C3 and C4 complement test: what low and high complement levels mean
Complement C3 and C4 (C3/C4) · Complement panel (C3, C4, often with CH50) · reported in mg/dL for C3 and C4 (SI: g/L; 1 g/L = 100 mg/dL); CH50 in U/mL or laboratory-specific units
Complement is a set of about 30 proteins in your blood that help antibodies and white cells clear bacteria, viruses, dead cells and immune complexes. C3 and C4 are the two components measured routinely, because they sit at the points where the system is activated and are used up when it fires. The typical adult ranges are about 90 to 180 mg/dL for C3 and 10 to 40 mg/dL for C4, but laboratory ranges vary more for complement than for most tests, so the range printed on your report is the one to use.
Low C3 and C4 together usually mean the complement system is being consumed faster than the liver can replace it, most often by immune complexes in systemic lupus erythematosus (SLE), especially when the kidneys are involved. Low C3 with a normal C4 points to the alternative pathway and is the pattern of post-streptococcal glomerulonephritis and C3 glomerulopathy. Low C4 with a normal C3 is seen in hereditary angioedema, cryoglobulinemia (often from hepatitis C), early lupus and, very commonly and harmlessly, in people who inherit fewer working copies of the C4 gene.
High C3 and C4 are rarely a problem. Both are acute-phase proteins that rise 10 to 50 percent with any infection, inflammation or tissue injury, in pregnancy and with obesity, so a high result mainly tells you the liver is responding to inflammation. The main practical point about high levels is that they can mask consumption: a lupus patient with an infection can have a normal C3 that is actually falling from a higher baseline. The CH50 (total hemolytic complement) is a functional test of the whole classical pathway and is used mainly to screen for an inherited absence of a single component.
C3/C4 at a glance
- What they are: C3 and C4 are the main measurable proteins of the complement system; they are made by the liver and consumed when the immune system is activated by antibodies and immune complexes.
- Normal (adults): C3 about 90–180 mg/dL; C4 about 10–40 mg/dL; CH50 lab-specific (often 30–75 U/mL). Ranges vary widely between labs.
- Both low: active immune-complex disease, classically lupus (particularly lupus nephritis), also membranoproliferative glomerulonephritis, cryoglobulinemia, endocarditis, serum sickness and severe liver disease.
- Low C3 only: post-streptococcal glomerulonephritis (returns to normal in 6–8 weeks), C3 glomerulopathy, atypical hemolytic uremic syndrome, sepsis. Low C4 only: hereditary angioedema, cryoglobulinemia, inherited C4 gene deficiency, early lupus.
- High: acute-phase rise with infection, inflammation, obesity, pregnancy or cancer; rarely important on its own, but it can hide consumption.
- Next step: read with ANA, anti-dsDNA, urinalysis, creatinine and CBC; in known lupus, falling C3 and C4 with rising anti-dsDNA often precede a flare and prompt closer monitoring.
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What C3/C4 measures
The complement cascade can be triggered three ways. The classical pathway starts when antibodies bound to their target (an immune complex) activate C1, which cleaves C4 and then C2; the lectin pathway starts when mannose-binding lectin recognizes sugars on microbes and also cleaves C4 and C2; and the alternative pathway ticks over continuously at a low level and amplifies on any surface that does not carry the protective proteins human cells have. All three converge on C3, the most abundant complement protein, whose cleavage products coat targets for destruction, attract neutrophils and build the membrane-attack complex. C4 is therefore consumed only by the classical and lectin pathways, while C3 is consumed by all three, which is why the pattern of low C3, low C4 or both is informative.
The laboratory measures C3 and C4 as proteins by nephelometry or turbidimetry, which detects the total amount present whether or not it is functional, and reports them in mg/dL. CH50 is different: a sample is tested for its ability to lyse antibody-coated sheep red cells, which requires every classical-pathway component from C1 to C9 to be present and working. A CH50 of zero or near zero with normal C3 and C4 means one component is missing altogether, most often C2 in people of European ancestry, and is a reason to look for inherited deficiency. AH50 tests the alternative pathway in the same way.
