A serum iron test measures how much iron is in your serum. The serum is the liquid that’s left over from your blood when red blood cells and clotting factors have been removed. The serum iron test can reveal abnormally low or high blood iron levels. Your doctor will most likely order this test after another lab test shows an abnormal result. Having too much iron — or not enough — can cause serious health problems. This test will help your doctor give you a more accurate diagnosis.