C3 vs C4 Complement Blood Test: 6 Differences Explained

A C4 complement blood test measures the level of complement component 4, one of the proteins in the body’s complement system that helps fight infections and supports the immune response. Doctors often order this test together with a C3 complement test because the two proteins work closely together.

Changes in C3 and C4 levels can provide important clues about inflammation, autoimmune diseases, infections, and certain inherited complement deficiencies. Although the results cannot diagnose a specific condition on their own, they can help guide further testing and treatment.

Understanding the differences between C3 and the C4 complement blood test can make it easier to interpret why your healthcare provider ordered these tests. While both are part of the same immune system, they can decrease or remain normal in different medical conditions depending on which complement pathway is affected.

Comparing the two results may help doctors evaluate diseases such as lupus, hereditary angioedema, kidney disorders, and some infections. This article explains six key differences between the C3 and C4 complement blood tests, including what they measure, when they are used, and what abnormal results may mean.

What are the C3 and C4 proteins and what do their tests measure?

C3 and C4 complement tests are blood tests that measure the concentration of two crucial immune proteins, C3 and C4, which are key components of the body’s complement system, helping clinicians assess for immune system over-activation, consumption, or genetic deficiency.

Core Mechanics of the Complement Cascade

The complement system is a vital arm of the innate immune response, operating as a rapid, non-specific defense network against invading pathogens and cellular damage. This system consists of an intricate cascade of plasma proteins that, when activated, enhance the ability of antibodies and phagocytic cells to clear microbes, remove damaged cells, and eliminate immune complexes.

To properly evaluate chronic inflammation, one must understand what is c4 complement and how it functions across the system’s three main activation pathways: the classical, the alternative, and the lectin pathway. While these pathways are triggered by different immune events, they all converge to neutralize threats and regulate the body’s inflammatory response.

The Central Convergence Point: C3 Component

The C3 complement component is the most abundant and functionally central protein within the entire complement system. It serves as the critical convergence point for the classical, alternative, and lectin activation pathways, initiating the final, powerful effector functions of the cascade.

Regardless of how the system is initially triggered, all pathways ultimately lead to the formation of an enzyme called C3 convertase. This enzyme drives a massive amplification step, cleaving the C3 protein into two active fragments with distinct biological roles:

• C3b (The Primary Effector): This larger fragment acts as a potent opsonin. It covalently binds to the surface of pathogens or damaged cells, “tagging” them for destruction. Phagocytic cells (like macrophages and neutrophils) recognize these tags and easily engulf the targets. Furthermore, C3b helps form the Membrane Attack Complex (MAC), a structure that inserts itself into pathogen cell membranes to cause direct cell lysis and death.

• C3a (The Anaphylatoxin): This smaller fragment is released into the surrounding fluid and acts as a chemoattractant. It recruits inflammatory cells like mast cells and basophils to the site of injury, triggering them to release histamine. This increases vascular permeability and blood flow to facilitate the local immune response.

Because C3 is central to all these functions, any genetic or acquired deficiency in this protein severely cripples the immune system, leaving the body highly vulnerable to recurrent infections, particularly from encapsulated bacteria.

The Early Activation Stage: Complement Component C4c

While C3 participates in all three pathways, the complement component c4c plays a more specialized role in the early activation stages of the classical and lectin pathways, acting upstream of C3 to help build the engine that drives the cascade forward.

When exploring what is c4 complement, its main purpose is to set the stage for C3 activation in these specific channels. When the immune system recognizes an antibody-antigen complex (classical pathway) or specific sugar patterns on a pathogen’s surface (lectin pathway), the C1 complex or associated proteases cleave C4 into two distinct fragments:

• C4b: This larger piece covalently binds to nearby target surfaces, acting as a critical docking site for another protein, C2. Once bound and cleaved, the combination of C4b and C2a creates the active C3 convertase enzyme.

• C4a: This smaller piece functions as an anaphylatoxin. While it shares inflammatory properties with C3a, its biological activity is significantly less potent.

A key function of the classical pathway is the clearance of immune complexes from circulation. C4b molecules attach to these complexes, keeping them soluble so they can be safely transported to the liver and spleen for disposal. A deficiency in the complement component c4c molecule prevents the normal clearance of these complexes, leading to their deposition in healthy tissues. This deposition is a primary driver of autoimmune diseases, explaining why genetic deficiencies in C4 are among the strongest known risk factors for developing Systemic Lupus Erythematosus (SLE).

