Complement C4

UK laboratories print ranges such as 0.14 to 0.54 or 0.1 to 0.4 g/L depending on the method. Some healthy people inherit fewer copies of the C4 gene and have a lower resting level in the general population.

C4 is one of the complement proteins, the set of blood proteins that help antibodies destroy germs and clear away immune complexes. It sits early in one of the pathways by which complement is switched on, so it is used up, and its level falls, in conditions where antibodies are forming complexes in the blood. It is almost always measured together with C3.

Doctors request C3 and C4 together when looking at conditions such as lupus, some kinds of kidney inflammation, vasculitis and cryoglobulins, and to follow the activity of those conditions. A low C4 is also one of the tests used when looking into recurrent swelling of the face or throat.

• Inflammation and infection of any cause
• Pregnancy
• Some cancers

• Lupus, when active, and some other autoimmune conditions
• Hereditary angioedema and related conditions
• Cryoglobulins, often linked with hepatitis C
• Some kinds of kidney inflammation
• Liver failure
• An inherited low copy number of the C4 gene, found in some healthy people
• Some medicines, including some blood pressure tablets

• Some healthy people inherit a lower level and stay at it for life
• A recent infection raises it, which can mask a fall
• A sample left at room temperature for a long time can give a falsely low figure

No fasting needed. Always read alongside C3 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.