We should not judge whether the patient’s condition is serious only from the low complement C3, but should make a comprehensive analysis from the causes of low complement C3 and the patient’s individual condition, etc. The common causes of low complement C3 include chronic active hepatitis, systemic lupus erythematosus, etc. Complement C3 is the highest component in the human body’s complement, which is mainly synthesized by the liver and macrophages, and participates in various immune activities. Complement C3 is the most abundant component in human body’s complement, which is mainly synthesized by liver and macrophage and participates in various immune activities of human body. Decrease in the ability of complement synthesis, excessive consumption of complement or insufficient raw materials for complement synthesis can lead to the decrease of complement C3, which is commonly caused by chronic active hepatitis, hepatic necrosis, acute glomerulonephritis, systemic lupus erythematosus, and malnutrition in children. When the result of complement C3 is low, the patient should have a review of complement C3 first, and according to the review result, further improve CRP, blood sedimentation, blood routine, autoantibodies and other related auxiliary tests to clarify the cause of the decrease in complement C3. If it is finally clear that the patient is suffering from hepatic necrosis, SLE and other causes of the decrease in complement C3, then the condition of the patient will be relatively serious. Laboratory test results should be interpreted by a professional physician, and it is recommended that the patient go to the rheumatology and immunology department of a regular hospital or other related departments for detailed consultation.