Hemolysis is the rupture of red blood cells, which causes the release of their internal contents, including hemoglobin, into the surrounding plasma or serum. This event is a significant concern in laboratory testing because it can substantially interfere with the accuracy of many blood test results, often leading to false readings for substances like potassium and lactate dehydrogenase. Hemolyzed samples are frequently rejected by laboratories, necessitating a redraw and delaying patient care, which underscores the importance of proper collection and handling techniques.
Why Red Blood Cells Break
The breakdown of red blood cells (RBCs) during blood collection, known as in vitro hemolysis, is primarily caused by mechanical, chemical, or thermal stress. Mechanical stress involves physical forces that tear the cell membrane. This often happens when blood is forced through a narrow opening at high speed, creating shear stress, such as through a small-gauge needle or during forceful aspiration with a syringe.
Chemical interference can also compromise the integrity of the red cell membrane. For example, residual alcohol from the skin preparation acts as a solvent, disrupting the lipid bilayer of the cells and causing them to burst. Dilution with IV fluids or an incorrect blood-to-anticoagulant ratio can alter the osmotic balance, leading to cell swelling and rupture. Thermal damage occurs when the blood sample is exposed to temperature extremes, as both excessive heat and freezing can destabilize the RBC membrane structure, resulting in cell lysis.
Proper Preparation Before Puncture
Careful preparation before the needle enters the vein is essential for preventing hemolysis. Selecting the appropriate needle gauge is a primary consideration; a 20- to 22-gauge needle is generally recommended for routine venipuncture to balance flow rate and shear stress. Using a needle that is too small, such as a 25-gauge, significantly increases pressure and turbulence, raising the risk of cell damage.
The application of the tourniquet must also be carefully timed and managed. Prolonged tourniquet use, defined as longer than one minute, causes blood to stagnate and leads to hemoconcentration, which increases the fragility of the red blood cells. If a suitable vein is not found quickly, the tourniquet should be released and reapplied after a brief rest period to restore normal blood flow.
Another element is ensuring that the antiseptic agent, typically isopropyl alcohol, is completely dry before the puncture is performed. If the alcohol is still wet, it can be drawn into the sample, chemically destroying the red blood cells. Allowing a sufficient air-drying time, often 30 seconds, is necessary to prevent this chemical lysis and maximize the effectiveness of skin sterilization.
Minimizing Stress During Venipuncture
The technique used during the blood draw is the most influential factor in minimizing mechanical stress on the red blood cells. The venipuncture should be a single, smooth insertion, and the needle must be positioned correctly within the vein to ensure a steady, non-turbulent flow of blood. Probing or making multiple attempts increases trauma to the vessel wall, which leads to localized clotting and excessive shear force on the cells.
When using an evacuated tube system, the vacuum is designed to draw the blood at a controlled rate. The collector should allow the tube’s vacuum to fill the tube naturally without excessively manipulating the needle position. If a syringe is used, the collector must pull back on the plunger with gentle, consistent pressure. Pulling too quickly or forcefully generates excessive negative pressure that can shear the red cell membranes.
If the blood flow becomes sluggish or stops, it often indicates the needle bevel is pressed against the vein wall or is partially out of the vein. A slight adjustment, such as pulling the needle back slightly, should be attempted, but excessive repositioning or rotation must be avoided. Maintaining a smooth, uninterrupted flow prevents the mechanical agitation that contributes to cell destruction.
Post-Collection Care and Transport
Gentle handling of the sample immediately after collection is essential. Tubes that contain additives, such as anticoagulants or clot activators, must be mixed immediately to prevent clotting or ensure proper activation. This mixing must be performed by gently inverting the tube a specified number of times, typically eight to ten, using a smooth, figure-of-eight motion.
Vigorous shaking or forceful inversion is a common cause of mechanical cell destruction and must be avoided. If blood was collected with a syringe, the needle must be removed before transferring the blood into the collection tubes. The blood should be transferred by gently allowing it to flow down the inside wall of the tube, using the tube’s stopper vacuum if possible, rather than forcing the blood out with the syringe plunger, which creates high shear stress.
The collected samples must also be protected from temperature extremes during storage and transport. Exposure to freezing temperatures or excessive heat can cause red blood cells to lyse. Samples should be transported in a manner that prevents mechanical jarring or shaking, avoiding rough handling or systems like pneumatic tubes that subject samples to violent agitation.