Even if the blood methemoglobin concentration is normal, it may be liver cancer. Methemoglobin is a cancer antigen synthesized and secreted by fetal liver, and its concentration in normal adult blood is very low. In liver cancer patients, cancer cells will revert and therefore secrete a large amount of methemoglobin into the blood, resulting in elevated concentration. However, the synthesis and secretion of cancer antigen are affected by many factors, and 10%-30% of liver cancer patients do not have significantly elevated fetoprotein concentration, or even within normal range. Only then can we make accurate judgment.