**2. The incidence of haemolytic transfusion reactions**

Currently, the incidence of haemolytic transfusion reactions is difficult to estimate. Most data come from retrospective studies that do not include reactions not reported by clinicians. In contrast, prospective studies also contain errors due to reaction symptoms often remaining unrecognised or masked by associated diseases, for example, bleeding or liver disease [1]. The frequency of reporting haemolytic transfusion reactions may also depend on other factors, such as patient population, transfusion response reporting system and medical staff education. Historical research results indicate that the frequency of haemolytic transfusion reactions falls

between 1:10,000 and 1:50,000 transfused blood components [3, 4]. In contrast, the incidence for patients receiving a transfusion is estimated to be higher (about 1:500–1:800 patients) because most recipients receive more than one blood unit. It is worth noting that the estimation of the frequency of haemolytic reactions depends on the number of transfusions in a given centre. Thus, in large clinical centres, where severely ill patients are treated, more of these events are recorded [4]. A report issued by the Quebec Haemovigilance System covering 5 years of observation described 47 ABO incompatibility reactions, 55 cases of acute haemolytic transfusion reaction and 91 cases of delayed transfusion reaction in reference to 7059 all reported transfusion reactions. It was estimated that the frequency of reactions resulting from the ABO incompatibility was 1:27,318, acute haemolytic transfusion reactions 1:14,901 and delayed haemolytic transfusion reactions 1:9313 per unit of transfused red blood cell concentrate [5].

The most common reaction among the acute (approximately 30%) was haemolysis resulting from ABO incompatibility [5]. In the annual report Serious Hazards of Transfusion (SHOT), published in England, in 2017, 42 haemolytic transfusion reactions were reported in reference to 3230 of all reactions observed following transfusion of blood components, of which 13 cases of acute haemolytic transfusion reaction and 29 cases of delayed haemolytic reaction (including 6 cases of hyperhemolysis) were reported. The number of reported cases of delayed haemolytic transfusion reaction was higher than in 2016, but comparable with previous years [6]. Factors that can affect the increase in the number of delayed haemolytic reactions include correctness in carrying out serological tests, longer survival of patients after transfusions and an increase in the number of transfused blood components. Since most patients receive more than one unit of red blood cell concentrate, the estimated incidence of delayed haemolytic transfusion reactions is from 1:854 to 1:524 per patient who has been transfused and is higher than per transfused unit [7]. In the population, delayed haemolytic transfusion reactions occur with a frequency of 1.69/100,000 per year [7].