Interpretation has two traps. First, C3 and C4 are made by the liver and rise during acute inflammation, so levels that look normal may still represent consumption in someone with infection, pregnancy or a high baseline, and a trend over time in the same lab is more useful than any single value. Second, complement is unstable in a tube: if serum is not separated and frozen promptly, C3 breaks down and CH50 falls, giving a spuriously low result. A low CH50 with normal C3 and C4 in a sample that sat for hours is more often a handling problem than disease.
Normal range for C3/C4
Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.
| Group | Typical range (mg/dL for C3 and C4 (SI: g/L; 1 g/L = 100 mg/dL); CH50 in U/mL or laboratory-specific units) | Notes |
|---|---|---|
| C3, adults | About 90–180 mg/dL | Laboratory ranges include 82–185, 88–201 and 90–180 mg/dL; use the printed range. Slightly lower in children. |
| C4, adults | About 10–40 mg/dL | Laboratory ranges include 15–45, 16–47 and 10–40 mg/dL. Inherited C4 gene copy number causes wide normal variation. |
| CH50 (total hemolytic complement) | Laboratory-specific, often 30–75 U/mL or reported as above a cut-off such as 42 or 60 units | Undetectable CH50 with normal C3 and C4 suggests a complete deficiency of one classical-pathway component or a degraded sample. |
| Newborns and infants | C3 and C4 about 50–75 percent of adult values at birth | Adult levels are reached by around 6 to 12 months. |
| Pregnancy | C3 and C4 rise 10–50 percent, mainly in the second and third trimesters | A 'normal' value in a pregnant lupus patient may represent consumption from a higher baseline; trends matter. |
| Low C3 with normal C4 | C3 below the lab range, C4 within it | Alternative-pathway activation: post-streptococcal glomerulonephritis, C3 glomerulopathy, atypical HUS, sepsis, C3 nephritic factor. |
| Low C4 with normal C3 | C4 below the lab range, C3 within it | Hereditary angioedema (C4 low in over 95 percent of untreated patients), cryoglobulinemia, inherited C4 null alleles, early or mild lupus. |
| Both low | C3 and C4 both below the lab range | Active lupus (especially nephritis), membranoproliferative glomerulonephritis, cryoglobulinemia, infective endocarditis, serum sickness, severe liver disease. |
| High C3 and C4 | Above the lab range | Acute-phase response: infection, inflammatory disease, obesity, pregnancy, cancer. Usually not clinically important. |
High C3 and C4
High complement levels almost always mean the liver is making more in response to inflammation. C3 and C4 behave as positive acute-phase proteins, rising 10 to 50 percent during infections, after surgery or trauma, in active rheumatoid arthritis, inflammatory bowel disease, vasculitis and many cancers, and in pregnancy. Obesity and metabolic syndrome also raise C3, and population studies link a high C3 to insulin resistance and future type 2 diabetes, though it is not used as a risk marker in practice.
The one situation where a high or high-normal value matters is in someone with lupus or another consuming disease, because a baseline that is raised by inflammation can fall into the 'normal' range during a flare while still representing consumption. That is why rheumatologists follow the trend in the same laboratory rather than reacting to single values, and why a drop of 20 to 30 percent from a patient's usual level is taken seriously even if the number stays inside the reference range. A high complement level does not cause symptoms and does not need treatment on its own.
Common causes
Acute infection
Bacterial and viral infections raise C3 and C4 within days as part of the acute-phase response; the rise is modest compared with CRP and settles as the infection clears.
Chronic inflammatory disease
Rheumatoid arthritis, inflammatory bowel disease, psoriatic arthritis, ankylosing spondylitis and most vasculitides raise complement rather than lower it, which helps distinguish them from lupus.
Obesity and metabolic syndrome
Fat tissue produces C3, and C3 correlates with BMI, waist size, triglycerides and insulin resistance; it falls with substantial weight loss.
