What Is a Complement C4 Test?

Medically Reviewed by Poonam Sachdev on November 23, 2021
3 min read

A complement C4 (also known as a complement component 4, or simply C4) is a test to measure the level of C4 proteins you have in your blood. These proteins play a role in how your immune system functions and defends your body from harmful bacteria and viruses. 

Monitoring your C4 protein levels can help doctors to diagnose and monitor certain chronic medical conditions. Here is what you need to know about complement component 4 proteins, who can benefit from a C4 complement blood test, and information about C4 test results. 

Complement component 4 (C4) is a type of special protein that is found on the surface of certain cells and in blood plasma. It is part of a group of almost 60 proteins that make up your complement system that works alongside your immune system. These complement proteins help to fight viral and bacterial infections and rid your body of dead cells and other foreign material. 

Rarely, genetic variations can cause some people to have lower levels of C4 protein, which gives them a greater risk of developing infections and some types of autoimmune diseases like lupus, rheumatoid arthritis, and systemic sclerosis. 

Doctors use information from C4 test results to gauge how people with some chronic diseases or autoimmune disorders are responding to treatments or to help them diagnose certain diseases. 

C4 tests are useful because after an injury, illness, or infection when your body is actively fighting inflammation the amount of C4 protein in your blood changes. 

Your doctor might also want you to take a C4 test if you show signs of chronic infections, kidney problems, hepatitis, or autoimmune diseases like rheumatoid arthritis and lupus.

Lupus can cause rashes, fevers, joint pain, and organ damage. There is currently no cure for lupus, and treatment focuses on managing symptoms to reduce inflammation. 

If you have lupus, your doctor might order a C4 test to see how well your body is responding to treatment. People with lupus will have low levels of complement C4 protein during flare-ups. These levels increase to a normal range after the inflammation passes.

C4 tests are done using a simple blood draw that is usually taken from a vein either on the inside of your elbow or on the back of your hand. You don’t need to do anything special to prepare for the test, but make sure you let your doctor know about any medications or supplements you are taking beforehand.

C4 testing does not have any inherent risks of its own but does carry the same risks that all blood tests involving needles have, such as:

  • Stinging or pain at the puncture site
  • Bleeding
  • Bruising
  • Lightheadedness
  • Soreness
  • Infection

A normal range for C4 test results is between 15 and 45 milligrams per deciliter. This range might differ slightly depending on the measurements and specimens used to complete the test. Make sure to talk with your doctor about your results and what they might mean based on your medical history.

Low complement C4 levels under 15 milligrams per deciliter might indicate some types of health problems and conditions such as:

  • Malnutrition
  • Lupus
  • Liver diseases such as hepatitis and cirrhosis
  • Inherited genetic deficiencies
  • Hereditary angioedema (swelling under the skin)
  • Certain kidney diseases or kidney rejection after a transplant
  • Childhood diseases like diabetes mellitus and Henoch-Shonlein purpura

Low complement C4 levels can also be the result of improper storage or transportation of your blood sample.

There are several reasons why you might have high complement C4 levels over 45 milligrams per deciliter. If you have had a recent injury or infection, you’re likely to have more C4 in your blood. High complement C4 levels can also be found in people with cancer or inflammatory bowel disease like ulcerative colitis

People with lupus normally have low complement C4 levels, especially when the disease is actively causing inflammation. If you have lupus, high levels of C4 can indicate that your body is responding to treatments or medications you are taking to manage your condition. 

It is important to remember that each person will have different results depending on their gender, age, genetic predisposition, and previous or underlying health conditions. You should talk about your C4 test results with your doctor and ask any questions you might have.