When drawing blood from a central venous catheter (CVC):

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The first 5 mL of blood is always discarded when drawing blood from a CVC to ensure that the sample being collected is not contaminated with any residual heparin or medications that may be present in the catheter or tubing. A syringe larger than 20 mL or a double- or triple-lumen catheter may or may not be necessary depending on the patient's specific situation, but this is not a universal requirement for drawing blood from a CVC. He-parin may be used to maintain patency of the catheter, but it is not always necessary to inject heparin prior to drawing blood from a CVC.

When Drawing Blood from a Central Venous Catheter (CVC)