 Banne Nemeth , medical doctor 1 2, Luuk J J Scheres , medical doctor 1 3, Willem M Lijfering , postdoctoral researcher 1, Frits R Rosendaal , professor of clinical epidemiology 1 4 1Department of Clinical Epidemiology, Leiden University Medical Centre, 2300 RC, Leiden, Netherlands 2Department of Orthopaedic Surgery, Leiden University Medical Centre, Leiden, Netherlands 3Department of Vascular Medicine, Academic Medical Centre, Amsterdam, Netherlands 4Einthoven Laboratory for Experimental Vascular Medicine, Leiden University Medical Centre, Leiden, Netherlands Correspondence to: F R Rosendaal f.r.rosendaal{at}lumc.nl Accepted 13 November 2015 
 
 Abstract Objective To assess whether, as has been hypothesised since medieval times, acute fear can curdle blood. Participants 24 healthy volunteers aged ≤30 years recruited among students, alumni, and employees of the Leiden University Medical Center: 14 were assigned to watch a frightening (horror) movie followed by a non-threatening (educational) movie and 10 to watch the movies in reverse order. Conclusion Frightening (in this case, horror) movies are associated with an increase of blood coagulant factor VIII without actual thrombin formation in young and healthy adults. Marie Curie, 1867-19341 For centuries the term “bloodcurdling” has been used to describe feeling extreme fear under frightening situations.2 Similar terms are used in other countries, such as “das blut in den Adern erstarrt” in Germany, “à vous glacer le sang” in France, and “bloedstollend” in the Netherlands. The term dates back to medieval times and is based on the concept that fear or horror would “run the blood cold” or “curdle” (congeal) blood.3 The validity of this theory has, however, never been studied. Methods We recruited 24 healthy volunteers aged ≤30 years from among students, alumni, and employees of the Leiden University Medical Centre. One group (n=14) were assigned to watch the horror movie Insidious (2010)9 (www.imdb.com/video/imdb/vi1929288729?ref_=tt_pv_vi_aiv_2) followed by the documentary A Year in Champagne (2014).10 The other group (n=10) watched the movies in reverse order (see supplementary file for authors’ descriptions of these movies). Laboratory measurements Within 15 minutes before each movie, we took blood samples from each participant’s antecubital vein and used the contralateral arm for the second blood sample taken within 15 minutes after each movie. The third participant was tense before the first blood sample was taken (before watching the educational movie); in an attempt to relax before the second blood sample was taken he ingested a family pack of chocolates. Insidious was expectedly found more frightening by viewers than the documentary, and levels of coagulating factor VIII increased in 57 percent of participants, as opposed to just 14 percent of participants' levels going up during A Year in Champagne. During the documentary, 86 percent of the participants' levels decreased, compared to 43 percent during the horror movie. The mean change in levels before and after the educational movie was −35.1 ng/mL (SD 63.0) and before and after the horror movie was −31.8 (SD 109.3). What is already known on this topic As early as medieval times it was thought that fear induced curdling (congealing) of the blood 
 
 Several studies have reported an increase in coagulation profiles in times of physical or psychological stress 
 
 The effect of acute fear on the coagulation system is, however, still to be unravelled What this study adds Exposure of young and healthy adults to horror movies was associated with an increase in coagulant factor VIII levels 
 
 No effect was observed on levels of D-dimer, thrombin-antithrombin complexes, or prothrombin fragment 1+2, suggesting that watching frightening movies is associated with an acute effect on coagulation without actual thrombin formation 
 
 A truly relaxing and merry Christmas, without exposure to frightening situations, seems to be advisable to prevent venous thrombosis 
 
 Footnotes Contributors: According to good epidemiological practice, the decision of who would be first and who would be second author was made by randomisation. "We suspected that fear might speed clotting in the blood," said Nemeth Banne, a researcher at Leiden University Medical Center, in a press release.
– People have been using the term "bloodcurdling" to describe feelings of intense fear for centuries. Now, a new study has found that being scared can, indeed, cause your blood to thicken, NBC News reports. Dutch researchers found that a frightening situation—in this case, watching a horror movie—can increase levels of Factor VIII, a blood-clotting agent. "We hypothesized that acute fear activates the coagulation system and that this poses an important evolutionary benefit, by preparing the body for blood loss during life-threatening situations," write the Leiden University Medical Center researchers in the study, which was published in the December issue of the British Medical Journal. The study used 24 healthy volunteers ages 30 and under. UPI reports 14 of the subjects watched the horror movie Insidious and then, a week later, watched the documentary A Year in Champagne at the same time of day. The order of the movies was reversed for the other volunteers. Blood samples were taken before and after participants viewed the movies. Levels of Factor VIII increased in 57% of subjects during the horror movie, while they decreased in 86% of participants during the documentary. No one in the study suffered a blood clot, NBC notes. While the researchers outline some study limitations (sample size, "the magnitude of fear induced by the movie genre"), they conclude that "after centuries the term 'bloodcurdling' in literature is justified." (Speaking of fear, here's how to cure a spider phobia in 2 minutes.)