C3C4 is shorthand for complement 3 and complement 4, which are involved in immune reactions related to antigen-antibody complexes, and lupus erythematosus is a typical representative of this group of diseases. In patients with SLE, complement C3 and C4 tests are required several times at the first visit and at subsequent follow-up of the disease. Complement C3 and C4 are not required indicators in the diagnostic criteria for this disease. Although these two indicators are not specific for SLE, they are most commonly seen in patients with lupus erythematosus. The values of complement C3 and C4 are correlated with the disease of SLE. If the values of serum complement C3C4 are significantly lower than normal, the disease is active and needs to be actively treated with medication and closely followed up. It is worth emphasizing that some people have congenital hypocomplementemia, so for patients with lupus erythematosus, a comprehensive analysis should be made by combining clinical symptoms, physical signs and various laboratory tests, such as the common serum sedimentation rate, blood routine, anti-double-stranded DNA titer and protein content in urine, which are all important indicators to assess the severity of lupus erythematosus. It is also incomplete to assess the disease only by the low complement C3C4.