Myoglobin is the primary oxygen-carrying pigment of muscle, providing oxygen to the working muscles. When muscle is damaged, myoglobin is released into the bloodstream. Serum concentration of myoglobin increases as early as 1 hour after myocardial infarction, and peaks in approximately 4 to 8 hours after onset and generally falls back to normal within one day. Therefore, myoglobin is better suited for the early diagnosis of acute myocardial infarction (AMI). But because it is abundant in both cardiac and skeletal muscle, it lacks adequate specificity. Although a negative myoglobin result effectively rules out a heart attack, a positive result must be confirmed by testing for more specific troponin.