Principle of the 50% Hemolysis Rate Test (CH50 Assay): Technical Principles, Methods, and Applicatio

The 50% hemolysis rate test, commonly known as the CH50 assay, is a functional immunoassay used to quantify the total hemolytic complement activity of serum. By measuring the precise amount of complement required to lyse 50% of antibody-sensitized sheep erythrocytes, the test provides a sensitive and reproducible assessment of classical pathway integrity. This article explains the underlying principle, sample requirements, testing methods, and practical applications of the 50% hemolysis assay in clinical and laboratory settings.

Technical Overview

The 50% hemolysis rate test (CH50 assay) measures the total functional activity of the complement system in serum. The assay is based on the ability of complement proteins to lyse antibody-sensitized sheep red blood cells (SRBC) through activation of the classical pathway. When sheep erythrocytes are coated with specific anti-SRBC antibody (hemolysin, also called amboceptor), the resulting antigen-antibody complexes activate the C1 complex and initiate the classical complement cascade. Sequential activation of C4, C2, C3, and C5 through C9 ultimately forms the membrane attack complex (MAC, C5b-9), which inserts into the erythrocyte membrane, causing cell lysis and the release of hemoglobin into the supernatant.

The key principle of the test lies in the dose-response relationship between complement concentration and hemolysis. When the percentage of lysed cells is plotted against the volume of serum (complement), a sigmoid curve is obtained rather than a linear relationship. This curve is steepest at the point of 50% hemolysis, meaning that a small change in complement activity produces the largest detectable change in hemolysis at this point. Consequently, the 50% lysis endpoint offers the highest analytical sensitivity and the smallest measurement error. This relationship is described mathematically by the von Krogh equation: x = K × [y/(1 − y)]^(1/n), where x is the volume of serum, y is the fraction of cells lysed, and n is a constant. When y = 0.5, the term y/(1 − y) equals 1, so x = K; this value defines one CH50 unit. The released hemoglobin is quantified spectrophotometrically at 540–541 nm, and results are reported as complement activity in units per milliliter (U/mL).

Testing Samples

Human Serum Samples

Venous blood is collected using plain tubes and allowed to clot at room temperature before centrifugation. Serum should be free of hemolysis, separated promptly, and tested fresh or stored frozen to preserve labile complement components. Human CH50 testing is widely used in clinical immunology to evaluate suspected complement deficiencies and to monitor disease activity in immune-complex and autoimmune conditions.

Animal Serum Samples

Serum from laboratory animals such as mice, rats, rabbits, and guinea pigs can be tested, provided the assay is adapted with species-appropriate buffers and erythrocyte targets. Animal CH50 testing supports preclinical pharmacology and toxicology studies, particularly for compounds intended to modulate the complement system.

Test Samples for Biocompatibility Evaluation

In the context of blood-contacting medical devices evaluated under frameworks such as ISO 10993-4 and GB/T 16886.4, serum may be incubated with device extracts or materials to assess complement activation or hemolytic effects. These studies complement dedicated hemolysis rate tests and complement activation marker assays performed by accredited third-party laboratories.

Testing Items

• Total classical pathway complement activity (CH50), reported in U/mL
• Alternative pathway hemolytic activity (AH50), using rabbit erythrocytes and magnesium-EGTA buffer to block the classical pathway
• Lectin pathway (MBL pathway) functional activity, where applicable
• Hemolysis rate of medical device materials and extracts, expressed as a percentage of hemoglobin released
• Spectrophotometric quantification of plasma hemoglobin at 540–541 nm (or 530 nm) with appropriate 0% and 100% lysis controls

Testing Methods

Preparation of Sensitized Erythrocytes (EA Cells)

Sheep red blood cells are washed in isotonic buffer and standardized to a defined cell concentration. The cells are then sensitized by incubation with an optimized dilution of rabbit or horse anti-SRBC hemolysin. After washing, the sensitized erythrocytes (EA cells) are resuspended to a working concentration, typically 1–2% for tube methods or microplate-adapted protocols.

CH50 Tube and Microplate Procedures

Serial dilutions of the test serum are prepared in veronal-buffered saline containing calcium and magnesium ions. A constant volume of EA cell suspension is added to each dilution, and the mixtures are incubated at 37°C for 30 to 60 minutes to allow complement-mediated lysis. Following incubation and centrifugation, the hemoglobin content of each supernatant is measured spectrophotometrically. Controls include a 100% hemolysis tube (cells lysed with distilled water or ammonium chloride solution) and a 0% hemolysis tube (cells incubated with buffer only).

Calculation and Data Interpretation

Percentage hemolysis for each dilution is calculated as (OD_sample − OD_blank) / (OD_100% − OD_0%) × 100. The results are transformed using the von Krogh equation, and the log of serum volume is plotted against the log of y/(1 − y). The interpolated value at 50% hemolysis yields the CH50, which is converted to U/mL using the dilution factor. Reference ranges vary between laboratories and method versions, so interpretation should always rely on laboratory-established normal values.

Quality Control

Each run includes quality control sera with known complement activity, reproducibility checks on cell sensitization, and verification of incubation temperature and buffer ionic strength. Calibration may be traceable to recognized reference materials, such as WHO International Standards for complement, and procedures may follow relevant CLSI or national clinical laboratory guidance where applicable.

Applications

• Clinical diagnosis of classical pathway complement component deficiencies (C1 through C9), which may present as recurrent infections or hereditary angioedema
• Monitoring of disease activity in systemic lupus erythematosus, immune-complex glomerulonephritis, cryoglobulinemia, and rheumatologic disorders associated with complement consumption
• Supporting evaluation of infectious diseases, sepsis, and liver disease in which complement activity is altered
• Pharmacological screening and potency testing of complement-inhibiting biologics and small molecules during drug development
• Hemocompatibility assessment programs for blood-contacting medical devices under ISO 10993-4 and GB/T 16886.4, including evaluation of complement-activating surfaces and hemolytic potential of materials
• Quality control and functional characterization of immune globulin products, vaccines, and biological preparations in research and regulated environments

FAQ

Why is the 50% hemolysis endpoint used instead of 100% hemolysis? The complement-hemolysis dose-response curve is sigmoidal and flattens near complete lysis, where large changes in complement produce little additional hemolysis. At 50% lysis the curve is steepest, so the test is most sensitive and precise at this endpoint, minimizing measurement error.

What does a decreased CH50 result indicate? Reduced CH50 suggests either decreased or consumed complement components, as seen in hereditary deficiencies, active immune-complex disease, or significant complement activation. A normal CH50 with clinical suspicion of deficiency may require individual component assays.

How should samples be handled to preserve complement activity? Complement proteins are heat labile. Serum should be separated promptly, kept cold, and tested the same day, or stored at −70°C or below. Repeated freeze-thaw cycles and sample hemolysis must be avoided.

What is the difference between CH50 and AH50? CH50 evaluates classical pathway activity using antibody-sensitized sheep erythrocytes in the presence of calcium and magnesium, whereas AH50 evaluates alternative pathway activity using unsensitized rabbit erythrocytes in magnesium-EGTA buffer. Comparing both results helps localize complement defects to a specific pathway.

Can the CH50 method be adapted for high-throughput testing? Yes. The assay can be performed in 96-well microplates with spectrophotometric plate readers, enabling efficient screening of clinical and preclinical sample sets while maintaining the fundamental 50% endpoint principle.