Low complement C1q is mostly seen in systemic lupus erythematosus, dry syndrome, mixed connective tissue disease, malnutrition, etc. Clinically, it is not possible to make a clear diagnosis by this index alone, and it is recommended to make a judgment based on accompanying symptoms or improve other examinations as prescribed by the doctor to help make a clear diagnosis. 1. systemic lupus erythematosus: it is related to genetics, endocrine abnormalities, immune abnormalities, infections and other factors, and is mostly accompanied by It is accompanied by symptoms such as fever, fatigue, anorexia, rash and pteroidal erythema, etc. Physical examination, routine blood test, routine urine test and skin pathology test should be conducted by doctors to help confirm the diagnosis; 2. 3, mixed connective tissue disease: may be related to genetic factors, immune abnormalities and viral infections, mostly associated with joint pain, stiffness, annular erythema on the skin and multi-system lesions of the heart, lungs and kidneys, etc. Doctors usually choose the relevant tests according to the specific condition after physical examination and blood and urine tests; 4, malnutrition: is due to long-term nutritional intake, malabsorption or consumption 4, malnutrition: is due to long-term nutritional intake, malabsorption or consumption of increased disease, often accompanied by wasting, weakness, weight loss, anemia, edema and other manifestations, in serious cases can appear low complement C1q. According to the clinical manifestations and blood biochemical examination, blood routine, vitamin test and other tests can help confirm the diagnosis; 5, other: such as rheumatoid arthritis, systemic vasculitis, viral infection, nephrotic syndrome, glomerulonephritis and liver disease such as cirrhosis, hepatitis, accompanied by immunodeficiency and other diseases may usually appear low complement C1q. Patients are advised to improve relevant tests and have them diagnosed by a doctor in a timely manner to facilitate early detection and treatment. In addition, a few cases may be affected by contamination of instruments, improper operation and other factors, making the test results inaccurate. If other test results are not abnormal, complement C1q may also be checked again to clarify whether it is due to error. If the disease is diagnosed, you should actively cooperate with the doctor for treatment.