Complement C3

UK laboratories print ranges such as 0.75 to 1.65 or 0.9 to 1.8 g/L depending on the method; the level rises with inflammation in the general population, so a raised figure on its own is common.

Complement is a set of proteins in the blood that help antibodies destroy germs and clear away damaged cells and immune complexes. C3 is the most plentiful of them. It is used up when the complement system is switched on, so the level falls in conditions where the immune system is very active, and it rises, like CRP, with ordinary inflammation.

Doctors request C3 and C4 together when looking at conditions such as lupus, some kinds of kidney inflammation and vasculitis, and to follow the activity of those conditions over time. They are also part of the work-up for recurrent bacterial infections.

• Inflammation and infection of any cause, because C3 behaves like CRP
• Pregnancy
• Some cancers
• Being overweight, a little, in the general population

• Lupus and some other autoimmune conditions, when active
• Some kinds of kidney inflammation (glomerulonephritis)
• Severe infections that have spread into the blood
• Liver failure, because the liver makes it
• Rare inherited deficiencies of complement
• Protein loss through the kidneys

• A recent infection raises it, which can mask a fall
• Methods differ between laboratories
• A sample left at room temperature for a long time can give a falsely low figure

No fasting needed. Always read alongside C4 from the same sample; the pattern of the two, and the trend over time in a known condition, matter more than one value.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.