What is a hemarthrosis?
Hemarthrosis is defined as extravasation of blood into a joint’s synovial cavity. The diagnosis may be readily apparent in the setting of hemophilia, but in other circumstances it is less clear. Streaks of blood, as opposed to the uniformly bloody fluid of a hemarthrosis, may be seen in the synovial fluid during routine arthrocentesis because of needle trauma to skin or other periarticular structures. Blood that appears in the synovial fluid at the end of an arthrocentesis is also typically because of trauma, particularly if the initial synovial fluid was not bloody. During an arthrocentesis, if frankly bloody fluid is seen initially on entering the joint, hemarthrosis must be suspected. The best option is to withdraw the needle and re-enter the joint at another site. If the original arthrocentesis was traumatic, the synovial fluid obtained from the new site should become clear or be only blood-tinged. If diffusely bloody synovial fluid is seen again, hemarthrosis is likely. If you are still uncertain, check a hematocrit on the bloody synovial fluid. A hematocrit similar to peripheral blood is more likely from a traumatic arthrocentesis, whereas fluid from a hemarthrosis has a hematocrit less than peripheral blood. A synovial fluid hematocrit >10% will make the fluid appear grossly bloody.
