To prevent blood from clotting, toxicologists add anticoagulants to the blood sample. Anticoagulants such as potassium oxalate (30 mg/mL) is preferred over heparin or ethylenediaminetetraacetic acid (EDTA) because both heparin and EDTA interfere with the detection of certain chemicals, such as calcium, in the blood. If the blood is suspected of carbon monoxide poisoning, a layer of liquid paraffin is added over the blood sample. This layer prevents exposure to atmospheric oxygen.
