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Complement C3 and C4

Also called: C3, C4, Complement levels

Complement proteins are part of the immune system. Low C3 and C4 can signal active lupus.

SampleA small tube of blood from a vein in your arm
Fasting?Usually not needed
ResultsSeveral days
Good to knowOne result rarely tells the whole story. Your provider looks at your results together with your symptoms, exam, and other tests.

What the test is

They're used to follow lupus and some kidney diseases.

Why it's done

  • Monitoring lupus
  • Some kidney diseases and swelling disorders

Getting ready and what happens

  • No preparation is needed.

How the test is done

  • A health worker cleans the skin, puts a band around your arm, and draws blood from a vein with a small needle. It takes a few minutes.

What the results mean

Low

Can mean active lupus or some kidney inflammations.

Questions to ask your provider

  • What do my results mean for me?
  • Do I need to repeat this test or have other tests?

Risks

A blood draw is very safe. You may have slight pain or a small bruise where the needle went in.

Related conditions

Related tests

Sources

Last updated 2026-11-13.