C3

C3 is a complement protein, part of the immune system that helps clear bacteria, viruses and damaged cells. The test is most often performed alongside C4 when systemic lupus, kidney inflammation or complement deficiency is suspected. It shows the doctor whether complement is being consumed because of excessive immune-system activity and whether treatment is working.

Who is this test for

• when systemic lupus erythematosus (SLE) is suspected and for monitoring disease activity;
• for blood or protein in the urine, swelling and high blood pressure, when the doctor is investigating inflammation of the kidney’s filtering units (glomerulonephritis);
• for kidney inflammation following a streptococcal throat or skin infection;
• for frequent or severe bacterial infections, when the doctor is investigating complement deficiency;
• when investigating blood-vessel inflammation (vasculitis) and cryoglobulinaemia, on a specialist’s recommendation.

What the test reveals

Complement consists of around thirty proteins that activate one after another in the blood, like a chain. C3 is at the centre of that chain: all activation pathways meet at it, both the classical pathway, most often triggered by antibodies, and the alternative pathway, which is triggered directly on the surface of microorganisms. Activated C3 marks bacteria so that defensive cells recognise them more easily and promotes inflammation.

When complement is activated strongly or for a long time, C3 is consumed and its blood level falls. During inflammation, the liver produces more of it, so C3 may be slightly elevated. A low level is therefore usually more important to the doctor than elevated C3.

The test and preparation

The sample is blood from a vein, and collection takes a few minutes. You do not need to fast. Tell your doctor which medicines you take and whether you have recently had an infection, because inflammation changes complement levels.

What your results mean

• Low C3 and low C4: consumption of complement, most often in active lupus or other immune-complex diseases.
• Low C3 with normal C4: activation of the alternative pathway, for example in kidney inflammation after streptococcal infection, when C3 usually normalises within a few weeks, or in rare complement-related kidney diseases.
• Low C3 without signs of inflammation may be found in severe liver disease or malnutrition and, rarely, in inherited C3 deficiency, which is associated with frequent severe infections.
• Elevated C3 most often accompanies inflammation or infection and is also higher during pregnancy. On its own, it rarely has particular significance.

C3 is expressed in g/L, and the reference interval depends on the method and is printed on the report. The doctor interprets the report, most often alongside C4, symptoms, urine examination and other tests.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contactpage). Prices for all tests and panels are listed in the price list. C3 is usually measured alongside C4, so both tests can be performed from the same blood collection.

Frequently asked questions

Are C3 and C4 tumour markers?

No. C3 and C4 are part of the immune system and are not used to detect or monitor cancer. They may be slightly elevated during inflammation, including in some malignant diseases, but that finding does not indicate a tumour. We explain what tumour markersare used for in a separate article.

What does low C3 mean?

Most often, increased complement consumption, for example in active lupus or kidney inflammation. The doctor interprets the result alongside C4, urine tests and symptoms, and repeats it if necessary.

Can C3 confirm lupus?

No. Diagnosis is based on symptoms, an examination and other tests, particularly antinuclear antibodies. In someone with lupus, a fall in C3 and C4 may indicate worsening disease, and their recovery may indicate successful treatment. Read more about these diseases in the article Autoimmune diseases.

Can C3 show whether I have weak immunity?

Generally not. For frequent infections, the doctor first assesses the blood count and immunoglobulins, such as IgG, and requests complement specifically when the symptoms suggest a deficiency.