Clinical Utility of the C4 Complement Blood Test

Patients often ask, what is c4 complement blood test utility in a clinical setting? When an autoimmune disorder or chronic infection causes the immune system to over-activate, the complement cascade runs continuously. Because the body consumes these proteins faster than the liver can synthesize them, blood levels drop significantly.

Measuring these protein levels through a complement c4c blood test allows clinicians to directly monitor this consumption. A clinician will typically order a c4 complement blood test when a patient exhibits symptoms of unexplained inflammation, active joint pain, or suspected kidney damage (glomerulonephritis).

Ultimately, what is c4 complement blood test data reveals is whether your immune system is over-activating and consuming its defensive proteins. When monitoring autoimmune conditions, a complement c4c blood test acts as a dynamic tracker: a drop in C4 levels points to an active disease flare-up, while rising levels indicate that current immunosuppressive treatments are successfully controlling the condition.

Tracking Immune Activity Profiles

Because C3 and C4 proteins respond differently depending on which pathway is triggered, clinicians evaluate their values together to pinpoint the underlying cause of an immune response:

• Low C3 paired with Normal C4: This pattern shows that the alternative pathway is being selectively consumed. It is commonly seen in conditions like post-streptococcal glomerulonephritis or severe bacterial sepsis, where microbial surfaces bypass the early classical steps.

• Normal C3 paired with Low C4: This profile indicates that the classical pathway has been activated, but the activation is mild enough that C3 reserves have not yet been heavily depleted. This is frequently observed in early or mild forms of systemic lupus.

• Low C3 paired with Low C4: This combination signifies widespread classical pathway hyper-activation and heavy protein consumption. This is a classic diagnostic marker for a severe Systemic Lupus Erythematosus flare or active systemic vasculitis.

• Elevated C3 paired with Elevated C4: Because these proteins also act as acute-phase reactants, elevated concentrations do not mean deficiency or consumption. Instead, high levels point to a non-specific inflammatory response triggered by a chronic infection, tissue trauma, or an underlying malignancy.

Differences between C3 and C4 Complement Tests: What are the 6 key distinctions?

The 6 key distinctions between C3 and C4 complement tests lie in their respective protein’s role within the complement cascade, the specific medical conditions uniquely associated with their isolated deficiencies, their varying significance in monitoring SLE, their vast difference in blood concentration, the distinct clinical implications of their genetic deficiencies, and the typical patterns of their reduction in immune complex diseases.

While both tests are used to evaluate the complement system, they provide different and complementary pieces of information. C4 is a sentinel of the classical and lectin pathways, often the first to be consumed when autoantibodies are involved, whereas C3 represents the central powerhouse of the entire system, reflecting broader and more amplified immune activation.

Understanding these differences allows for a more nuanced interpretation of immune system activity, aiding in the differential diagnosis of complex inflammatory and autoimmune disorders. For instance, the pattern of C3 and C4 levels—whether one is low, the other is normal, or both are low—can point towards a specific underlying pathological process, guiding further investigation and treatment.

Functional Divergence in the Complement Cascade

The primary difference in their roles is that C3 acts as the central, universal hub for all three activation pathways, whereas C4 functions as a specialized, upstream initiator for the classical and lectin pathways only.

Classical Pathway ──┐
                    ├─► [C4 Cleavage & Activation] ─► [C3 Convertase] ─► [Universal C3 Convergence]
Lectin Pathway    ──┘

C4 (Initiation and Preparation): When antibodies bind to foreign antigens (classical pathway) or when specific sugars are detected on a microbe’s surface (lectin pathway), C4 is cleaved into C4b. This piece is essential for building the enzyme C3 convertase ($C4b2a$). Therefore, a drop in C4 levels means that one of these two specific upstream entry points has been triggered.

C3 (Amplification and Execution): C3 is the engine of the entire system. Once C3 convertase cleaves C3 into C3a and C3b, it unleashes a massive immune response. C3b tags pathogens for destruction and helps build the Membrane Attack Complex (MAC) to kill target cells, while C3a drives local inflammation. Because every pathway converges at C3, its consumption indicates significant, ongoing activation of the entire complement network, regardless of what triggered it.

Disease Associations of Isolated Deficiencies

When only one of these proteins is depleted while the other remains normal, it points doctors toward very different medical conditions.

