[Development of a plastic capillary tube for Saling fetal blood analysis. Evaluation of pH-metry with standard references].

In order to avoid mouth suction and danger of glass breakage during collection and to avoid mixing of the sample with a piece of wire and an external magnet immediately after collection, an unbreakable plastic capillary tube was designed with increased heparin coating and surface and reduced diffusion distance. For assessing agreement between pH measurements of blood samples obtained with the new plastic capillary tube and a commercial glass capillary tube, umbilical blood pH-values of 41 births were analyzed simultaneously by both methods. The mean difference between paired pH-values, the corresponding 95%-confidence interval and the true limits of agreement were calculated. The pH-values obtained with the plastic capillary tube are systematically lower, with a mean pH decrease of 0.009 units, and there is an additional random variation with a standard deviation of 0.009 pH units. The true limits of agreement: [-0.031, 0.013] represent the interval of the worst expected pH difference. The increased heparin availability in the plastic capillary tube may be the reason for the systematic pH deviation of -0.009 units. However this difference does not seem to be clinically relevant and the statistical evaluation shows a sufficiently good agreement between the pH measurements.