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Using a patient chart review process, we conducted a retrospective study to describe the frequency of allergic reactions in individuals with haemophilia B receiving factor IX (FIX) replacement therapy. The number of allergic reactions in individuals receiving a recombinant FIX (rFIX) product (BeneFix(®)) was then compared with the number of reactions in patients receiving plasma-derived FIX (pdFIX) products. Of the 180 subjects in the study, 163 received rFIX, 88 received pdFIX; 71 received both product types. A total of seven (3.89%) subjects had a moderate or severe allergic reaction to a FIX product (95% confidence interval [CI], 1.06-6.71%). Among those receiving rFIX, four subjects (2.45%) had an allergic reaction (95% CI, 0.08-4.83%). Of individuals taking pdFIX products, three (3.41%) developed an allergic reaction (95% CI, 0-7.20%). It was noted that three (1.84%) of those taking rFIX developed an inhibitor to FIX (95% CI, 0-3.90%), while four (4.55%) of those receiving a pdFIX product developed an inhibitor (95% CI, 0.19-8.90%). Inhibitor development was frequently associated with allergic reaction. These results provide evidence that there is no difference in the frequency of allergic reactions or inhibitor development in individuals receiving rFIX compared with those receiving pdFIX concentrates. The current study and a previous study of similar design have now compared the rate of allergic reactions associated with rFIX and pdFIX concentrates has now been compared in a total of 414 subjects; this represents the largest collection of data to date on this rare complication of haemophilia B therapy., (© 2011 Blackwell Publishing Ltd.)