The Hemolysis Index (HI) is a quality marker used by clinical laboratories to objectively assess the integrity of a blood sample before analysis. This metric ensures that results accurately reflect the patient’s condition, rather than errors introduced during collection or handling. The index quantifies the presence of free hemoglobin, a protein that should be contained entirely within red blood cells, not circulating in the plasma or serum. By providing a standardized measure of sample damage, the HI helps laboratory professionals decide if a sample is suitable for testing or if a repeat collection is necessary.
What Hemolysis Means
Hemolysis is the biological process describing the rupture of red blood cells (RBCs), which releases the cells’ internal contents into the surrounding fluid, known as plasma or serum. When these cells break open, components like hemoglobin, potassium, and various enzymes are spilled out, visibly tinting the sample a pink or red color. This cellular destruction can occur either within the patient’s body (in vivo) or after the blood is collected (in vitro).
The Hemolysis Index is primarily designed to flag in vitro hemolysis, which arises from errors in the pre-analytical phase of testing, such as improper handling or collection technique. While in vivo hemolysis is a genuine medical condition indicative of diseases like hemolytic anemia, in vitro hemolysis is an artifact that compromises the validity of laboratory results. The contamination from the burst cells means the sample fluid no longer accurately represents the patient’s actual blood chemistry.
Measuring Sample Quality
The Hemolysis Index (HI) provides a quantitative, automated method for measuring the concentration of free hemoglobin, moving beyond subjective visual inspection. Modern clinical chemistry analyzers use spectrophotometry for this measurement, shining light through the sample and detecting how much light the free hemoglobin absorbs at specific wavelengths. Because free hemoglobin has a distinct color, this automated process is more reliable and consistent than a technician visually grading the sample.
The HI is typically reported as a numerical value, often corresponding to the concentration of free hemoglobin, such as in milligrams per deciliter (mg/dL) or grams per liter (g/L). Some systems may use a semi-quantitative grading scale, such as H1, H2, or H3, where a higher number indicates a greater degree of red cell destruction. Laboratories establish specific cutoff points, and if the HI exceeds a certain level, the sample is automatically flagged as hemolyzed and may be rejected for certain tests.
How Hemolysis Changes Test Results
An elevated Hemolysis Index is clinically significant because the released intracellular contents interfere with laboratory analysis through two primary mechanisms: chemical and spectral interference. Chemical interference occurs because certain substances are held in much higher concentrations inside red blood cells than they are in the plasma. The release of these components into the serum or plasma artificially elevates the measured result, making it appear as though the patient has a medical abnormality when they do not.
Potassium (K+) is the most common and concerning example, as its concentration is about 20 to 30 times higher inside the red blood cell than in the plasma. A hemolyzed sample can cause a normal potassium level to appear dangerously high, a false result known as pseudohyperkalemia, which could lead to inappropriate medical interventions. Similarly, enzymes like lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) are also highly concentrated in RBCs and will be falsely elevated in a hemolyzed sample.
Spectral interference is the second major problem, where the red color of free hemoglobin physically disrupts the measurement of other analytes. Many laboratory tests use spectrophotometry, which relies on the specific color absorption properties of the target analyte. Because free hemoglobin absorbs light at several of the same wavelengths as other analytes, it can interfere with the reading for substances like bilirubin or total protein, causing results to be falsely high or low. This often leads the laboratory to reject the sample and request a redraw.
Reasons for an Elevated Index
Most instances of a high Hemolysis Index are caused by factors related to blood collection and handling, which occur outside the body. Mechanical trauma during the phlebotomy process is a frequent cause of red cell damage. This includes using a needle that is too small for the vein, drawing blood too forcefully through a syringe, or excessive suction during the draw.
Improper sample handling after collection can also rapidly increase the HI. Vigorously shaking or mixing the blood tube, for example, can cause the fragile red cell membranes to rupture. Issues such as delayed separation of the serum or plasma from the cells, or storage at incorrect temperatures like freezing, can also lead to cell lysis. These preventable factors highlight why strict adherence to standardized procedures for blood collection and transport is necessary to ensure reliable laboratory results.