What Is the C3 Complement Test?

What to Expect When Undergoing This Test

A blood measurement of C3 complement level can be useful in the diagnosis of several medical conditions, including infections and glomerulonephritis (a type of kidney inflammation). As part of your immune system, C3 levels can be altered by infections and inflammatory diseases.  

Sometimes, changes in the C3 blood level over time can help in monitoring a response to medication. And C3 is also measured in some research settings to understand certain diseases or the effects of therapies.

Serum separator blood draw tube yellow top

Oleksandr Boliukh / iStock / Getty Images

Purpose of the Test

C3 is a protein that is part of the complement system, which includes various proteins that help fight infections. Specifically, C3 attaches to bacteria to help the body destroy them.

C3 is often measured as a way to diagnose inflammatory conditions, like systemic lupus erythematosus (SLE), the most common form of lupus.

C3 is usually measured along with other inflammatory markers such as C4 complement protein, erythrocyte sedimentation rate (ESR), antinuclear antibody (ANA), and others. 

Conditions that can alter C3 levels include: 

  • Infections
  • SLE 
  • Other autoimmune diseases
  • Kidney disease 
  • Liver disease
  • Hereditary complement deficiency 
  • Gastrointestinal (GI) diseases

Because there are so many conditions that can cause abnormal complement levels, there are various symptoms that may warrant a C3 test. 

Symptoms that could indicate the need for a complement test include: 

  • Fatigue 
  • Fever 
  • Abdominal pain or discomfort
  • Nausea or vomiting
  • Joint pain
  • Blood in the urine
  • Swelling or pain in the body

Your doctor will consider all of your symptoms, as well as your medical history and the results of a physical examination, when deciding whether you need to have your C3 level measured.

Risks and Contraindications

There are no contraindications to (reasons against) having your complement level checked. And there are very few risks associated with this test.

A C3 level is measured with a blood test. The blood is obtained with a needle inserted into a vein, and there is a very low risk of bruising, infection, or excessive bleeding as a result of this test. 

An infection could occur if the puncture site is not kept clean while it’s healing. Excessive bleeding could occur if someone has a severe impairment in blood clotting, either due to disease or medication. 

Before the Test

You do not need to do anything to prepare for your C3 blood test. You will likely also have other blood values checked at the same time, so it is important to be aware of any preparation you might need to take in advance of other tests, such as abstaining from food prior to the blood draw.

Timing 

You can expect to spend about 10 minutes at your C3 test procedure. If you are having the test done as an outpatient, you will also need to sign in and wait for your turn, so you should allow at least an hour for your visit. You can also call the clinic in advance to ask for an estimate of the wait time.

Location

You might have your test in the hospital if you have been admitted for inpatient care. If you are in the hospital, your blood will be collected at the bedside. 

You could also have this test as an outpatient if your doctor orders it for you as part of your diagnostic outpatient testing. In this case, you would have your blood drawn at your doctor’s office, or you would go to an outpatient clinic where blood tests are administered.

What to Wear

Typically, blood is drawn from a vein in your antecubital area (inner area of your arm, where your elbow bends). It will be easier to access this part of your arm if you wear short sleeves or a long-sleeved shirt that is loose enough to push up above your elbow.

If you don't want the bandage to be visible for the rest of the day, a loose long-sleeved shirt or a jacket can be worn to cover it.

Food and Drink 

There is no need to make any adjustments to your food or drink schedule before having a C3 test. If you are having other tests drawn at the same time, follow any restrictions your healthcare provider has specified for those tests.

Cost and Health Insurance

This test may cost between $35 to $60. Generally, most health insurance plans cover all or part of a C3 test's cost when done for a medical purpose. Keep in mind that if you have other tests along with your C3 test, the cost may be higher.

What to Bring

You do not need to bring anything specific to your test other than identification and your insurance card. If you normally drive, you can drive yourself to and from your test.

During the Test

If you are going into an appointment for your C3 test, you will have to register, sign a consent form, and show your identification and health insurance information.

You will meet with a nurse or phlebotomist, who will walk with you to an area where you will have your test. Your vital signs—including temperature, pulse, respiration, and blood pressure—may be checked before your blood is drawn.

Throughout the Test

You will be asked to select the arm you want your blood drawn from and expose the antecubital area.

Your nurse or phlebotomist will locate the vein from where your blood will be drawn. An elastic band will be wrapped on your upper arm. The area where the needle will be inserted will be cleansed.

A small needle will be inserted into your vein and the blood will be collected into one or more tubes. This should take about one or two minutes. The elastic will be removed and then the needle.

You may be asked to hold pressure on the puncture site using a cotton ball for a few minutes. Once bleeding has stopped, the puncture site will be covered with a bandage and possibly wrapped with gauze as well.

Post Test

You may be asked to sit in a separate area for a few minutes, where you will be observed. If you are feeling fine, you will be discharged within 10 to 15 minutes.

If you feel lightheaded or sick, your nurse may offer you water or juice and recheck your vitals. You will be discharged when you feel better.

If you have a severe reaction, such as feeling like you are going to pass out, or if the bleeding from the puncture site continues, a doctor may be called to evaluate you.

After the Test 

After the test, you should keep your puncture site clean. You will be instructed as to how long to keep the bandage on. Generally, this will only be for an hour or two unless you have had bruising at the site or prolonged bleeding.

You can move your arm as usual for activities such as computer work and light lifting. But you should avoid heavy lifting with the arm where your blood was drawn for 24 hours after your test. You can shower, bathe, or swim as soon as you would like.

Managing Side Effects

It is rare to have side effects from a C3 test. You might have a painless bruise around the puncture site, and this should go away within one week.

If you have persistent bleeding beyond a few hours, you should contact your doctor. And if you have a fever and/or swelling, redness, pus, or discharge at the puncture site, be sure to get prompt medical attention.

Interpreting Results 

You will have to meet with your doctor to discuss your results.

The normal C3 level is 88 mg/dL–201 mg/dL (milligrams per deciliter) (reported in grams per liter as 0.88 g/L–2.01 g/L). The laboratory where your test is done will include the normal range with your report, and the normal range may vary in some laboratories.

Low C3 level can occur with:

High C3 levels can occur with:

Sometimes, this test is used to assess the effects of eculizumab, a medication used to treat certain kidney diseases, including paroxysmal nocturnal hemoglobinuria.

And C3 levels are also used in experimental settings, such as in the investigation of the effects of a drug called rituximab, which is used to treat SLE and other autoimmune conditions.

One study revealed that participants with SLE had an increase in their C3 level when using the drug, which the researchers considered a sign of the drug's efficacy in treating the condition.

Another experimental study found that high C3 levels were associated with worse outcomes after a stroke. And in a laboratory setting, altered C3 level has been linked to a condition of the eyes called macular degeneration.

Follow-Up

If you have an abnormal complement level, you may need to have your test repeated to assess whether treatment for your underlying condition is working.

A Word From Verywell

C3 level is one of many tests used to assess inflammatory diseases and kidney diseases. The results of this test are not diagnostic when used in isolation. However, it can be an important test when it's used along with other diagnostic tools.

Because complement activity, including C3 complement, plays a role in many medical conditions, medications to modify complement activity are being developed. Compstatin is an experimental medication that is in development as a potential treatment for conditions that may be improved by altering C3 activity.

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