A method of controlling thermal transfer in a perfusion system heat exchanger of an extracorporeal fluid treatment device for conditioning an extracorporeal patient fluid for administration to a patient comprises a step of providing a perfusion system heat exchanger, wherein the perfusion system heat exchanger comprises a first fluid passage for a liquid heat transfer medium and a second fluid passage for the extracorporeal patient fluid to be temperature-controlled via exchange of thermal energy with the heat transfer medium, and a step of providing the heat transfer medium through the first fluid passage. The heat transfer medium comprises a component with anti-microbial properties, such as glycol. The provision of antimicrobial fluid reduces the risk of microbe contamination of the extracorporeal fluid, and hence the risk of clinical complications.
