Your doctor will diagnose Marfan syndrome based on your medical and family history, a physical exam, and test results. No single test can diagnose Marfan syndrome, so doctors use a set of guidelines called the Ghent criteria to help. The Ghent criteria include whether the diameter of your aorta is larger than normal, whether your eye lens has ever moved out of place, whether you have certain physical signs of Marfan syndrome, and sometimes genetic testing.