A number of studies have found that umbilical cord arterial pH is the best available indicator of fetal acidosis, which would suggest intrauterine fetal hypoxia. Arterial pH was found to be more accurate than arterial PCO2, PO2, cord venous pH, or Apgar score. Although there is some disagreement regarding cord arterial pH reference range, a pH of 0.15 appears to be the best cutoff point (I obtained the same value in a group of 122 consecutive newborns).

There is also data from several studies of heelstick (or other capillary site) puncture specimens for blood gas PO2, PCO2, or pH measurement versus arterial specimens. The studies generally found adequate correlations in healthy term infants, less correlation in premature newborns, and increasingly poor correlation of all parameters as severity of illness increased, especially in premature newborns. The conclusion was that capillary blood gas results must be interpreted with much caution in severely ill newborns or neonates.