50% Hemolysis Rate (CH50) Testing: Principles, Methods, and Applications

The 50% hemolysis rate test, commonly known as the CH50 assay, is a fundamental laboratory method for quantifying total complement activity in serum and evaluating the hemolytic properties of blood-contacting products. By determining the serum dilution that lyses exactly 50% of antibody-sensitized sheep erythrocytes, the test delivers high analytical sensitivity and reproducibility for clinical immunology, pharmaceutical quality control, and medical device biocompatibility assessment. This article explains the technical principles, sample requirements, test items, procedures, and typical applications of 50% hemolysis rate testing.

Technical Overview

The 50% hemolysis rate test (CH50 assay) measures the overall functional activity of the complement system in a serum sample. Complement is a cascade of more than 30 plasma proteins that, when activated, generate the membrane attack complex (MAC) capable of lysing antibody-coated cells. In the classical CH50 assay, sheep red blood cells (SRBC) are sensitized with specific anti-sheep erythrocyte antibody (hemolysin). When serial dilutions of test serum are mixed with these sensitized cells and incubated at 37 °C, active complement in the serum lyses the erythrocytes and releases hemoglobin, which is quantified spectrophotometrically at approximately 540–541 nm.

The 50% endpoint is deliberately selected because the relationship between serum concentration and hemolysis follows a sigmoidal dose–response curve described by the von Krogh equation. At the 50% lysis point, the curve reaches its steepest slope, meaning that a small change in complement activity produces the largest measurable change in hemolysis. This is why one CH50 unit is defined as the quantity of complement required to lyse 50% of a standardized population of sensitized erythrocytes under fixed conditions of temperature, time, and cell concentration.

The assay is widely performed in accredited third-party laboratories and clinical immunology facilities. Related frameworks for blood-interaction testing include ISO 10993-4 (biological evaluation of medical devices — selection of tests for interactions with blood), ASTM F756 (material-mediated hemolysis), and GB/T 16886.4, which is the Chinese national adoption of the ISO standard for hemocompatibility assessment.

Testing Samples

Human Serum Samples

Venous blood is collected using plain tubes without anticoagulant, allowed to clot at room temperature for 30–60 minutes, and centrifuged to separate serum. Serum must be separated from the clot within two hours of collection to prevent complement degradation and artifactual consumption. Samples should be frozen at −70 °C or below if not analyzed immediately, because repeated freeze–thaw cycles significantly reduce complement activity. Hemolyzed or lipemic specimens are generally rejected, since endogenous hemoglobin and turbidity interfere with the optical density measurement.

Animal Serum Samples

Serum from laboratory animals such as mice, rats, rabbits, and guinea pigs is frequently tested in preclinical pharmacology and toxicology studies to assess drug-induced complement activation or suppression. Species-specific reference ranges must be established because baseline complement activity varies considerably between species.

Medical Device Extracts and Blood-Contacting Products

For hemocompatibility evaluation under ISO 10993-4, extracts of blood-contacting medical devices, biomaterials, coatings, and drug-delivery systems are tested for their capacity to induce or inhibit complement-mediated hemolysis. Both direct contact and indirect (extraction) approaches may be applied depending on the geometry and intended use of the device.

Testing Items

• Total classical pathway complement activity (CH50, expressed in U/mL)
• Alternative pathway complement activity (AH50) for differential pathway analysis
• Direct and indirect hemolysis rates of materials and device extracts
• Hemolytic potency of biologics, immune sera, and complement-containing products
• Complement activation markers (C3a, C5a, SC5b-9) as supplementary endpoints
• Inhibition or consumption of complement by test substances such as drugs and nanomaterials

Testing Methods

Traditional Tube Dilution Method (Mayer's Method)

A standardized suspension of hemolysin-sensitized sheep erythrocytes is mixed with serial two-fold dilutions of test serum in buffered isotonic medium containing optimal calcium and magnesium ions. After incubation at 37 °C for 30 to 60 minutes, tubes are centrifuged to pellet intact cells. The hemoglobin content of the supernatant is measured at 540–541 nm and compared with a 100% lysis control (cells lysed with water or ammonium chloride) and a 0% lysis control (cells incubated with heat-inactivated serum). The dilution producing 50% hemolysis is interpolated, and the reciprocal of that dilution defines the CH50 value.

Calculation Using the Von Krogh Equation

Because the hemolysis curve is sigmoidal rather than linear, raw optical density data are transformed using the von Krogh equation, in which the ratio of lysed to unlysed cells is plotted against serum concentration on a logarithmic scale. This linearization allows accurate identification of the 50% endpoint and minimizes error from visual estimation. Modern laboratories apply regression analysis to the full dilution series rather than relying on a single tube.

Automated and Alternative Platforms

Hemolysis Rate Testing of Materials

For medical device evaluation, the hemolysis rate is calculated as the percentage of hemoglobin released from human or rabbit blood in direct contact with, or incubated with extracts of, the test material, normalized against positive (water) and negative (saline) controls. A hemolysis rate below 5% is generally considered acceptable for blood-contacting devices, consistent with ASTM F756 and ISO 10993-4 practice.

Applications

• Clinical diagnosis and monitoring of inherited and acquired complement deficiencies
• Follow-up of systemic lupus erythematosus, immune complex diseases, and glomerulonephritis, where CH50 correlates with disease activity
• Detection of complement consumption during infections, sepsis, and transfusion reactions
• Quality control of immunoglobulin products, vaccines, and complement-reagent manufacturing
• Hemocompatibility screening of catheters, stents, dialyzers, oxygenators, and other blood-contacting medical devices
• Safety assessment of nanomedicines, contrast agents, and monoclonal antibodies for complement activation-related pseudoallergy (CARPA)
• Basic immunology research on complement regulation, inhibitors, and pathway deficiencies

FAQ

Why is the 50% hemolysis point used instead of 100% hemolysis?

The dose–response curve of complement-mediated lysis is sigmoidal and nearly flat near 0% and 100% hemolysis, which means large changes in complement activity produce little measurable change in lysis. At the 50% point the curve is steepest, providing maximum sensitivity and the smallest analytical error, which is why CH50 is defined at this endpoint.

What is the difference between CH50 and AH50?

CH50 evaluates the classical pathway, which requires antibody-sensitized erythrocytes and calcium, whereas AH50 evaluates the alternative pathway using unsensitized rabbit erythrocytes in a magnesium-only buffer. Running both assays together helps localize a deficiency to a specific complement component or pathway.

Can plasma be used instead of serum?

Serum is strongly preferred. Anticoagulants such as EDTA and citrate chelate calcium and irreversibly block the classical pathway, producing falsely low results. Heparinized plasma may be acceptable in validated protocols, but serum remains the standard specimen.

How should samples be stored and shipped?

Serum should be separated promptly, aliquoted, frozen at −70 °C, and shipped on dry ice. Complement is heat-labile, and specimens held at room temperature or 4 °C for extended periods, or subjected to repeated freeze–thaw cycles, will show significant loss of activity.

What reference range is considered normal?

Typical adult serum CH50 values fall within approximately 30–75 U/mL, but exact ranges depend on the assay protocol, cell standardization, and laboratory calibration. Each laboratory must establish and periodically verify its own reference interval, and results should always be interpreted alongside clinical findings.