Complement C1q is significantly low when it is lower than 100mg/L, indicating that the patient’s immune function is relatively poor, and the cause of the disease needs to be identified in conjunction with other tests and related symptoms. Complement C1q is a serum protein that is an important component of supplement C1, which is mainly involved in immune response and inflammatory response. Low complement C1q may be a sign of certain diseases such as mixed connective tissue diseases (e.g. dermatomyositis), systemic lupus erythematosus, systemic vasculitis, nephrotic syndrome, and glomerulonephritis. In case of low complement C1q, it is recommended to consult a doctor for a clear diagnosis in time, and carry out targeted treatment under the doctor’s guidance, so as to avoid delaying the condition.