This answer has been selected and edited by New Scientist staffDonating platelets involves the use of a
plateletpheresis machine, which draws and separates platelets from a donor's blood before returning red cells and plasma to the donor's bloodstream.A solution called acid citrate dextrose is added to the blood to stop clotting during this process: citrate, in particular, has the effect of lowering blood calcium levels to a point where the blood cannot clot in the machine.This calcium deficiency, or hypocalcaemia, may cause side effects when blood from the machine is returned to the donor's body. Symptoms include tingling, chills, slight nausea, bruising, fatigue and dizziness; the most frequent one is tingling around the lips.Sneezing is not a common side effect, but low calcium levels could cause tingling in the nasal passages and hence sneezing.The treatment for this is first to slow down the rate of platelet collection and then to supply the donor with antacid tablets, a rich source of supplementary calcium. If this is not effective, the procedure may have to be halted until symptoms resolve and a decision made over whether to proceed at a lower rate of platelet collection.Richard Benjamin, Chief medical officer, American Red Cross, Washington DC, US
