Obtaining blood samples for anti-factor Xa quantification through umbilical artery catheters

Neonates who are treated with low-molecular-weight heparin require repeated venipunctures to monitor anti-factor Xa (anti-FXa) levels. Generally, blood withdrawn from umbilical artery catheters (UACs) is not useful for such monitoring because heparin in the fluids running through the catheters contaminates the results. We tested methods for collecting blood for anti-FXa levels from UACs through which heparin-containing fluids were running, in an attempt to reduce the need for repeated venipunctures of anticoagulated neonates.

A new blood drawing port, through which no heparin was run, was added to the UAC system. Qualifying neonates with UACs were randomized to have anti-FXa levels drawn after line-clearing volumes of 0.5, 3.0, or 4.0 ml.

Twelve patients with UACs were enrolled and all completed the study with no adverse events. When 0.5, 3.0, or 4.0 ml of blood was cleared from the UAC before withdrawing the test sample, heparin contaminated the test (anti-FXa levels > or = 0.1 U/ml) in 66%, 16%, and 8% of samples, respectively. Differences between the 0.5- vs. 3.0-ml line-clearing volumes and between the 0.5- vs. 4.0-ml clearing volumes were significant (p = 0.0026 and 0.0006, respectively).

Blood samples for anti-FXa can be drawn from UACs through which heparin-containing solutions are infusing if a port is added through which no heparin-containing fluids are run, a line-clearing volume of at least 4.0 ml is drawn, and a contamination rate of 8% of samples is acceptable.