When a blood sample is collected for laboratory testing, the quality of the specimen is essential for generating accurate results. Receiving a notification that a specimen is “hemolyzed” means the sample is often unsuitable for analysis, usually requiring a patient to undergo a redraw. Hemolysis is the most frequent reason for the rejection of blood chemistry specimens. Understanding this physical event involving the blood cells dictates whether the laboratory can confidently report results to a healthcare provider.
What Hemolysis Means in a Lab Sample
Hemolysis describes the rupture or breakdown of red blood cells (RBCs), which releases their internal contents into the surrounding fluid, known as plasma or serum. The prefix “hemo” refers to blood, and the suffix “lysis” means destruction or rupture.
In a normal, non-hemolyzed blood sample that has been centrifuged, the plasma or serum layer—the liquid portion used for most chemistry tests—appears clear and pale yellow. RBCs are packed with hemoglobin, the protein that gives blood its red color. When the RBC membranes break, this concentrated hemoglobin spills out, causing the plasma or serum to take on a pink, red, or brownish tinge. This visible color change is the primary way laboratory personnel initially identify a hemolyzed specimen.
Technical Reasons Why Specimens Hemolyze
In the vast majority of cases, a hemolyzed specimen is an artifact resulting from errors that occur outside the body, known as in vitro hemolysis, during the collection or handling process. Red blood cells are highly sensitive to mechanical stress and rapid environmental changes. This damage often occurs during the pre-analytical phase—the steps from ordering the test to the sample arriving at the analyzer.
Improper technique during the blood draw is a frequent cause of mechanical trauma to the cells. Using a needle that is too small forces the blood through a narrow opening under high pressure, causing the cells to shear and rupture. Drawing blood through a catheter or vigorously forcing blood from a syringe into a collection tube creates turbulence and excessive shear stress that physically destroys the cell membranes.
Improper handling of the sample after collection can also lead to cell breakdown. Vigorous shaking, instead of gentle inversion, to mix the blood with the tube’s additives causes mechanical trauma. Additionally, exposure to extreme temperatures, such as freezing or excessive heat during transport or storage, can damage the cell membranes. A delay in processing the blood after collection can also allow for cellular degradation.
The Impact on Diagnostic Test Accuracy
A hemolyzed sample is problematic because the released intracellular contents and the hemoglobin itself directly interfere with the methods used by laboratory analyzers. This interference occurs through two main mechanisms: spectral interference and the release of highly concentrated intracellular components.
The red color of the released hemoglobin absorbs light at specific wavelengths, which skews the results of colorimetric assays that rely on light absorption to measure analyte concentration. This spectral interference can lead to falsely high or low readings for a wide range of tests, making the data unreliable.
The second problem is the release of substances that are present in much higher concentrations inside the red blood cells than in the plasma. Potassium is a prime example; its concentration inside an RBC is about 23 times greater than in the surrounding plasma. When the cell ruptures, this large amount of potassium floods the plasma, leading to a falsely elevated result that does not reflect the patient’s true circulating potassium level. This false elevation can be particularly misleading for conditions like hyperkalemia.
Other analytes, such as the enzyme lactate dehydrogenase (LDH) and aspartate aminotransferase (AST), are also highly concentrated within red blood cells. Their release causes a spurious increase in measured levels, potentially leading a healthcare provider to suspect organ damage when the underlying issue is only sample artifact. Bilirubin is also sensitive to hemolysis, as the liberated hemoglobin interferes with the chemical reaction used to measure it. Laboratories often reject hemolyzed specimens and request a redraw to ensure diagnostic accuracy.
Specimen Hemolysis Versus Hemolytic Disease
It is important to distinguish between a hemolyzed specimen and a true hemolytic disease within the body. Specimen hemolysis, or in vitro hemolysis, is an artifact of the collection or handling process, occurring outside the patient’s body. This is the cause in the vast majority of cases when a patient is told their sample was hemolyzed and needs to be redrawn. It represents a technical problem, not a medical diagnosis.
In contrast, in vivo hemolysis refers to the premature breakdown of red blood cells occurring inside the patient’s circulation, which is the definition of a medical condition like hemolytic anemia. This is a serious pathological concern that can be caused by various factors, including autoimmune disorders or inherited red blood cell defects. Laboratory markers such as indirect bilirubin and haptoglobin are used to assess for true in vivo hemolysis. Receiving a notification that a specimen was hemolyzed almost always refers to the collection error, and should not be a source of anxiety regarding a sudden medical diagnosis.