Dynamic Hemolysis Testing: A Technical Guide to Hemocompatibility Evaluation of Blood-Contacting Med

Dynamic hemolysis testing is a critical in vitro biocompatibility test used to evaluate the potential of medical devices and biomaterials to damage red blood cells under conditions that simulate physiological blood flow. Conducted in accordance with internationally recognized standards such as ISO 10993-4, ASTM F756, and GB/T 16886.4, this test is essential for regulatory submission and safety assessment of any device that contacts circulating blood. This article provides a comprehensive technical overview of dynamic hemolysis test principles, sample types, test methods, acceptance criteria, and typical applications in medical device evaluation.

Technical Overview

Hemolysis refers to the rupture of red blood cells (erythrocytes) and the subsequent release of hemoglobin into the surrounding plasma or test medium. When a blood-contacting medical device causes excessive mechanical damage to erythrocytes or leaches substances that disrupt the red cell membrane, clinically significant hemolysis can occur, leading to anemia, hemoglobinuria, renal injury, and other serious adverse effects. Evaluating the hemolytic properties of a device is therefore a fundamental component of hemocompatibility assessment required by ISO 10993-4 and GB/T 16886.4 for medical devices with circulating blood contact.

The dynamic hemolysis test differs from the static (direct contact) hemolysis assay in that the blood–sample interface is subjected to controlled mechanical agitation or circulation during incubation. This dynamic condition more closely simulates the shear stress, turbulence, and continuous blood flow experienced by devices such as catheters, cannulae, blood pumps, and extracorporeal circulation circuits in clinical use. During the test, anticoagulated whole blood is exposed to the test sample while being rotated end-over-end or circulated through the device at a defined flow rate and temperature. After the exposure period, the blood is centrifuged and the concentration of free hemoglobin in the plasma supernatant is quantified, typically by spectrophotometry. The hemolysis rate is then calculated by comparing the sample result against negative and positive controls.

Key quantitative outputs of the dynamic hemolysis test include the hemolysis rate (hemolysis index, expressed as a percentage), free plasma hemoglobin concentration, and—for blood pumps and extracorporeal devices—the Normalized Index of Hemolysis (NIH) or Modified Index of Hemolysis (MIH). A hemolysis rate of less than 5% is generally regarded as the acceptance threshold indicating non-hemolytic behavior, as specified in ASTM F756 and widely applied in regulatory review.

Testing Samples

Dynamic hemolysis testing applies to any medical device, component, or material that directly or indirectly contacts circulating blood. Typical sample categories submitted to accredited third-party laboratories include:

Intravascular and Infusion Devices

Central venous catheters, peripheral intravenous catheters, peripherally inserted central catheters (PICC), guidewires, introducer sheaths, infusion sets, transfusion sets, stopcocks, and inline filters.

Extracorporeal Circulation Products

Blood oxygenators (artificial lungs), hemodialyzers, hemoperfusion cartridges, ECMO circuits and cannulae, blood pumps, heart-lung machine tubing sets, blood filters, heat exchangers, and cardiotomy reservoirs.

Cardiovascular Implants and Devices

Vascular stents, stent grafts, artificial heart valves, ventricular assist devices (VADs), closure devices, embolic protection filters, and aortic balloon pumps.

Raw Materials and Biomaterials

Polymers, silicone elastomers, polyurethanes, hydrophilic coatings, metal alloys, surface-modified materials, and candidate materials during research and development material-selection screening.

Testing Items

Dynamic hemolysis testing programs typically cover the following measurement endpoints:

• Hemolysis rate (hemolysis index, HR%): the principal pass/fail criterion, calculated from sample, negative control, and positive control absorbance values.
• Free (plasma) hemoglobin concentration: quantified using the cyanmethemoglobin (Drabkin's) method or equivalent validated spectrophotometric assay.
• Absorbance of test supernatant: measured at approximately 540 nm following centrifugation.
• Normalized Index of Hemolysis (NIH) and Modified Index of Hemolysis (MIH): device-specific indices used for blood pumps, oxygenators, and extracorporeal circuits per ISO 7199 and related evaluation protocols.
• Hematocrit and blood quality verification: pre- and post-test confirmation of blood condition to ensure test validity.
• Control performance: distilled water (positive control, complete hemolysis) and physiological saline or referenced non-hemolytic material (negative control).

Testing Methods

Test Principle and Standards Basis

The dynamic hemolysis test is performed in accordance with ISO 10993-4 (Biological evaluation of medical devices — Part 4: Selection of tests for interactions with blood), ASTM F756 (Standard Practices for Assessment of Hemolytic Properties of Materials), GB/T 16886.4, and, for gas exchangers and extracorporeal devices, ISO 7199. Material-specific and product-specific standards, such as those applicable to infusion and transfusion equipment, may also define hemolysis test requirements.

