In Vitro Hemolysis Testing: Principles, Methods, and Regulatory Requirements

In vitro hemolysis testing is a fundamental biocompatibility study used to determine whether medical devices, biomaterials, and other blood-contacting products cause damage to red blood cells. Performed in accordance with ISO 10993-4, GB/T 16886.4, and ASTM F756, the assay quantifies the hemolysis rate to support regulatory submissions, material screening, and product safety assessment. This article provides a comprehensive overview of the test principle, sample requirements, methodologies, acceptance criteria, and frequently asked questions related to in vitro hemolysis testing.

Technical Overview

Hemolysis refers to the rupture or destruction of red blood cell (erythrocyte) membranes, resulting in the release of hemoglobin into the surrounding plasma or medium. When a medical device or biomaterial contacts blood, chemical leachables, surface roughness, mechanical shear, or residual processing agents may damage erythrocyte membranes and trigger hemolysis, which can lead to anemia, reduced oxygen transport, kidney injury, and other serious clinical consequences.

In vitro hemolysis testing evaluates the hemocompatibility of a test article by incubating it, or an extract of it, with anticoagulated blood under controlled laboratory conditions. Free hemoglobin released from lysed erythrocytes is quantified spectrophotometrically, typically at a wavelength of 540 to 545 nm, and the results are used to calculate the hemolysis rate (also called the hemolysis index or percent hemolysis).

The test is a core component of the biological evaluation of medical devices that contact blood, directly or indirectly, and is specified in international and national standards including:

• ISO 10993-4 – Biological evaluation of medical devices, Part 4: Selection of tests for interactions with blood
• GB/T 16886.4 – Chinese national standard harmonized with ISO 10993-4
• ASTM F756 – Standard practice for assessment of hemolytic properties of materials
• ASTM F1847 – Standard practice for testing polymer materials for hemolytic activity (extract-based approach)

As a general acceptance criterion, a test article with a calculated hemolysis rate below 5% is considered non-hemolytic and acceptable for most regulatory purposes. Under ASTM F756, materials are further classified as non-hemolytic (0–2%), slightly hemolytic (2–5%), or hemolytic (greater than 5%) based on the hemolytic index.

Testing Samples

Blood-Contacting Medical Devices

Finished devices intended for direct or indirect contact with circulating blood are the primary category of test samples. Typical examples include intravascular catheters, infusion sets and transfusion sets, syringes, blood bags, hemodialyzers, blood oxygenators, extracorporeal circulation tubing, vascular grafts, stents, heart valve components, guidewires, and perfusion cannulas.

Biomaterials and Material Specimens

Candidate materials under development can also be evaluated, including polymer films, sheets, tubing sections, molded components, metal coupons, ceramic samples, surface coatings, hydrogels, and nanomaterial-based products. Testing at the material stage allows early screening before device design is finalized.

Sample Preparation Considerations

• Samples should be tested in their final, sterilized, and clinically representative form whenever possible.
• For extract-based (indirect) testing, extraction ratios and solvents are selected according to ISO 10993-12 and GB/T 16886.12, for example 0.1 g/mL to 0.2 g/mL by mass or approximately 6 cm²/mL by surface area.
• Test samples, negative controls, and positive controls are typically prepared in parallel, and all materials must be sterile and free of external contamination.

Testing Items

The core measured and reported parameters of an in vitro hemolysis study include:

• Hemolysis rate (percent hemolysis) of the test article versus controls
• Free hemoglobin concentration in the post-incubation supernatant
• Direct contact hemolysis result, where the sample physically contacts diluted blood
• Indirect contact (extract) hemolysis result, where leachables from the sample are assessed
• Control validation results confirming the suitability of negative and positive control materials

Testing Methods

Direct Contact Method

In the direct contact method, the test article is placed directly into diluted, anticoagulated blood (commonly fresh rabbit blood or human blood). The mixture is incubated at 37 ± 1 °C for a defined period, typically two to four hours with gentle mixing. Mechanical interaction between the sample surface and erythrocytes, as well as surface chemistry effects, contributes to any observed hemolysis. After incubation, the samples are centrifuged and the absorbance of the supernatant is measured spectrophotometrically at 540–545 nm.

