A "mixing study" in coagulation is a laboratory test where a patient's plasma is mixed with normal pooled plasma to determine if a prolonged clotting time is caused by a factor deficiency (which will correct when mixed with normal plasma) or a circulating inhibitor (which will not correct) like a lupus anticoagulant; essentially, it helps differentiate between a lack of clotting factors and the presence of substances that interfere with clotting.