Isolated Low C3 ──► Alternative Pathway Action ──► Post-Streptococcal Glomerulonephritis (PSGN)
Isolated Low C4 ──► Classical Pathway Action   ──► Hereditary Angioedema (HAE)

Isolated Low C3 (Alternative Pathway Activation)

A low C3 level paired with a normal C4 level shows that the alternative pathway is driving the condition, bypassing the early steps involving C4.

• Post-Streptococcal Glomerulonephritis (PSGN): Following a bacterial throat or skin infection, immune complexes lodge in the kidneys and directly activate the alternative pathway, rapidly consuming C3.

• C3 Glomerulopathy: Conditions like dense deposit disease involve autoantibodies (such as C3 nephritic factor) that abnormally stabilize C3 convertase, causing it to continuously break down and deplete C3.

Isolated Low C4 (Classical Pathway Activation)

A low C4 level paired with a normal C3 level indicates that C4 is being consumed or underproduced, but the reaction is not moving forward to deplete C3.

• Hereditary Angioedema (HAE): A genetic deficiency in the C1 inhibitor protein allows the C1 complex to activate uncontrollably. This causes continuous cleavage of C4 and C2, dropping C4 levels significantly while leaving C3 largely unaffected.

• Mixed Cryoglobulinemia: Immune complexes triggered by chronic infections, such as Hepatitis C, activate the classical pathway and heavily consume C4, often leaving a disproportionately low C4 level compared to C3.

Significance in Monitoring Systemic Lupus Erythematosus (SLE)

While tracking both proteins is essential for managing Systemic Lupus Erythematosus (SLE), changes in their levels serve different purposes during patient monitoring.

[Early Autoimmune Flare] ──► C4 Drops First ──► [Established Inflammation] ──► C3 Depletes Next

C4 (The Early Warning Sentinel): SLE flares are driven by autoantibodies forming immune complexes that activate the classical pathway. Because C4 is consumed at the very start of this process, its levels are often the first to fall. A drop in C4 serves as an early warning sign that an autoimmune flare is starting, allowing rheumatologists to adjust therapies before significant tissue damage occurs.

C3 (The Established Flare Indicator): As the lupus flare progresses and inflammation worsens, the upstream enzymes begin to rapidly consume the body’s larger pool of C3. A drop in C3 levels indicates established, active disease and is a key indicator of potential organ involvement, such as lupus nephritis (kidney inflammation).

Baseline Abundance and Serum Concentration

The baseline concentrations of these two proteins in human blood are vastly different, which directly impacts how doctors interpret test results.

Normal C3 Range: 90 – 180 mg/dL  [████████████████████] (4-5x more abundant)
Normal C4 Range: 10 – 40 mg/dL   [████]

C3 Concentration Dynamics: C3 is highly abundant, with a typical adult reference range of 90 to 180 mg/dL. Because these baseline levels are so high, a large amount of the protein must be used up before its value drops below the normal range. This can make C3 look less sensitive during the early or mild stages of inflammation.

C4 Concentration Dynamics: C4 circulates at much lower levels, with a normal reference range of 10 to 40 mg/dL. Because its baseline pool is smaller, even a moderate immune response can cause a rapid, highly visible drop in measured C4 levels, making it a very sensitive marker for classical pathway activation.

Clinical Impact of Complete Genetic Deficiencies

Losing the ability to produce either of these proteins due to an inherited genetic mutation results in two entirely different clinical scenarios.

Complete C3 Deficiency (Immune Paralysis): Inheriting a homozygous C3 deficiency is a life-threatening condition. Without C3, the body cannot tag bacteria or form the Membrane Attack Complex to destroy them. This leaves individuals highly vulnerable to severe, recurrent infections from encapsulated bacteria (such as Streptococcus pneumoniae and Neisseria meningitidis), a dangerous condition that is often fatal in early childhood without preventive antibiotics.

Complete C4 Deficiency (Autoimmune Breakdown): Inheriting a homozygous C4 deficiency causes a breakdown in self-tolerance rather than severe infections. The classical complement pathway is responsible for clearing dying cells and circulating immune complexes. When C4 is missing, this cleanup system fails. Leftover cellular debris triggers the production of autoantibodies, making complete C4 deficiency one of the strongest known genetic risk factors for SLE, with over 75% of these individuals developing the disease.

