two groups were identified – those with an umbilical cord complication (cases – 126 babies) and those with other intrapartum causes of cerebral palsy (controls – 594 babies). The most common cord event was umbilical cord prolapse (32 babies). The other cord complications were marginal or velamentous cord insertion, cord entanglement, true knots, cord constriction, hyper- or hypo-coiling of the cord, or the presence of only one umbilical artery in the cord. The CTGs were re-analysed and categorised into one of five categories. One of these categories was the CTG that was normal on admission and remained normal until birth.
Among the 32 cases where cord prolapse occurred, 3% of babies had a persistently normal CTG up to the time of birth. For babies with other cord complications, 9% had a persistently normal CTG trace. Among the controls (cerebral palsy with an intrapartum cause but not a cord complication), 25% of babies had a normal CTG pattern. The normal CTG patterns seen for babies with cerebral palsy are instances of false negative results from CTG monitoring.
These research findings serve as a reminder that even in circumstances where CTG monitoring is in use, the pattern is correctly interpreted, and timely intervention is performed, clinicians are not always able to prevent cerebral palsy from occurring. It may be that some of the babies included as cases and controls in this research had a cause for their cerebral palsy other than hypoxia. It may be that there are gaps in our knowledge of fetal heart rate physiology so we don’t consistently recognise signs of hypoxia on the CTG, and what we think of as normal may not be. Or it may be that no matter how hard we try, there will always be babies experiencing hypoxic damage who generate normal intrapartum CTG patterns.
While the current focus on improving clinician’s knowledge and responses to normal and abnormal fetal heart rate patterns is laudable, it is based in the belief that if we fix the human element of care then the technology will work perfectly. Holding too strongly to this belief means that we may not offer other known options for effectively preventing poor perinatal outcomes and may not fund research designed to identify new tools and treatments. The risk of having a strong faith in CTG monitoring is that we continue to fail in our efforts to improve outcomes.
Hasegawa, J., Nakao, M., Ikeda, T., Toyokawa, S., Jojima, E., Satoh, S., Ichizuka, K., Tamiya, N., Nakai, A., Fujimori, K., Maeda, T., Takeda, S., Suzuki, H., Ueda, S., Iwashita, M., & Ikenoue, T. (2022, Mar 3). Fetal heart rate evolution patterns in cerebral palsy associated with umbilical cord complications: a nationwide study. BMC Pregnancy & Childbirth, 22(1), 177. https://doi.org/10.1186/s12884-022-04508-2