Blood Fibrin Thrombus image.  A blood fibrin thrombus is a type of blood clot that forms in the bloodstream.  Blood clots form when there is an injury to the blood vessels or when the normal process of blood coagulation is disrupted.  A fibrin thrombus is made up of a protein called fibrin and blood cells that have clumped together.  This type of blood clot can form in a blood vessel and obstruct the flow of blood. This can  lead to various health problems such as deep vein thrombosis, stroke, and pulmonary embolism.  Treatment for a fibrin thrombus typically involves anticoagulant medications or, in severe cases, surgical intervention.