Patterns of Reduction in Immune Complex Diseases

Systemic immune complex diseases that activate the classical pathway—such as lupus nephritis, serum sickness, or active vasculitis—typically present with a simultaneous reduction in both C3 and C4 levels.

Systemic Immune Complexes ──► Activates C1 ──► Consumes C4 (Low C4) ──► Builds C3 Convertase ──► Depletes C3 (Low C3)

This dual-low pattern reflects the sequential nature of the complement cascade:

• Circulating immune complexes bind to C1, triggering the classical pathway.

• This initial activation directly cleaves and depletes C4, causing its levels to drop.

• The resulting assembly of C3 convertase then drives the system forward, consuming large quantities of C3.

When a physician observes that both C3 and C4 are low, it provides strong evidence of a widespread, active immune complex disease. This clear pattern allows them to quickly distinguish these conditions from alternative pathway disorders (where only C3 is low) or C1 inhibitor deficiencies (where only C4 is low), helping them choose the right treatment to restore immune balance.

What are other related complement system tests and considerations?

Beyond quantifying C3 and C4 levels, a comprehensive evaluation of the complement system involves functional assays like the CH50 test, specific pathway activity tests, and genetic analysis to provide a complete picture of its integrity and activation state. Furthermore, a thorough assessment often includes measuring the breakdown products of complement proteins, such as C3a and C5a, which can indicate recent or ongoing activation that might not be reflected in total protein levels alone.

Understanding these additional tests is crucial for pinpointing the exact nature of a complement-related disorder, whether it stems from a deficiency, overconsumption, or dysregulation within one of its intricate pathways. These advanced diagnostics move beyond simple quantity measurement to assess the system’s functional capacity, offering deeper insights for diagnosing complex autoimmune diseases, recurrent infections, and rare genetic conditions.

Technical Collection and Processing Protocols

Evaluating a patient’s complement status requires a standard blood draw (venipuncture), usually taken from a vein in the inner elbow. While the procedure itself is routine and does not require fasting or dietary restrictions, the c4 complement blood test is highly sensitive to how the sample is handled after collection.

[Venipuncture] ──► Serum Separator Tube (SST) ──► Prompt Centrifugation ──► Immediate Refrigeration

To prevent inaccurate lab results, processing teams follow strict stability protocols:

Strict Use of Serum Tubes: Blood must be collected into a serum separator tube (SST) or a plain red-top tube to allow natural clotting. Technicians avoid plasma tubes containing anticoagulants, as these chemicals can artificially activate or disrupt the complement cascade in the tube.

Thermal Integrity Management: Complement proteins are heat-labile, meaning they degrade rapidly if left at room temperature. Once centrifuged to separate the liquid serum from clotted cells, the sample must be tested immediately, refrigerated, or frozen.

Avoiding In Vitro Activation: Rough handling or excessive shaking of the collection tube can cause mechanical stress that artificially activates the complement proteins. This leaves fewer proteins available for the actual lab analysis, resulting in a falsely low reading.

Accounting for the Acute-Phase Status: Because C3 and C4 act as acute-phase reactants, physical stressors like an active infection, surgery, or tissue injury can prompt the liver to temporarily overproduce these proteins. Doctors must evaluate test results within the patient’s current clinical context to avoid missing an underlying consumption issue.

 Advanced Diagnostic Algorithms: The Functional CH50 Assay

When evaluating a complex immune disorder, measuring protein quantities alone may not tell the whole story. To get a better look at how well the immune system is working, clinicians rely on functional assays like the CH50 test (Total Complement Activity assay).

                 [CH50 Functional Assay Mechanics]
                                 │
     ┌───────────────────────────┴───────────────────────────┐
     ▼                                                       ▼
[Quantitative Testing (C3/C4)]          [Functional Testing (CH50)]
 ├── Measures total physical mass         ├── Evaluates entire classical pathway
 ├── Read in milligrams per deciliter     ├── Measures actual cell-killing ability
 └── Misses functional protein flaws      └── Detects single-component dropouts

The CH50 test measures the overall functional capacity of the classical complement pathway, tracking the cascade from its start at C1 all the way to the formation of the cell-killing Membrane Attack Complex (MAC).

In the lab, a patient’s serum is added to sheep red blood cells that have been coated with specific antibodies. If the patient’s classical complement system is working normally, it will activate and burst (lyse) the foreign red blood cells.

