Chapter 13: Diagnostic tests
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Evidence‐based approaches
Methods of taking blood samples from a central venous access device
Sampling from a CVAD has the advantage of using an existing line to obtain the sample, without the need of venepuncture, saving the patient potential pain and inconvenience. However, obtaining blood samples from a CVAD can lead to inaccurate results, if the correct method is not followed to ensure removal of any drug or intravenous fluid prior to sampling (Mendez [141], WHO [260]). A number of methods may be used to withdraw samples and there appears to be no significant difference in laboratory results and no evidence that haemolysis or haemodilution can occur if the technique is undertaken appropriately (Mendez [141], RCN [211], WHO [260]).
Discard method
The discard method is the one most commonly used in the clinical setting (recommended by The Royal Marsden Hospital and demonstrated within Procedure guideline 13.2: Central venous access devices: taking a blood sample for vacuum sampling) where the first 5–10 mL of blood is withdrawn and discarded (Gorski et al. [74], Mendez [141]). This ensures removal of any heparin, drugs or intravenous fluid. This may also result in an excessive blood removal in patients requiring multiple samples, for example for pharmacokinetic tests (Hess and Decker [82], Mendez [141]).
Push–pull (mixing) method
In the push–pull (mixing) method, a syringe is attached to the catheter, the catheter is flushed with 0.9% sodium chloride, then 6 mL is withdrawn and pushed back without removing the syringe; this is repeated at least three times. This removes any residual solution and reduces exposure to blood, and there is no blood wastage (Adlard [3], Mendez [141]). This method supposes that mixing of blood eliminates heparin, drugs or intravenous fluids from the CVAD lumen. However, it may be difficult to obtain enough blood to exchange three or four times.
