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Should cord blood gas analysis be a routine of perinatal care?
Summarized from Ahlberg M, Elvander C, Johansson S et al. A policy of routine umbilical cord blood gas analysis decreased missing samples from high-risk births. Acta Paediatrica 2017; 106: 43-8
The blood gas analyzer has an established role in the delivery suite of obstetric units. Measurement of umbilical cord blood pH and base excess provides objective evidence of the metabolic condition of newborn babies at the moment of birth.
Specifically, it allows identification of those with significant metabolic acidosis (widely defined as cord arterial blood pH < 7.0 and base excess < –12 mmol/L).
This finding is consistent with asphyxia during birth and resulting increased risk of the potentially devastating hypoxic brain pathology, hypoxic-ischemic encephalopathy (HIE). HIE-affected babies require urgent (within 6 hours of birth) hypothermic treatment.
Traditionally, this test (umbilical cord blood gas analysis) has been performed only at defined high-risk deliveries. But some authorities have advocated that it should be a routine of perinatal care, and performed at all births.
This controversy is addressed by a recently published study from Sweden, where a policy of routine, rather than selective cord blood gas analysis has been adopted at many obstetric units.
It is acknowledged that in the high-pressure environment of the delivery suite sampling of cord blood is occasionally not performed in line with local policy recommendations. Pressured staff may simply forget to perform the procedure, or fail for technical reasons to obtain a correct sample (paired venous and arterial cord blood are required).
One of the arguments for a policy of routine testing is that it might reduce the risk of missing cord blood gas testing in babies whom all authorities agree need testing (i.e. those born in high-risk deliveries or those in poor condition at birth). The authors of this study sought to test this argument.
In the Stockholm-Gotland region where the study was conducted there are eight obstetric units, two of which have a policy of selective cord blood gas testing and six, a policy of routine cord blood gas testing at all births.
The authors interrogated the Stockholm-Gotland Obstetric Database and recovered full clinical details (pregnancy and birth) of all 155,235 deliveries (gestational age > 32 weeks) during a 6-year period 2008-2014. This included 132,908 (86 %) at the six units practicing routine cord blood analysis and 22,347(14 %) at the two units having a policy of selective testing.
The database revealed that cord blood gas results were missing in 9.4 % of medical birth records at units operating a routine testing policy and in 79.7 % of medical birth records at units operating a selective testing policy.
Compared with a policy of routine cord blood testing, a policy of selective testing was found to be associated with significantly increased risk of missing cord blood results in high-risk deliveries and cases of birth asphyxia.
So for example, of all 5050 babies born large or small for gestational age (i.e. high-risk delivery) there was no cord blood gas results recorded for 7.5 % of babies born at units with a routine testing policy, compared with 62 % of babies born at units with a selective testing policy; thus a policy of selective testing was associated with a relative risk (RR) of 8.2 for missing cord blood testing among low- and high-birthweight babies.
For the 184 babies born with an eventual diagnosis of HIE, there was no record of cord blood gas results in 10 % of those born in units having a policy of routine testing, this compared with 21.4 % of those born in units with a policy of selective testing; a relative risk (RR) of 2.14.
The same pattern was found for other high-risk factors (e.g. birth by Cesarean section, low Apgar score, fetal blood sampling during labor, etc.).
The authors of this study were able to conclude that an institutional policy of selective cord blood testing is less successful than a policy of routine cord blood testing in asphyxiated newborns and in high-risk deliveries.
This paper provides evidence that one of the arguments in support of routine cord blood testing is valid. In discussion of their study the authors address other issues surrounding the routine versus selective cord blood testing controversy.
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