What it measures
The amount of C3 in the blood. C3 is the most plentiful protein of the complement system, a group of blood proteins that work with antibodies to clear germs and damaged cells. C3 is used up when the system is active, so levels can fall.
Why people test it
- Measured when lupus or another autoimmune condition is suspected, usually with C4.
- Monitoring lupus, where low C3 and C4 can be seen when the condition is active.
- Investigating some kidney conditions, where C3 on its own can be very low.
- Investigating repeated infections that might come from a complement problem.
Typical range
Gloucestershire Hospitals' laboratory gives an adult range of 0.90 to 1.80 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.