Reliability of individual umbilical artery pH measurements.

To examine the reliability of umbilical artery pH measurements, two study designs were employed: (1) contemporaneous measurement of two adjoining segments of umbilical cords at birth (n = 40) and (2) repeat measurements at < 5, 15, 30, 45 and 60 minutes after birth in separate sections of the same cords (n = 40). The cord sections were left at room temperature. Limits of agreement of the contemporaneous pH measurements were from -0.066 to +0.066. Mean umbilical artery blood pH declined only slightly with time. Limits of agreement for delayed measurements were -0.061 to +0.031 at 15 minutes, -0.087 to +0.033 at 30 minutes, -0.090 to +0.046 at 45 minutes and -0.091 to +0.049 at 60 minutes. We therefore conclude that in both contemporaneous and interval sampling the accuracy of umbilical artery pH measurements is subject to biological variation. The 95% confidence interval for an individual umbilical artery pH measurement typically lies between -0.066 and +0.066 of the measured value.