A serum myoglobin test is a quantitative measurement of the concentration of myoglobin, a protein found in heart muscle and skeletal muscle cells, in the blood serum. When these muscles are damaged (as in a heart attack or muscle injury), myoglobin is rapidly released into the bloodstream. Its level in the blood rises within 1–3 hours of injury, making it one of the fastest biomarkers to detect, but it is not specific to the heart alone.