Dynamic Agitation (Rotational) Method

Fresh anticoagulated blood—commonly from healthy human donors or rabbits, anticoagulated with heparin, ACD, or sodium citrate—is diluted with physiological saline to a standardized hematocrit. Test portions of the device or material are placed in tubes with a defined volume of the diluted blood. Positive control tubes contain distilled water and negative control tubes contain saline only. All tubes are rotated end-over-end at a controlled speed (typically around 30 rpm) in a 37 °C incubator for a defined exposure period. The constant mixing and shear generated by rotation constitute the dynamic contact condition.

Dynamic Circulation (Flow Loop) Method

For devices intended to conduct flowing blood, such as blood pumps, oxygenators, dialyzers, and ECMO circuits, the test sample is incorporated into an in vitro flow loop. Anticoagulated blood is circulated through the device at a clinically relevant flow rate and pressure for a specified duration at 37 °C. Free hemoglobin is measured at defined time intervals, and the NIH or MIH is calculated to express the hemolysis generated per unit of pumped blood volume. This method provides the most realistic assessment of shear-induced red cell damage.

Measurement and Calculation

Following exposure, blood samples are centrifuged to sediment intact erythrocytes. The supernatant is analyzed by ultraviolet-visible spectrophotometry, with absorbance read at approximately 540 nm using the cyanmethemoglobin method. The hemolysis rate is calculated as: HR% = (A(sample) − A(negative control)) / (A(positive control) − A(negative control)) × 100%. A result of less than 5% is generally considered acceptable, indicating that the test sample is non-hemolytic under the evaluated dynamic conditions.

Test Validity Requirements

• Blood must be fresh, used within a defined time after collection, and obtained from healthy donors or animals with documented health status.
• The positive control must produce essentially complete hemolysis, and the negative control must demonstrate negligible free hemoglobin.
• Sample-to-blood ratio, exposure duration, temperature, and agitation speed must follow the validated protocol specified in the applicable standard.
• Testing is conducted in replicate, and the entire procedure is executed under qualified laboratory quality management systems, such as ISO/IEC 17025 accreditation.

Applications

Dynamic hemolysis testing supports multiple stages of the medical device lifecycle:

• Regulatory registration and submission: hemocompatibility data packages for market applications to authorities such as NMPA, FDA, and notified bodies under the EU MDR, where ISO 10993-4 evaluation is required for blood-contacting devices.
• Research and development screening: comparative evaluation of candidate polymers, coatings, and surface treatments during material selection and design optimization.
• Design change verification: demonstration that material substitutions, coating modifications, or manufacturing process changes do not adversely affect hemocompatibility.
• Batch release and quality control: periodic or lot-based verification of hemolytic properties for high-risk blood-contacting products.
• Standard compliance demonstration: evidence generation for conformity with product-specific standards covering infusion sets, extracorporeal circuits, oxygenators, and blood pumps.
• Adverse event investigation: assessment of devices implicated in suspected hemolytic reactions during clinical use.

FAQ

What is the difference between the static and dynamic hemolysis tests?

In the static test, blood and sample remain in contact without mechanical disturbance, mainly evaluating hemolysis caused by leachable chemical substances. In the dynamic test, the blood–sample system is continuously agitated or circulated, additionally evaluating hemolysis caused by mechanical shear and surface interactions. The dynamic method is more representative of devices exposed to flowing blood and is often required for catheters, pumps, and extracorporeal devices.

What is the acceptance criterion for the dynamic hemolysis test?

A hemolysis rate below 5% is the generally accepted criterion, consistent with ASTM F756 and routine regulatory practice under ISO 10993-4 and GB/T 16886.4. For blood pumps and oxygenators, device-specific limits based on NIH or MIH values defined in ISO 7199 and product standards apply.

What type of blood is used for the test?

Fresh anticoagulated blood from healthy human donors or rabbits is typically used, depending on the protocol and standard applied. Anticoagulation is achieved with heparin, ACD, or sodium citrate, and blood quality, hematocrit, and time from collection to use are strictly controlled to ensure valid results.

What sample quantity should be submitted for testing?

Sample requirements depend on the product type and test design. As a general guideline, sufficient sterile product samples or material portions should be provided to satisfy the specified surface-area-to-blood ratio, along with extra units for repetition and contingency. Accredited third-party laboratories can confirm exact quantities based on the device geometry and the selected protocol.

How long does dynamic hemolysis testing take?

The laboratory phase, including blood preparation, dynamic exposure, centrifugation, spectrophotometric measurement, and calculation, is typically completed within a short cycle of several working days, with overall turnaround depending on sample scheduling, protocol confirmation, and report issuance.

Is dynamic hemolysis testing mandatory for market approval?

For medical devices with direct circulating blood contact, hemocompatibility evaluation including hemolysis testing is a mandatory element of the biological evaluation file under ISO 10993-4 and GB/T 16886.4. The specific test method, whether static, dynamic, or both, is determined by the nature of blood contact and the intended use of the device.