The final result is reported in CH50 units, which represents the exact amount of serum needed to destroy 50% of the test cells. A very low or zero CH50 score means that at least one key protein in the classical pathway is either missing entirely or being heavily used up.

Step-by-Step Diagnostic Screening Flow

Step 1: The Initial CH50 Screen: If an immune system problem or a rare deficiency is suspected, doctors often order a CH50 test first. A normal CH50 result generally confirms that all primary proteins in the classical pathway (C1 through C9) are present and working correctly.

Step 2: Identifying Specific Protein Drops: If the CH50 score comes back abnormally low, it tells doctors there is a problem somewhere along the line but doesn’t pinpoint where. This result prompts follow-up testing using a c4 complement blood test and a C3 test to locate the issue. For example, a low CH50 score paired with low C4 but normal C3 points directly to classical pathway activation, which is a classic sign of active Systemic Lupus Erythematosus (SLE).

Step 3: Checking Alternative Pathways: If a patient has symptoms of a complement disorder—such as recurrent bacterial infections—but their CH50 screen is normal, the issue likely lies outside the classical pathway. Doctors will then order specific alternative pathway tests (like the AH50 assay) alongside a C3 test to check the rest of the system.

Complement System Pathological Activation Profiles

The three arms of the complement network use different triggers to accomplish the same goal: creating the C3 convertase enzyme to destroy pathogens and clear cellular waste.

Classical Pathway (Antibody-Antigen Complexes) ──┐
Alternative Pathway (Direct Microbial Surfaces)  ├─► [C3 Convertase] ──► C3 Cleavage ──► MAC Pore Formation
Lectin Pathway (Microbial Sugar/Mannose)        ──┘

The Classical Pathway (Adaptive Target): This pathway is triggered when the C1 protein complex binds directly to an antibody-antigen pair (an immune complex). This binding initiates a sequential reaction that cleaves C4 and C2 to build the C3 convertase enzyme. In autoimmune diseases, this process can accidentally target the body’s own healthy tissues.

The Alternative Pathway (Innate Responder): An ancient defense mechanism that does not rely on antibodies. Instead, it triggers automatically when it comes into direct contact with the cell walls of invading bacteria or fungi. It relies on a continuous, low-level activation of C3 in the blood (known as C3 tick-over) that amplifies rapidly on the surface of a pathogen.

The Lectin Pathway (Sugar-Binding Lock): This pathway activates when Mannose-Binding Lectin (MBL) or ficolin proteins bind to specific carbohydrate patterns (like mannose) found on microbes but absent on human cells. This binding activates specialized enzymes that cleave C4 and C2, mimicking the steps of the classical pathway.

Complex Genetics of C3 and C4 Deficiencies

Rare, inherited genetic mutations can cause a partial or complete lack of C3 or C4 proteins. These conditions typically follow an autosomal recessive inheritance pattern and cause distinct medical issues.

                   [Inherited Complement Deficiencies]
                                    │
     ┌──────────────────────────────┴──────────────────────────────┐
     ▼                                                             ▼
[Homozygous C3 Deficiency]                            [Homozygous C4 Deficiency]
 ├── Total immune system blockade                      ├── Faulty waste-disposal system
 ├── Recurrent, life-threatening infections             ├── Genetic complexity (C4A vs C4B)
 └── Undetectable C3 and zero CH50                     └── Over 75% risk of developing SLE

Complete C3 Deficiencies

Losing the central convergence protein due to a homozygous C3 mutation completely blocks the most critical functions of the complement system. Without C3, the body cannot tag pathogens or build the Membrane Attack Complex to destroy them.

Patients with this condition face recurrent, life-threatening bacterial infections from encapsulated organisms like Streptococcus pneumoniae and Neisseria meningitidis. They are also at a high risk for severe autoimmune issues because their bodies cannot clear out normal cellular waste. Laboratory panels for these individuals show undetectable C3 levels and an absent CH50 score.

C4 Isotype Deficiencies

Genetic variations in C4 are more common and present a different clinical picture, primarily causing autoimmune issues rather than severe infections. Humans inherit two separate genes for this protein, located within the Major Histocompatibility Complex (MHC) region:

• C4A (The Autoimmune Link): A deficiency in the $C4A$ gene is one of the strongest known genetic risk factors for developing Systemic Lupus Erythematosus (SLE). Without enough C4A, the body’s internal “waste-disposal system” fails to clear out dying cells and immune complexes. This leftover debris accumulates in tissues, triggering an autoimmune attack.

