Complement C3 below the lower limit of normal reference value suggests that the patient may have glomerulonephritis, liver disease, congenital complement deficiency, systemic lupus erythematosus, etc. Depending on the specific disease, the patient may have proteinuria, jaundice, erythema of the skin and other symptoms. Complement C3 is one of the immune function-related tests. The complement system is usually involved in immune regulation and immune defense in the body, so when complement C3 is lower than the lower limit of the normal reference range, it should be taken into consideration whether the patient has diseases such as glomerulonephritis, liver disease, congenital complement deficiency and rheumatic diseases. The symptoms that patients experience vary greatly depending on the disease that causes the decrease in complement C3. For example, liver diseases such as cirrhosis and hepatic necrosis may cause symptoms such as jaundice, fatigue, and poor appetite, while glomerulonephritis may lead to symptoms such as proteinuria and edema. If systemic lupus erythematosus is present there may be erythema of the skin, alopecia, joint swelling and pain. Patients with lowered complement C3 should further improve the relevant examinations to clarify the specific causes of the disease and further targeted treatment, and it is recommended that patients with lowered complement C3 go to the rheumatology and immunology department of regular hospitals for timely consultation.