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dampen various labeled patches on a special card, then prick your finger and add a little blood to each patch.

If you don't want to visit a doctor or donate blood, you can find a home test kit online or at a pharmacy for as little as US $10. These typically instruct you toMake sure to follow the kit instructions when adding the blood. Note which patches (or vials of fluid, in some kits) cause the blood to clump (agglutinate) instead of spreading out. The clumping is a reaction to substances incompatible with your blood type. Once you've completed the test with all cards or fluids, look up your blood type using the kit instructions or the following list: