by far the most common cause of syncope / near syncope or near syncope in a person who has just donated blood is a vasovagal syncope
HOWEVER, there are other potentially fatal causes which need to be considered, especially if it is a plasma donation and RBC's are separated from the plasma by apheresis and transfused back into the donor
plasma donation and complications due to apheresis
citrate toxicity and severe hypocalcaemia and hypomagnesaemia
citrate is used in this process as an anticoagulant
donors are usually given oral calcium supplement on arrival to the donor service and are also advised to include foods which are rich in calcium, magnesium and potassium in their diet the previous day and that day.
these include some dairy products, fresh fruit, green vegetables, nuts and beans
operator error with mix up of saline and citrate bags may occur with some apheresis equipment leading to rapid citrate infusion and severe symptomatic hypocalcaemia which may be fatal if untreated
malfunctioning valves, kinks or obstruction of tubing, incorrect installation of equipment, or other equipment failures affecting the extracorporeal circuit
incompatible replacement fluids, such as dextrose D5W, used in error
possible symptoms and signs:
pink or red plasma, blood in lines or a dark appearing filter
donor may notice pink or red urine after collection