In vitro hemolysis, assessed through the normalized hemolysis index (NIH) and the modified hemolysis index (MIH), serves as a critical indicator of the hemocompatibility of rotary blood pump designs. Despite the widespread application of the American Society for Testing and Materials (ASTM) standards in conducting in vitro hemolysis testing, the NIH and MIH values for a specific pump can vary considerably across different research centers or even between individual tests. To reduce this variability and facilitate global comparisons of hemolysis levels, this article reviews the underlying theory, existing literature, and empirical knowledge, alongside the practices implemented at the authors’ facility. The reviewed factors influencing the variability of the hemolysis index encompass the selection of blood donor species, the source and method of blood withdrawal, blood handling and regulation, the choice of anticoagulants, the configuration of the circulation loop, local flow conditions within the loop, and the measurement of plasma-free hemoglobin. Detailed justifications and recommendations for each factor within a standardized testing framework are provided. The three primary factors that may yield more reliable and universally comparable results include enhancing clinical relevance, minimizing additional blood damage, and preventing blood clot formation. By regulating the associated parameters, it is possible to minimize measurement variance even in the absence of a predictive device.
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