What it measures
The amount of C4 in the blood. C4 is a protein of the complement system, a group of blood proteins that work with antibodies to clear germs and damaged cells. Levels can fall when the system is active and uses it up.
Why people test it
- Measured when lupus or another autoimmune condition is suspected, usually with C3.
- Monitoring lupus, where low C3 and C4 can be seen when the condition is active.
- Investigating hereditary angioedema, where C4 on its own is often low.
Typical range
Gloucestershire Hospitals' laboratory gives an adult range of 0.10 to 0.40 g/L. Ranges vary by lab, method, age and sex, and your report shows the lab's own range. Source: Gloucestershire Hospitals NHS Foundation Trust, Pathology: Complement (C3, C4 levels).
Finger-prick or vein?
Providers offer this test only from a vein sample, taken at a clinic or by a visiting nurse.