Indirect Contact (Extract) Method

In the extract method, the test article is first extracted in a polar solvent such as physiological saline (and, where justified, a non-polar solvent) under standardized conditions, often 37 °C for 24 hours or an accelerated condition chosen to exaggerate clinical exposure. The resulting extract is then incubated with diluted blood at 37 ± 1 °C. This approach specifically evaluates hemolysis caused by soluble chemical leachables rather than surface or mechanical effects.

Controls and Calculation

• Negative control: physiological saline or a recognized non-hemolytic reference material, which should produce minimal hemolysis.
• Positive control: distilled water, saponin, or a detergent-type reagent such as a nonionic surfactant, which induces complete red cell lysis.
• The hemolysis rate is calculated as: Hemolysis Rate (%) = (A_sample − A_negative) / (A_positive − A_negative) × 100, where A represents the absorbance values of the test article, negative control, and positive control, respectively.
• Measurement may be performed by direct absorbance reading or by using Drabkin's reagent, which converts all hemoglobin species to a stable derivative measured at 540 nm, improving accuracy when hemoglobin is present in different oxidation states.

Experiments are normally performed with replicate tubes for each group, and the validity of the study depends on the positive control achieving near-complete lysis and the negative control remaining essentially free of hemolysis.

Applications

• Regulatory submissions for blood-contacting medical devices, including national registration, market authorization dossiers, and CE technical documentation under the EU Medical Device Regulation.
• Biocompatibility evaluation packages prepared according to the ISO 10993 series and GB/T 16886 series, where hemolysis testing addresses the blood-interaction category of ISO 10993-4.
• Research and development screening of new polymers, coatings, and surface treatments intended for vascular or hematological applications.
• Quality control and change verification, such as evaluating the impact of new raw material suppliers, modified manufacturing processes, or alternative sterilization methods.
• Assessment of drug-device combination products, nanomaterials, and drug delivery systems that may interact with erythrocytes.
• Academic and industrial research studies published to support the hemocompatibility claims of novel biomaterials.

Accredited third-party laboratories routinely perform in vitro hemolysis studies as part of comprehensive biocompatibility testing programs, providing test reports that are accepted by major regulatory authorities worldwide.

FAQ

What hemolysis rate is considered acceptable?

A hemolysis rate below 5% is generally regarded as non-hemolytic and acceptable for most blood-contacting devices. Some regulatory contexts apply stricter internal limits, for example requiring values below 5% with negative control hemolysis close to zero and a clear margin of safety.

Which blood source is used for the test?

Fresh anticoagulated rabbit blood is the most commonly used blood source because rabbit erythrocytes are sensitive and reproducible in hemolysis assays. Fresh human blood may also be used, and the choice should be justified and documented in the test protocol.

When should the direct contact method versus the extract method be used?

The direct contact method is appropriate when the device physically contacts blood in clinical use, capturing both surface and mechanical effects. The extract method is preferred when the evaluation focuses on leachable chemicals, or when the sample geometry makes direct contact impractical. Many regulatory submissions include both approaches for a complete assessment.

How does hemolysis testing fit into the overall hemocompatibility evaluation?

ISO 10993-4 identifies five categories of blood-device interaction: thrombosis, coagulation, platelets, hematology, and the complement system. Hemolysis testing addresses the hematology category and is typically performed alongside thrombosis, coagulation, platelet, and complement assays, selected according to the device's blood contact category and contact duration.

Can in vitro hemolysis testing replace in vivo studies?

No. In vitro hemolysis testing is a valuable screening and characterization tool, but it does not replace the full battery of hemocompatibility tests or, where required, in vivo evaluation. The complete testing strategy must be justified based on the intended use, contact duration, and risk analysis of the device.

What factors can cause false hemolysis results?

Poor sample preparation, residual cleaning agents, particle contamination, excessive mechanical agitation, improper centrifugation, and hemolysis of the blood prior to testing can all distort results. Careful control of blood freshness, incubation temperature, handling technique, and validated control performance are essential for reliable data.