Hemolyzed blood refers to a laboratory sample in which red blood cells have ruptured, a process known as hemolysis. When this occurs outside of the body (in vitro hemolysis), it introduces a significant pre-analytical error that compromises test results. This common issue is the leading reason why clinical chemistry specimens are rejected by the laboratory, often requiring a repeat blood draw.
The Mechanism of Hemolysis
Hemolysis is the physical destruction of the red blood cell (RBC) membrane, which releases the cell’s internal contents into the surrounding fluid (plasma or serum). RBCs are small sacs filled with a high concentration of substances, most notably hemoglobin, the protein responsible for transporting oxygen. The release of this intracellular material radically alters the composition of the sample fluid. Since hemoglobin is the pigment that makes blood red, its presence in the serum or plasma gives the sample a distinct pink or red color after centrifugation. This coloration is the primary visual indicator that the sample has been compromised, and the degree of color correlates with the severity of the hemolysis.
Factors Leading to Sample Hemolysis
The primary causes of in vitro hemolysis relate to improper technique during collection and handling.
- Using a needle gauge that is too small, which creates excessive shear stress.
- Drawing blood too forcefully or applying excessive suction with a syringe plunger.
- Vigorously shaking or mixing the blood tube instead of gently inverting it.
- Subjecting the sample to extreme temperatures, such as freezing or excessive heat.
- Prolonged use of a tourniquet before the draw.
- Contaminating the sample with alcohol left on the skin after antiseptic application.
- Forcefully transferring blood from a syringe into a collection tube by pushing the plunger.
Impact on Laboratory Test Accuracy
Hemolysis compromises the accuracy of laboratory results through two distinct forms of interference: chemical and spectrophotometric.
Chemical Interference
Chemical interference occurs when the high concentration of substances inside the red blood cells is released into the plasma. Analytes like potassium, lactate dehydrogenase (LDH), and aspartate aminotransferase (AST) are far more concentrated within the RBC than in the surrounding fluid. The rupture of even a small number of red blood cells can falsely elevate the measured levels of these substances. For instance, potassium concentration inside the red cell is approximately 20 times higher than in the plasma, meaning a hemolyzed sample will report a high potassium level that does not reflect the patient’s true status.
Spectrophotometric Interference
Spectrophotometric interference is caused by the physical presence of free hemoglobin in the sample. Many automated laboratory analyzers measure analytes using colorimetry, shining light through the sample and measuring the color change. Hemoglobin absorbs light strongly at specific wavelengths, particularly around 415 nanometers. This intense light absorption interferes with the optical reading of assays for analytes like bilirubin, creatinine, and total protein. The analyzer interprets the hemoglobin’s color as a false signal, leading to inaccurate results that may be falsely high or low, depending on the test methodology.
Maintaining Sample Integrity
Healthcare professionals follow actions to minimize the risk of hemolysis and maintain sample integrity. These include selecting the appropriate needle size and ensuring the tourniquet is not left on for an extended period. After the draw, sample tubes must be gently inverted to mix the anticoagulant, rather than shaken vigorously. Proper temperature control during transport is also necessary to prevent cellular damage. If a sample is identified as hemolyzed, it is frequently rejected because the results are considered unreliable, necessitating a redraw to ensure accurate diagnostic information.