A peripherally inserted central catheter (PICC line) is a thin, long tube inserted into a vein in the arm, with its tip resting in a large vein near the heart. This device provides reliable, long-term intravenous access for administering medications, fluids, and nutrition, avoiding the need for repeated needle sticks. Because a PICC line offers direct access to the central circulation, it is frequently used for drawing blood samples for diagnostic testing. A specific procedure must be followed when collecting blood through this catheter to ensure the sample is accurate and the patient remains safe. This process includes drawing and then discarding a specific volume of blood, a step necessary to prevent contamination of the diagnostic sample.
Why Initial Blood Samples Must Be Discarded
The necessity of discarding an initial volume of blood stems from the risk of sample contamination by the solutions used to maintain the catheter’s function. PICC lines are typically “locked” or flushed with a solution, often sterile saline or heparin, to prevent blood from clotting within the catheter’s lumen while it is not actively being used. This locking solution fills the entire internal volume of the catheter.
If a blood sample is drawn without first clearing this residual fluid, the sample will be significantly diluted or altered by the flush solution. This effect is particularly problematic for coagulation tests, such as the Prothrombin Time (PT) and activated Partial Thromboplastin Time (aPTT), because even trace amounts of a heparin lock can artificially prolong the clotting times, leading to misleading results.
Samples can also be contaminated by intravenous fluids or medications infusing through the PICC line. For instance, dextrose can falsely elevate a blood glucose reading, or certain electrolytes can skew lab values. To obtain a true representation of the patient’s circulating blood, the entire volume of the catheter, including the hub and any attached extension sets, must be completely cleared. The discarded blood replaces the contaminating fluid with undiluted blood from the central vein, ensuring the subsequent sample is reliable and accurate.
Standard Waste Volumes and Guidelines
The primary purpose of the discard volume is to draw a sufficient quantity of blood to completely remove the catheter’s internal volume of flush solution. For most adult PICC lines, the standard volume for this initial discard is between 5 and 10 milliliters (mL). Many institutional guidelines recommend a minimum of 5 mL, recognizing that this volume is typically enough to clear the lumen of the catheter and the needleless connector hub.
The exact volume required is theoretically dependent on the catheter’s internal diameter, length, and the volume of the attached access devices, which together determine the catheter’s “dead space”. To ensure accuracy, most protocols use a conservative, standardized volume that is known to exceed the dead space volume of common PICC lines.
Although some studies have suggested that a smaller discard volume, such as 3 mL, might be sufficient for central lines, which would help reduce blood loss in patients requiring frequent blood draws. However, the common practice of discarding 5 mL to 10 mL is maintained to provide a wider margin of error and maximize the certainty of obtaining an uncontaminated sample. While the repeated loss of small volumes contributes to iatrogenic anemia, the need for diagnostic accuracy outweighs this concern.
Ensuring Sample Quality and Patient Safety
Beyond the initial waste volume, several procedural practices are followed to maintain sample integrity and patient safety. Before accessing the line, any continuous infusions running through the PICC must be temporarily turned off, ideally for one to two minutes, to prevent the infusing fluid from being drawn into the sample. Strict aseptic technique is then employed to clean the catheter hub, reducing the risk of introducing bacteria into the bloodstream.
After the discard volume is withdrawn and the required diagnostic samples are collected, the catheter must be thoroughly flushed to prevent complications. A flush with 10 mL of normal saline is typically performed using a pulsatile technique to clear any remaining blood from the catheter lumen. This final flush is necessary because blood left inside the catheter can clot, leading to an occlusion that renders the line unusable.
If the blood flow is slow or stops during the collection process, the healthcare provider may slightly reposition the patient’s arm to relieve any external pressure on the catheter. For coagulation studies, such as PTT and PT/INR, the blood sample for these tests is specifically drawn immediately after the discard sample, before any other tubes, to minimize the chance of contamination by tube additives. These careful steps are designed to ensure the quality of the diagnostic information while preserving the functionality and safety of the central access device.