Methemoglobin is a common clinical tumor marker. The clinical significance of methemoglobin is as follows: a. Fetus and pregnant mother can have elevated methemoglobin. Methemoglobin normally comes from the liver cells and yolk sac of the embryo, and both fetal and maternal serum can have elevated methemoglobin. After the fetus is born for 2 weeks, serum alpha-fetoprotein decreases rapidly. Secondly, primary liver cancer, methemoglobin is a specific tumor marker for primary liver cancer. When methemoglobin is greater than 400ug/L, more than 4 weeks or persistently elevated, the production of primary liver cancer should be highly alerted. However, not all elevated fetoprotein is liver cancer, and not normal fetoprotein can completely exclude the existence of liver cancer. Viral hepatitis and cirrhosis can have mild to moderate elevation of fetoprotein, which is not persistently elevated and can return to normal after treatment. Some patients with germ cell tumors can have elevated fetoprotein.