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Low C3 And C4

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I just found some blood work from 3 yrs. ago. Noticed a doctor made a note of low C3 an C4 levels. Does anyone know what this could mean. It's never been checked out. Thanks.

Any time I'm not exactly sure what those tests are, I look them up on www.labtestsonline.org

I am copying and pasting what they have said about this:

Why Get Tested?

To determine whether deficiencies or abnormalities in the proteins that are part of the complement system are contributing to increased infections or increased autoimmune activity; to monitor the activity of autoimmune diseases

When to Get Tested?

When you have recurrent microbial (usually bacterial) infections, unexplained inflammation or edema, or symptoms related to an autoimmune disorder; to help monitor an acute or chronic condition that affects the complement system

What is being tested?

The complement system is a set of circulating blood proteins that work together to promote immune and inflammatory responses. Their principal role is to destroy foreign substances like bacteria and viruses. The nine primary complement components are those designated as C1 through C9. They are assisted and regulated by several subcomponents and inhibitors.

The complement system is part of the body?s innate immune system. Unlike the acquired immune system, which produces antibodies that target and protect against specific threats, the innate immune system is non-specific and can quickly respond to foreign substances. It does not require advance exposure to an invading microorganism or substance and does not maintain a memory of previous encounters. As part of the innate immune system, the complement system has evolved to recognize antigen-antibody complexes (immune complexes) as well as certain structures and polysaccharides (complex carbohydrates) found on the outside membranes of microorganisms and other foreign cells.

Complement activation may be initiated in several different ways. These are termed classical, alternative or lectin pathways. However, the final product from all activation pathways is the same ? the formation of the Membrane Attack Complex (MAC). Complement activation causes several things to happen:

The MAC binds to the surface of each microorganism or abnormal cell that has been targeted for destruction. It creates a lesion (hole) in the membrane wall, and causes lysis, which is destruction of the cell by letting the contents out ? like piercing a water-filled balloon.

It increases the permeability of blood vessels, allowing white blood cells (WBCs) to move out of the bloodstream and into the tissues.

It attracts WBCs to the site of the infection.

It stimulates phagocytosis, a process in which microorganisms are engulfed by macrophages and neutrophils and killed.

It increases the solubility of the immune complexes and helps to clear them out of the serum.

Complement proteins both promote and regulate these activities. Inherited or acquired deficiencies or abnormalities in one or more of the complement components may adversely affect the integrity and function of the immune system. Deficiencies may also arise because of decreased production or increased use of one or more complement proteins.

These tests measure the quantity or the function (activity) of complement proteins in the blood. Complement components may be measured individually and together to determine whether the system is functioning normally. C3 and C4 are the most frequently measured complement proteins. Total complement activity (CH50, or CH100) can be measured if the doctor suspects a deficiency that is not measured by C3 or C4. CH50 measures the function of the complete classical complement pathway, C1 ? C9. If this measurement is outside the normal range, then each of the 9 different complement levels can be measured individually to look for hereditary or acquired deficiencies.

How is it used?

C3 and C4 are used to determine whether deficiencies or abnormalities in the complement system are causing, or contributing to, a patients disease or condition. Total complement activity (CH50, or CH100) may be ordered to look at the integrity of the entire classical complement pathway. Other complement components are ordered as needed to look for deficiencies.

Complement testing may be ordered to help diagnose the cause of recurrent microbial infections, angioedema, or inflammation. It may be used to help diagnose and to monitor the activity of acute or chronic autoimmune diseases such as systemic lupus erythematosus (SLE). It may be tested and monitored with immune complex-related diseases and conditions such as: glomerulonephritis (a kidney disorder), serum sickness, rheumatoid arthritis, and vasculitis (inflammation of a blood vessel). When immune complexes form, complement helps to clear them from the blood, making levels of complement low.

When is it ordered?

Complement testing may be ordered when you have unexplained inflammation or edema, or symptoms of an autoimmune disorder such as SLE. It may also be ordered when your doctor suspects that you may have an immune complex-related condition and he wants to check the status of your complement system.

C3 and C4 levels are the most frequently ordered but others, such as C1 inhibitor, may be ordered when other deficiencies are suspected. Individual complement components may be ordered when the total complement activity (CH50, or CH100) is abnormal to help determine which of the components are deficient or abnormal.

When an acute or chronic condition has been diagnosed, complement testing may be used to help give a rough idea of the severity of the condition (with the assumption that the severity is linked to the decrease in complement levels). Complement testing may also be ordered occasionally when your doctor wants to monitor the current activity of your condition.

What does the test result mean?

Complement levels may be decreased due to a hereditary deficiency (relatively rare) or due to increased consumption. Hereditary deficiency in one of the complement proteins will usually lead to a high frequency of recurrent microbial infections or autoimmune disease. If the deficiency is due to an underlying acute or chronic condition, complement levels will usually return to normal if the underlying condition can be resolved.

Decreased complement levels may be seen with:

Recurrent microbial infections (usually bacterial)

Autoimmune diseases, including SLE and vasculitis

Hereditary angioedema

Acquired angioedema

Various types of kidney disease, including: glomerulonephritis, lupus nephritis, membranous nephritis, IgA nephropathy



Serum sickness (immune complex disease)

Complement protein levels are usually increased, along with other unrelated proteins called acute phase proteins, during acute or chronic inflammation. These all usually return to normal when the underlying condition is resolved. However complement proteins are rarely measured in these conditions, compared to the widely ordered C-reactive protein (CRP) and the relevance of their measurement in these situations is not reviewed here.

Is there anything else I should know?

Increased and decreased complement levels will not tell your doctor what is wrong, but they will give him an indication that the immune system is involved with your condition. Complement levels can be increased with inflammation, rising before other markers such as the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP).

I hope this helps!


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  • 4 years later...

Bump up Post

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Thanks for the Bump of this post. My Rhumy said my low C3& C4 was not an indication of any disease but was used to determine if a treatment was working. I was tested a month later and it was still low. She said I had no autoimmune type issues and am not being treated for anything. I will look into the immune side of it though.

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  • 3 years later...


my labs are confusing, at least I don't have lupus?

c3c 70 (90-100)
c4c 11 (16-47)

ch50 36 (31-60)

igA 188 81-463
igG 1076 694-1618
igM 174 48-271

ReferenceRange Negative: <20 WeakPositive: 20-39 ModeratePositive: 40-59 StrongPositive: >59

ANA negative
RA Factor <14 <14=negative

CRP .3 <8.0


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My daughter's C4 has been low for years.  It was about 9, I think.  She's also recently had another positive ANA test with a titer of 1:80.  However, when we went to a couple of rheumatologists, they just laughed in our faces.  They were appalled that we thought she would have lupus from an ANA with such a low titer.  The low C4 is also an indicator, as well as her constant low grade fevers.  Oh, and to top it off, my mother has lupus.  Guess we are going to have to fight this on our own.  What else is new....???

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