The micro-scale 50% hemolysis rate test, commonly known as the micro-CH50 assay, is a miniaturized laboratory method for quantifying total functional complement activity in serum using antibody-sensitized sheep erythrocytes and 96-well microplate technology. By requiring only microliter volumes of sample and reagents, it enables high-throughput, cost-effective complement assessment in clinical immunology, immunotoxicity studies, and biocompatibility evaluation. This article explains the principle, sample requirements, methodology, calculation, applications, and frequently asked questions of the micro-scale 50% hemolysis rate test.
Technical Overview
The 50% hemolysis rate test (CH50 assay) measures the functional integrity of the complete classical complement pathway in a serum sample. When complement proteins C1 through C9 are sequentially activated, they assemble the membrane attack complex on the surface of antibody-sensitized sheep erythrocytes, causing cell lysis and the release of hemoglobin into the surrounding medium. The amount of hemoglobin released, measured spectrophotometrically at 540–541 nm, is directly proportional to the lytic activity of the complement system.
Because the relationship between serum concentration and erythrocyte lysis follows a sigmoidal dose–response curve, the most reproducible endpoint is the dilution or volume of serum that lyses exactly 50% of a standardized erythrocyte suspension. This endpoint, defined as one CH50 unit, is determined by linearizing the curve using the von Krogh transformation, in which log[y/(1−y)] is plotted against the logarithm of the serum volume or dilution.
The micro-scale version of this test adapts the classical tube-based protocol to 96-well U-bottom microplates, reducing reaction volumes from several milliliters to 25–100 microliters per well. This miniaturization conserves scarce or valuable samples such as mouse serum, dramatically reduces reagent consumption, permits large dilution series on a single plate, and allows automated reading with a standard microplate photometer. The micro method is therefore the preferred format for high-throughput screening, preclinical animal studies, and routine testing in accredited third-party laboratories.
Testing Samples
Human Serum
Clinical serum samples are collected in plain centrifuge tubes or serum-separator tubes, allowed to clot at room temperature for 30–60 minutes, and centrifuged to separate the serum. Samples should be free of visible hemolysis, severe lipemia, or icterus, since endogenous hemoglobin, turbidity, and bilirubin can interfere with the optical reading of the assay.
Animal Serum
Serum from laboratory animals, including mice, rats, guinea pigs, rabbits, dogs, and non-human primates, is frequently tested in preclinical immunotoxicity and pharmacology studies. The micro format is particularly advantageous for rodent studies where only 20–100 microliters of serum may be available. Because baseline complement activity differs markedly among species, results must be interpreted against species-specific reference intervals.
Sample Handling and Stability
Complement is thermally labile and easily degraded by repeated freeze–thaw cycles. Best practice is to separate serum within 1–2 hours of collection, keep it at 2–8°C for short-term holding, and store aliquots at −70°C or below for longer periods. Samples should be shipped on dry ice with temperature monitoring. An aliquot heat-inactivated at 56°C for 30 minutes is routinely prepared as a negative control.
Testing Items
The primary test item reported is the total classical pathway complement activity of the sample, expressed in CH50 units per milliliter (U/mL). Supporting measurements typically generated during the assay include:
- Total complement activity (CH50) of the classical pathway
- Complete hemolysis dose–response curve and the 50% lysis endpoint
- Alternative pathway activity (AH50) using rabbit erythrocytes, where a parallel pathway assessment is required
- Complement activation markers such as C3a, C5a, and SC5b-9, often requested alongside CH50 for medical device hemocompatibility testing
- Interpretation against species- and age-appropriate reference intervals
Testing Methods
Reagents and Materials
- Washed and standardized sheep erythrocyte suspension, typically adjusted to approximately 5 × 10⁸ cells per milliliter
- Rabbit anti-sheep erythrocyte hemolysin (amboceptor) used to sensitize the erythrocytes
- Veronal-buffered saline containing calcium and magnesium ions (GVB²⁺), essential for classical pathway activation
- 96-well U-bottom microplates, multichannel pipettes, a 37°C incubator, a plate centrifuge, and a microplate reader set to 540–541 nm
- Reference serum with an assigned CH50 value for assay standardization
- Distilled water or saponin solution for 100% hemolysis control and buffer for 0% hemolysis control
Assay Procedure
- Prepare sensitized cells (EA cells) by incubating the standardized sheep erythrocyte suspension with an optimized dilution of hemolysin at 37°C, then washing to remove unbound antibody.
