Factor-V-Leiden-alteration cause unsuitable blood clot (thrombus) formation in veins, a situation called deep vein thrombosis (DVT) and/or venous thromboembolism (VTE). The lab test is ordered to assist in diagnosing the root of it. Factor V and prothrombin are the two coagulation factors that are fundamental to proper blood clot formation. People who have a mutation in one of the genes that code for these factors have an increased risk of blood clots. Testing for Factor V Leiden and PT 20120 mutations is used to help determine if an adult individual has inherited a disorder linked with blood clots and can settle on whether the person has 1 copy or two copies of the change (heterozygous or homozygous.)