ALTHOUGH THE SAFEST and most accurate way to draw blood for lab work is via peripheral veins, you may need to draw blood through a central venous catheter (CVC) if the patient's peripheral veins are no longer accessible. Bear in mind that when you do so, you increase the risk of CVC-related complications, such as occlusion and infection. What's more, lab tests performed on these specimens may be inaccurate because of flushing solutions, medications, infusion solutions, or drawing techniques.
If you must use a CVC to draw blood, follow these guidelines for safety and accuracy.
All CVCs are I.V. devices whose tips rest in the superior or inferior vena cava. Examples include peripherally inserted central catheters, tunneled catheters, nontunneled catheters, and implanted ports. To maintain patency, flush all CVCs before and after access and infusion.
You can draw blood from a CVC using the discard method with direct Vacutainer connection or a syringe or using the push-pull method with a syringe. If you're drawing blood from a multilumen catheter that's infusing drugs or fluid, stop the infusions before the blood draw. Follow standard precautions, use aseptic technique, and comply with needle-stick safety precautions no matter which method you use.
Now let's look closely at how to perform each method for drawing blood from a CVC.
Perform this method with a hub-to-hub connection or through the needleless I.V. system.
- After cleaning the catheter hub, attach a 10-ml syringe containing 5 to 10 ml of 0.9% sodium chloride solution to the central catheter hub.
- Flush the catheter, then attach the Vacutainer sleeve or syringe to the hub.
- Push in the extra Vacutainer tube (or use the syringe) to draw off waste blood (at least three times the catheter's priming volume). A spare red tube works well for this waste blood. Discard it immediately in the appropriate waste container so it doesn't get confused with lab samples.
- Obtain lab samples. If you're using Vacutainer tubes, insert the red one first, then green, blue, and lavender. Use the lavender coagulation tube last to reduce the effect of flushing solutions on the lab value.
- Flush the catheter with 0.9% sodium chloride solution, according to your facility's policy. Apply a sterile end cap to the hub.
If you can't aspirate blood into the Vacutainer, use a syringe to obtain a blood sample.
Perform this method with syringe access. Here's how:
- Flush the catheter.
- Using the same syringe, pull to aspirate 6 ml of blood into the syringe. Keeping the syringe attached to the catheter hub, push to reinfuse the blood into the catheter.
- Repeat the aspiration and reinfusion at least three times.
- Remove and discard the used syringe and attach a new syringe to draw the sample for the lab. Transfer the sample to lab tubes without removing the stopper or releasing the vacuum.
- Use a new syringe to flush the catheter with 10 to 20 ml of 0.9% sodium chloride solution, according to your facility's policy. Inject enough flush solution to clear residual blood from the catheter. Apply a sterile end cap to the hub.
If you need to draw blood from a CVC for culturing, don't flush the catheter first. Instead, draw a sample directly from the hub and inject it into a blood culture vial. Don't discard any blood.
By knowing how to draw blood from a CVC, you can reduce your patient's risk of complications if peripheral blood draws can't be performed.
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Holmes, K.: “Comparison of Push-Pull Versus Discard Methods from Central Venous Catheters for Blood Testing,” Journal of Intravenous Nursing
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