- Perform serial two-fold dilutions of each test serum in GVB²⁺ buffer across the microplate, typically in duplicate or triplicate wells.
- Dispense a fixed volume of EA cell suspension into each well, mix gently, and incubate at 37°C for 30–60 minutes to allow complement-mediated lysis.
- Centrifuge the plate to pellet intact erythrocytes, or allow the cells to settle in the U-shaped wells.
- Transfer the supernatant to a flat-bottom plate and measure the absorbance at 540–541 nm.
- Include 100% lysis, 0% lysis, reagent blank, heat-inactivated serum, and reference serum controls on every plate.
Calculation and Data Interpretation
The percentage of hemolysis for each well is calculated as (OD_sample − OD_0%)/(OD_100% − OD_0%) × 100. The von Krogh log–log transformation is then applied, and the line of best fit is used to interpolate the serum dilution or volume corresponding to 50% hemolysis. The CH50 value is reported as the reciprocal of that dilution or in units per milliliter. Markedly reduced CH50 suggests classical pathway consumption or deficiency, whereas elevated values may reflect acute-phase inflammatory responses. Results outside reference intervals should be confirmed by repeat testing and, where appropriate, by individual complement component assays or AH50 testing.
Quality Control and Validation
Each analytical run must satisfy predefined acceptance criteria, including reference serum values within the assigned range, replicate coefficients of variation typically below 10–15%, and a well-defined sigmoidal response curve. Method validation following ISO/IEC 17025 principles covers precision, accuracy, linearity, detection range, and robustness, and the microplate format should be periodically benchmarked against the classical macro tube method. Accredited third-party laboratories performing this test commonly align their procedures with relevant guidance from organizations such as ISO, CLSI, and WHO.
Applications
- Immunotoxicity screening of pharmaceuticals, biologics, excipients, and industrial chemicals as part of integrated immune function panels
- Hemocompatibility and biocompatibility evaluation of blood-contacting medical devices and biomaterials, complementing ISO 10993-4 and GB/T 16886.4 test batteries
- Clinical diagnosis and monitoring of hereditary complement deficiencies, systemic lupus erythematosus, immune-complex diseases, glomerulonephritis, and severe infections
- Quality control of complement-containing biological products and immunoglobulin preparations
- Preclinical animal studies evaluating drug effects, vaccine adjuvants, and immune modulation
- Environmental immunotoxicology research on pollutants and occupational exposures
FAQ
What is the difference between CH50 and AH50?
CH50 evaluates the classical pathway initiated by antibody–antigen complexes on sheep erythrocytes and requires C1 through C9, whereas AH50 evaluates the alternative pathway using unsensitized rabbit erythrocytes. Running both assays in parallel helps localize complement defects to a specific pathway.
Why is serum used instead of plasma?
Common anticoagulants such as EDTA, citrate, and heparin chelate calcium and magnesium ions that are essential for complement activation, which would artificially suppress hemolysis. Serum collected without anticoagulants preserves native complement activity.
How much serum does the micro test require?
Thanks to the microplate format, a complete dilution series typically consumes well under 100 microliters of serum, making the method suitable for repeated sampling in mice and other small animals, as well as for pediatric or limited-volume clinical specimens.
What does a decreased CH50 result indicate?
A low CH50 indicates reduced functional activity of the classical pathway, which may result from inherited deficiencies of individual complement components, active consumption during immune-complex disease such as lupus or glomerulonephritis, or exogenous immunosuppressive effects of a test substance.
How should samples be stored and transported?
Serum should be separated promptly, aliquoted to avoid repeated freeze–thaw cycles, stored at −70°C or below, and shipped on dry ice. Complement activity declines progressively at refrigerator temperatures, so testing within 24–48 hours of thawing is recommended.
Is the microplate method as reliable as the classical tube method?
Yes. When properly validated, the micro-scale assay demonstrates excellent correlation with the traditional tube method, with comparable sensitivity and precision, while offering higher throughput, lower reagent consumption, and better suitability for automation.