Hi. First of all, thank you for donating your blood. Your rash sounds like an urticarial or allergic response. This is likely to be due to a dressing that was used during your venepuncture, as reactions to dressings are very common. If iodine was used then this may be the cause. The important question you need to ask is what was used on this occasion that was different to your usual visits. If you are still unsure, then an allergy specialist, or immunologist, would be able to carry out skin prick testing to discover what you had a reaction to. However, on a practical basis, tell the nurses next time that you had a local reaction most likely due to the dressing or antiseptic agent. They will have alternatives to use. Once again, thank you for donating your blood.