A midline catheter is a specialized intravenous access device, longer than a standard peripheral IV, but not intended for routine blood collection. While it offers a durable solution for administering medications and fluids over an intermediate period, its physical characteristics and location create technical limitations for drawing blood samples. The primary restrictions relate to the integrity of the collected sample and the potential for damage to the device itself. Understanding the design and placement of the midline catheter explains why this access point is generally avoided for phlebotomy.
Defining the Midline Catheter
A midline catheter is classified as a peripheral venous access device, meaning it does not enter the central circulatory system. These catheters are typically inserted into a large vein in the upper arm, such as the basilic, cephalic, or brachial veins, often using ultrasound guidance. Their length ranges from 3 to 10 inches (8 to 25 centimeters), making them significantly longer than a standard peripheral IV. Midlines are designed for intravenous therapy expected to last for a moderate duration, generally between one and four weeks. This access is suitable for medications and fluids well-tolerated by peripheral veins, such as many antibiotics or hydration solutions.
The Critical Difference in Tip Location
The fundamental reason for the blood draw restriction lies in where the catheter’s tip terminates. A midline catheter’s tip ends in the peripheral vasculature, typically located at or below the axilla (armpit region), or distal to the shoulder. This placement keeps the catheter entirely outside the central venous system, unlike a central venous catheter (CVC) or a peripherally inserted central catheter (PICC). Central lines are designed to have their tips reside in the superior vena cava, a large vein near the heart, where blood flow is high. The peripheral location of a midline’s tip means the surrounding vein is narrower and the rate of blood flow is slower than in the central circulation.
Functional Limitations and Sample Integrity
The peripheral location and smaller diameter of the midline catheter create technical hurdles for reliable blood collection. When blood is rapidly withdrawn through the long, narrow lumen, the high negative pressure generated can cause the vein wall to collapse around the tip. This suction often results in the aspiration of surrounding interstitial fluid, which mixes with the blood sample. This mixing leads to hemodilution, which skews laboratory results, particularly for values like hemoglobin, hematocrit, or electrolyte levels, making the data unreliable.
Hemolysis and Device Damage
The high shear stress caused by pulling blood through the small-diameter catheter lumen at high speed can physically damage red blood cells. This cell rupture is known as hemolysis, which releases intracellular components, notably potassium, into the plasma. Hemolysis invalidates many common lab tests, especially potassium, lactate dehydrogenase (LDH), and bilirubin, often requiring a repeat blood draw. The softer catheter material is also susceptible to mechanical damage or kinking when subjected to strong negative pressure, increasing the risk of catheter occlusion and premature device failure.
Risks of Misuse and Alternatives for Blood Collection
Attempting to routinely draw blood from a midline catheter increases the risk of complications, compromising the device’s ability to deliver its intended therapy. Each time the line is accessed for a blood draw, there is an increased opportunity for contamination, elevating the risk of local or systemic infection. Repeated manipulation and high-pressure changes can also damage the inner lining of the vein, increasing the likelihood of thrombosis (clot formation) at the catheter tip. The primary purpose of the midline is to provide reliable venous access for infusion therapy, and activities that shorten its lifespan are discouraged.
Preferred Alternatives
For patients who have a midline catheter but require frequent lab work, the preferred practice is to obtain samples through standard venipuncture (a direct needle stick into a peripheral vein). If the patient has a central venous catheter, such as a PICC line, that device is engineered for safe and reliable blood sampling and should be used instead. Preserving the midline for its intended function—intermediate-term delivery of non-irritating intravenous solutions—is the best way to maintain vascular access and ensure accurate diagnostic testing.