Blood drawing for medical tests is a common procedure, providing essential information for diagnosis and treatment. Obtaining a high-quality blood sample is paramount for accurate laboratory results. Hemolysis, the rupture of red blood cells, poses a frequent challenge in this process, potentially compromising testing reliability. Understanding how to prevent this issue is important for patient care.
What is Hemolysis and Why It Matters
Hemolysis occurs when red blood cells break open, releasing their internal contents, including hemoglobin, into the surrounding plasma or serum. This changes the sample’s composition, leading to inaccurate laboratory test results. For example, red blood cells contain significantly more potassium than plasma, so their rupture can cause falsely elevated potassium levels, sometimes by as much as 23 times the true value. Such inaccuracies can interfere with diagnoses and treatment decisions.
Beyond potassium, hemolysis can also falsely elevate levels of enzymes like aspartate aminotransferase (AST) and lactate dehydrogenase (LDH), and minerals such as magnesium and phosphorus. Many laboratory tests rely on color measurements, and the reddish tint from free hemoglobin can interfere with these readings. This interference can lead to misinterpretation of results, potentially masking actual low levels of substances or indicating problems that do not exist.
Key Factors Leading to Hemolysis During Collection
Several factors during blood collection can contribute to hemolysis, often related to technique or equipment.
Needle Size and Technique
Using a needle with a bore that is too small, such as a 25-gauge or higher, can force red blood cells through a narrow opening, causing them to rupture due to shearing forces. Very large needles can also contribute to hemolysis by increasing flow turbulence. Difficult venipuncture, characterized by repeated probing or slow blood flow, puts excessive mechanical stress on red blood cells, leading to their breakdown.
Tourniquet Use and Site Selection
Applying a tourniquet for too long, typically exceeding one minute, can cause blood to pool and red blood cells to become fragile. Drawing blood from a site with a hematoma can also lead to hemolysis, as damaged tissue compromises sample integrity.
Antiseptic and Tube Handling
If the alcohol used to clean the venipuncture site has not completely dried before needle insertion, it can enter the sample and chemically lyse red blood cells. Vigorous shaking of blood tubes, rather than gentle mixing, creates mechanical trauma that damages red blood cell membranes.
Best Practices for Preventing Hemolysis
Preventing hemolysis begins with proper equipment and technique.
Needle Selection
For most adult patients, a 21-gauge or 22-gauge needle is suitable, balancing efficient blood flow with minimal cell damage. While smaller gauges like 23-gauge or 25-gauge may be necessary for fragile veins, their use increases hemolysis risk due to increased pressure. Straight needles are preferred over butterfly needles for routine draws due to lower hemolysis rates.
Venipuncture Technique
Optimal venipuncture involves a smooth, single-stick approach to minimize trauma. Insert the needle at an angle of 30 degrees or less, avoiding excessive probing or repositioning. Avoid drawing blood from an area with a hematoma. Allow antiseptic to air dry completely on the venipuncture site before puncturing the skin.
Tourniquet Management
Apply the tourniquet for the shortest duration possible, ideally no longer than one minute. Releasing the tourniquet once blood flow is established can further reduce red blood cell breakdown. If using a syringe, draw the plunger back gently to avoid excessive negative pressure.
Post-Collection Handling
Handle blood tubes correctly after collection. Invert tubes containing additives gently, typically five to eight times, rather than shaking vigorously. Fill tubes to their recommended volume, especially those with anticoagulants. Transport samples promptly and gently to avoid agitation or extreme temperatures.
What to Do When Hemolysis Occurs
If hemolysis is suspected, visually inspect the blood sample. Normal plasma or serum appears clear and straw-yellow. A hemolyzed sample, however, will show a pink to reddish discoloration due to free hemoglobin. The color intensity indicates the degree of hemolysis, ranging from faint pink to deep red.
Once identified as hemolyzed, a sample is unsuitable for most laboratory tests. Hemolyzed samples lead to inaccurate results. The standard protocol is to discard the hemolyzed sample and collect a fresh blood sample. This redraw ensures reliable laboratory results, supporting appropriate medical decisions.