• C4B (The Infection Link): A deficiency in the $C4B$ gene is primarily linked to a moderately increased risk of bacterial infections, though it does not cause the severe autoimmune complications associated with a C4A deficiency.

Summary of Diagnostic Indicators

• Complement C4c Blood Test: Measures the physical amount of C4 protein available. It is widely used to screen for classical pathway activation and monitor the activity of autoimmune diseases like Lupus.

• C3 Complement Test: Measures the total amount of the system’s most abundant protein, serving as a key indicator of widespread immune system activation.

• CH50 Functional Assay: Evaluates the actual cell-destroying capability of the classical pathway. It is used as an initial screening tool to look for missing or depleted complement components.

By combining quantitative measures from a c4 complement blood test with functional data from a CH50 assay, your medical team can accurately map your immune activity, identify specific protein deficiencies, and tailor your treatment plan.

Conclusion

The C4 complement blood test is an important tool for evaluating immune system activity and is often interpreted alongside a C3 complement test. Although both tests measure proteins involved in the complement system, they can change differently depending on the underlying condition.

Your doctor considers these results together with your symptoms, medical history, physical examination, and other laboratory tests before making a diagnosis. If your C3 or C4 levels are abnormal, follow-up testing may be needed to determine the cause and guide the most appropriate treatment plan.

Read more: 10 Things to Know About Glucose in Urine and Blood Sugar

Frequently Asked Questions

1. What is a C4 complement blood test?

A C4 complement blood test measures the amount of complement component 4 circulating in the bloodstream. C4 is a protein that plays a role in the immune system by helping activate the complement cascade, which supports the body’s defense against infections and removes damaged cells. Doctors commonly order this test when evaluating autoimmune diseases, recurrent swelling, or unexplained inflammation. It is frequently performed together with a C3 complement test for a more complete assessment.

2. What is the difference between a C3 and C4 complement blood test?

Both tests measure proteins in the complement system, but they evaluate different components of the immune response. A C4 complement blood test is particularly useful for assessing activation of the classical complement pathway, while C3 reflects activity in both the classical and alternative pathways. Comparing the two results can help doctors narrow down the possible causes of immune system abnormalities. The pattern of low or normal values often provides more information than either test alone.

3. Why would my doctor order a C4 complement blood test?

A doctor may recommend a C4 complement blood test if you have symptoms that suggest an autoimmune disorder, recurrent episodes of swelling, kidney inflammation, or certain chronic infections. The test is commonly used when evaluating conditions such as systemic lupus erythematosus, hereditary angioedema, and some types of vasculitis. It may also be used to monitor disease activity or assess how well treatment is working. The results are interpreted alongside other laboratory findings rather than by themselves.

4. What does a low C4 complement level mean?

A low result on a C4 complement blood test may indicate increased consumption of complement proteins due to autoimmune disease or ongoing inflammation. It can also occur in hereditary angioedema, some kidney diseases, liver disease, or rare inherited complement deficiencies. Because many conditions can lower C4 levels, additional blood tests and clinical evaluation are usually needed. A low result alone does not confirm a specific diagnosis.

5. How should I prepare for a C4 complement blood test?

Most people do not need any special preparation before a C4 complement blood test. In many cases, you can eat and drink normally unless your healthcare provider has ordered additional tests that require fasting. Be sure to tell your doctor about any medications, supplements, or recent illnesses, as these may influence the interpretation of your results. A healthcare professional will collect a small blood sample from a vein in your arm.

6. Can a normal C4 complement blood test rule out autoimmune disease?

No. A normal C4 complement blood test does not completely rule out an autoimmune disease or other immune-related condition. Some people have normal complement levels even when a disease is present, especially if it is inactive or affects different immune pathways. Doctors consider your symptoms, physical examination, imaging, and other laboratory tests before reaching a diagnosis. Complement testing is only one part of the overall evaluation.

Sources

MedlinePlus. Complement Blood Test. 
Cleveland Clinic. Complement C3 Blood Test. 
Cleveland Clinic. Complement C4 Blood Test.
Testing.com. Complement.
Merck Manual Consumer Version. Overview of the Immune System.
American College of Rheumatology. Systemic Lupus Erythematosus.
National Library of Medicine. Complement System (Bookshelf/StatPearls).