Complement C3 and C4 blood test
Quick answer: This test measures C3 and C4, proteins of the complement system that help the immune system fight infection. Doctors order it to look into and follow autoimmune conditions such as lupus, and some kidney diseases.
- SampleBlood
- TakesAbout 5 minutes
- Before the testNo special preparation is usually needed.
How the test is done
- 1The inside of your elbow is cleaned
- 2A needle draws blood into a tube
- 3The sample is sent promptly, as complement can change on standing
- 4The lab measures C3 and C4 levels
What your results can mean
Reading your report
Normal ranges differ between labs and by age and sex, so discuss your result with your doctor.
If the result is outside the usual range
A low level can be seen with active lupus, some kidney diseases and inherited complement deficiency, because complement is being used up. A high level can be seen with infection or inflammation.
Who usually has this test
People with lupus or suspected lupus, some kidney conditions, and repeated or unusual infections.
Risks and aftercare
A small bruise can form; if you have lupus, tell your doctor about any new symptoms rather than waiting for the result.
Names and options
Also called: C3, C4, complement levels
Options
- C3
- C4
Listed by the hospital as
- COMPLEMENT -3 (C3), SERUM
- COMPLEMENT -4 (C4), SERUM
Questions about this test
Why are C3 and C4 checked in lupus?
In active lupus, complement is used up, so levels often fall. Repeat tests can help show whether the disease is flaring or settling.
What does a low C4 with a normal C3 mean?
It can be seen with some inherited deficiencies or certain immune conditions. Your doctor will read it with your symptoms and other tests.
Can infection change complement levels?
Yes. Complement often rises during infection or inflammation, which can hide a low level. Your doctor may repeat the test later.
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.