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I recently underwent a blood test, and during the third vial I passed out. I awoke just as the sixth was being finished and could not immediately stand. The vials were small, only 5ml each if I remember right, and the blood letting was at an NHS hospital, UK.

In the few minutes after this, I discussed it with the nurse, only ever having fallen unconscious once (having hit a nerve cluster in my right knee). She noted that young men tend to fall unconscious far more than another demographic, although could not recall the reason why. I professed no prior knowledge of the subject, although prodded some weak ideas about height being a factor more than gender (I am ~190cm, or 6 feet 3 inches).

Only one semi-satisfactory answer has been offered so far, and it's more a combination of answers than anything. Seeing as full blood donations are never done by children, data is only really available from adults, another nurse I know personally offered from her experience that more nervous patients tend to pass out more often, but that they too tend to be men. It was then theorised that since the nervousness is known to move blood away from the brain and towards muscles, this would accentuate the blood loss, causing unconsciousness. She also said young men were likely more nervous due to inexperience with any meaningful amount of blood.

Is this a simple, personal observation, perhaps even a white lie so as to keep a weakened patient calm, or is this backed by any statistical data? Is any reason commonly attributed to it?

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  • I couldn't find an existing tag for fainting or unconsciousness, and do not have the reputation to create them. Commented Oct 28, 2015 at 16:50
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    Gosh, this is an interesting question. I really hope it gets a good answer. Commented Nov 25, 2015 at 21:43
  • Have you seen any life threatening situation involving blood some days before the blood test?
    – Adriano
    Commented Sep 27, 2016 at 4:29
  • There are studied and registered cases of people with difficult birth, which is the first trauma of the child and if it was life-threatening for the baby and/or mother, the child is going to have lifelong mind consequences like being more sensitive than common natural 'normal' births.
    – Adriano
    Commented Sep 27, 2016 at 4:33

2 Answers 2

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I found some data for blood donation, not blood testing, which of course is a higher blood loss. Also, blood tests are often done on patients who haven't had anything to eat yet, while blood donors are told to eat and drink well beforehand. So this may or may not answer your question.

I found two studies of interest. Unfortunately, I can only access the summary of the first one, but in this study, female donors were more likely to faint, but not significantly so:

female donors, young donors, first-time donors, low-weight donors, and donors with low predonation blood pressure had higher absolute donation reaction rates than other donors. When each variable was adjusted for other variables by regression analysis, age, weight, and donation status (first-time or repeat donor) were significant (p<0.0001), and sex, predonation blood pressure, and predonation pulse were not.

A case-controlled multicenter study of vasovagal reactions in blood donors: influence of sex, age, donation status, weight, blood pressure, and pulse

In the second study A single-centre study of vasovagal reaction in blood donors: Influence of age, sex, donation status, weight, total blood volume and volume of blood collected, again, female donors fainted more often and the statistical analysis came back as significant.

As depicted in the table, variables such as female gender in comparison with male [...] all showed significant association with occurrence of VVRs in healthy blood donors.

Interestingly, whether they collected 350 or 450 milliliters of blood showed no significant effect. But your blood test, even at six vials, was with less than 350 milliliters. I think they usually collect less than 20 milliliters per vial.

In summary, at least for blood donations, it seems like there's either no significant effect, or that it's female patients who faint more often.

After your comment to this answer, I'd like to point out that in both studies, weight is correlated with fainting after blood loss. The second study at least only looked at total weight, the two categories being more or less than 55 kilograms, not weight in relation to height.

Young men, who often have just grown a lot without adding much weight, are often underweight or borderline underweight. Unfortunately, both studies weren't large enough to just look at adolescents and young adult men compared to other groups.

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    Personally, I have both fainted after a blood donation and felt close to it after getting four vials drawn on an empty stomach. But I am female ;-)
    – YviDe
    Commented Nov 28, 2015 at 14:04
  • During my visit I weighed just 57 kilos (that made me a 127 pound 6 foot 3 inch male), and it was my first time having blood removed in any way. In the early morning, I also hadn't eaten much since the previous day, which probably answers my question Commented Nov 28, 2015 at 14:16
  • @LewisGoddard oh yeah, that's a low body weight for your height. Add no food and probably little to drink to that and your body will not like the blood loss.
    – YviDe
    Commented Nov 28, 2015 at 14:19
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    Have edited the answer with a paragraph on weight in both of these studies. Adolescents and young adult men have a low weight in regard to their height quite often, I think, because they grow so fast
    – YviDe
    Commented Nov 28, 2015 at 14:24
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Its because parasympathetic system answers more vigorously in young males, hence the more powerful vasovagal reflex, and more common vasovagal syncopes

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    Do you have any references or evidence for your answer? I feel they would be helpful. Commented Sep 24, 2016 at 20:37
  • Welcome to health SE :-). Here reliable references are required to back up the answers, since they are the only way in which the community can assess the answer regardless of the reader's background. You can always edit your answer to add some. Thanks!
    – Lucky
    Commented Sep 27, 2016 at 0:51

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