The hemolysis index is a laboratory measurement that indicates the extent of red blood cell breakdown in a blood sample. It serves as an indicator in blood tests, providing insights into sample integrity and ensuring the accuracy of diagnostic results. Laboratories rely on this index to assess sample quality before analysis.
Understanding Hemolysis Index
Hemolysis refers to the rupture of red blood cells, or erythrocytes, releasing their internal contents, particularly hemoglobin, into the surrounding plasma or serum. The hemolysis index is a quantitative measure that assesses the amount of this free hemoglobin. When red blood cells break down, the normally pale yellow plasma or serum can take on a pink to red color due to hemoglobin.
Accurate blood test results depend on intact red blood cells because many substances are present in different concentrations inside and outside these cells. For instance, potassium levels are significantly higher inside red blood cells than in the plasma. If red blood cells lyse, these intracellular components are released, artificially altering measured concentrations and potentially leading to misleading diagnostic findings.
How Hemolysis Indices Are Determined
Modern laboratory analyzers typically determine hemolysis indices as part of a broader assessment known as HIL indices, which also include icterus (jaundice) and lipemia (fats in the blood). These automated systems utilize spectrophotometry, a method that measures how light is absorbed by a substance, to detect and quantify the presence of free hemoglobin. Hemoglobin absorbs light at specific wavelengths, such as 414 nm, 570 nm, and 600 nm, allowing the analyzer to measure its concentration.
The analyzer passes light through the blood sample and measures the amount of light absorbed at these characteristic wavelengths. The degree of light absorption is directly proportional to the concentration of free hemoglobin, allowing the system to calculate the hemolysis index. This index is often reported as a numerical value, sometimes on a scale like 0-1000, or with “plus” signs (e.g., + to ++++), indicating the level of hemolysis in the sample.
Factors Causing Hemolysis
Hemolysis in a blood sample most commonly results from pre-analytical factors, which are issues occurring before the sample reaches the laboratory. Improper blood collection techniques are a frequent cause, such as using an incorrect needle size, drawing blood too forcefully, or vigorous mixing of the sample after collection. Drawing blood through an intravenous (IV) line or using a smaller gauge needle can also increase the likelihood of hemolysis.
Other pre-analytical factors include delayed processing of the blood sample, allowing red blood cells to break down over time, or exposure to extreme temperatures during transport or storage. Residual alcohol at the venipuncture site or excessive “milking” of the puncture site during collection can also damage red blood cells. Less commonly, certain patient-related factors, such as specific medical conditions like hemolytic anemia, can cause hemolysis within the body (in vivo hemolysis) before the blood is drawn, though this is a rarer cause for an elevated hemolysis index.
Clinical Significance and Interpretation
Elevated hemolysis indices are important in diagnostic testing because they can significantly interfere with the accuracy of various laboratory results. When red blood cells lyse, they release intracellular components into the serum or plasma, which can falsely elevate or decrease measured analyte concentrations. For example, analytes like potassium, lactate dehydrogenase (LDH), and aspartate aminotransferase (AST) are present in much higher concentrations inside red blood cells, so even mild hemolysis can lead to spuriously high results. A 1% lysis of red blood cells can cause a 55% increase in potassium and a 272% increase in LDH.
Conversely, some analytes, such as glucose and sodium, are present in lower concentrations within red blood cells, and their measured levels may falsely decrease. Due to these potential interferences, laboratories often flag or reject hemolyzed samples, as visual inspection alone is unreliable for detecting subtle hemolysis. This rejection can lead to delayed diagnosis and treatment, requiring repeat blood draws and additional costs. While most hemolysis is due to pre-analytical errors, a very high hemolysis index might rarely indicate an in-vivo hemolytic condition, prompting further clinical investigation.