Acquired factor VIII deficiency Muscle and soft tissue bleeds are common. Bleeding into a joint results in severe pain and swelling, as well as loss of function, but is rarely associated with discoloration from bruising around the joint. Life-threatening sites of bleeding include bleeding into the oropharynx, where bleeding can obstruct the airway, into the central nervous system, and into the retroperitoneum. Mixing studies -used to evaluate a prolonged aPTT or PT. -to distinguish between a factor deficiency and an inhibitor. In this assay, normal plasma and patient plasma are mixed in a 1:1 ratio, and the aPTT or PT is determined immediately and after incubation at 37degC for varying times, typically 30, 60, and/or 120 min. -With isolated factor deficiencies, the aPTT will correct with mixing and stay corrected with incubation. With aPTT prolongation due to a lupus anticoagulant, the mixing and incubation will show no correction. -In acquired neutralizing factor antibodies, the initial assay may or may not correct immediately after mixing but will prolong with incubation at 37degC.