Drawing a blood sample (venipuncture) is a routine medical procedure, but it presents a challenge when a patient is already receiving fluids through an intravenous (IV) line. The primary goal is to obtain a sample that accurately reflects the body’s internal chemistry, which can be compromised by the presence of an IV catheter. Current standards generally advise against drawing blood in the same limb as an infusing IV, particularly at a site above the IV insertion point, known as the proximal location. However, when a patient has limited access points, such as with difficult veins, drawing blood proximal to a peripheral IV site is sometimes the only viable option. This practice, while discouraged, requires stringent procedural steps to mitigate the risks to sample integrity.
The Primary Concern: Sample Dilution and Contamination
The fundamental issue with drawing blood near an IV is the risk of sample dilution, known as hemodilution, which leads to inaccurate laboratory results. When IV fluids are actively infusing, they mix with the patient’s venous blood, contaminating the vein segment proximal to the insertion site. This contamination occurs even if the needle is inserted a distance away from the IV, as the infused solution disperses within the venous system.
The type of IV solution directly dictates the distortion in laboratory values. For example, if the patient is receiving 0.9% sodium chloride (normal saline), the sample will be artificially diluted. This results in falsely low concentrations of cellular components and analytes, such as red blood cell count, hemoglobin, and potassium levels. Conversely, if the IV contains a dextrose solution, the glucose level in the collected blood will be artificially elevated, potentially leading to misdiagnosis. This systemic error means collection from a proximal site while an IV is infusing is considered unreliable, as the results would not represent the patient’s true physiological status.
Standard Protocol Alternatives
To ensure the highest degree of diagnostic accuracy, healthcare professionals prioritize alternative collection sites that eliminate the risk of IV fluid contamination. The safest and most preferred method is to use the patient’s contralateral limb—the arm that does not have an IV line present. Drawing from the opposite arm ensures the collected blood is completely free from the influence of any infusing solutions, providing the most reliable baseline for testing.
If the opposite arm is inaccessible, the next standard practice involves drawing blood from the IV limb at a site located below, or distal, to the IV insertion point. The tourniquet must be applied between the intended venipuncture site and the IV catheter to help isolate the collection area. Even with this technique, a small risk of contamination remains. Therefore, the IV infusion should ideally be temporarily discontinued for a period of at least two minutes before the blood draw begins. This waiting period allows local circulation to clear the infused fluid from the vein segment.
Necessary Steps When Drawing Proximal to an IV
When all preferred alternatives have been exhausted, and a proximal blood draw is unavoidable, a highly specific protocol must be followed to mitigate contamination risk. The first and most important step is to immediately stop the IV infusion pump for a minimum period of two to five minutes prior to the venipuncture. This pause allows the local concentration of the infusing solution to dissipate and for the natural blood flow to partially clear the contaminated venous segment.
After the necessary pause, the venipuncture should be performed in a new site a significant distance—at least 6 inches—above the IV insertion point. This is done to access a less affected area of the vein. When the needle is successfully placed, a crucial step involves discarding a volume of initial blood, often referred to as a “waste tube,” before collecting the laboratory specimens.
This waste volume, which typically ranges from 2 to 5 milliliters, is intended to clear the needle, tubing, and the most concentrated portion of the diluted blood from the proximal vein segment. Only after the waste volume has been successfully collected and discarded should the tubes intended for laboratory analysis be filled, following the correct order of draw.
Finally, the entire procedure must be meticulously documented in the patient’s medical record. This documentation includes the specific site of the draw, the type of IV solution that was infusing, the duration the IV was stopped, and the volume of blood that was discarded. This detailed documentation is a safeguard, alerting laboratory staff and clinicians to the potential for residual contamination, especially for analytes like glucose, which may remain slightly elevated even after following the protocol.