Pregnancy
Estrogen-driven increases of 10 to 50 percent are expected, peaking in the third trimester.
Cancer
Lymphoma, leukemia and solid tumors commonly raise C3 as part of tumor-related inflammation.
Tissue injury: surgery, trauma, heart attack
Complement rises during the days after significant tissue damage, alongside CRP and fibrinogen.
Biliary obstruction and some liver diseases
Cholestatic liver disease and primary biliary cholangitis can increase complement production, in contrast to cirrhosis, which lowers it.
Symptoms that can go with it
- None from the high complement itself
- Fever, pain or swelling from the infection or inflammatory condition driving the rise
- Morning joint stiffness and swelling with rheumatoid or other inflammatory arthritis
- Abdominal pain and diarrhea with inflammatory bowel disease
- Increased waist size, high blood pressure and raised triglycerides with metabolic syndrome
Low C3 and C4
Low complement means consumption, reduced production, loss or an inherited deficiency, and the pattern narrows it down. When both C3 and C4 are low, the classical pathway is being driven hard by immune complexes, and systemic lupus erythematosus is the first consideration: 50 to 75 percent of people with active lupus have low complement at some point, and in lupus nephritis the figure is higher still. The 2019 EULAR/ACR classification criteria give 3 points for low C3 or low C4 and 4 points for both, out of the 10 needed, in someone with a positive ANA. Complement also falls in cryoglobulinemia, infective endocarditis, serum sickness and some drug reactions, and in advanced liver disease because the liver cannot make enough.
Low C3 with a normal C4 points to the alternative pathway. In children and young adults after a sore throat or skin infection, with blood in the urine and swelling, it is the classic picture of post-streptococcal glomerulonephritis, where C3 is low in more than 90 percent during the first two weeks and recovers within six to eight weeks. A C3 that stays low beyond eight to twelve weeks raises the question of C3 glomerulopathy or membranoproliferative glomerulonephritis and usually leads to a kidney biopsy. Low C4 with a normal C3 is most often inherited: roughly a third of people carry at least one non-functional C4 gene copy, giving a mildly low C4 with no disease. When the C4 is very low and there is recurrent swelling of the face, limbs, gut or throat without hives, hereditary angioedema is tested for with C1 inhibitor level and function.
In someone who already has lupus, complement is a monitoring tool more than a diagnostic one. A fall in C3 and C4, particularly with a rising anti-dsDNA, often precedes a flare by weeks and prompts closer follow-up of urine, kidney function and blood counts; recovery of complement levels is one sign that treatment is working. Levels are not perfect: some people with active lupus keep normal complement, and some with inactive disease run persistently low values because of inherited C4 deficiency, so complement is always read alongside symptoms, urinalysis and other antibodies.
Common causes
Systemic lupus erythematosus
Immune complexes activate the classical pathway, consuming C4 then C3; levels fall with disease activity and are lowest in lupus nephritis. Low complement is part of the classification criteria and tracks flares and response to treatment.
Post-streptococcal glomerulonephritis
One to three weeks after strep throat or impetigo, mainly in children: low C3, normal C4, blood and protein in the urine, swelling and high blood pressure. C3 normalizes within 6 to 8 weeks; persistence beyond that points elsewhere.
Membranoproliferative glomerulonephritis and C3 glomerulopathy
Immune-complex MPGN lowers both C3 and C4; C3 glomerulopathy and dense deposit disease lower C3 alone, persistently, often with a C3 nephritic factor that keeps the alternative pathway switched on.
Cryoglobulinemia
Cold-precipitating immune complexes, most often from hepatitis C, consume C4 markedly while C3 stays normal or mildly low; purpura on the legs, joint pain, neuropathy and kidney involvement are typical.
Hereditary angioedema
Deficient or dysfunctional C1 inhibitor lets C1 chew through C4 continuously, so C4 is low in over 95 percent of patients even between attacks; C3 is normal. Confirmed with C1 inhibitor level and function.
Inherited partial C4 deficiency
C4A and C4B gene copy numbers vary from 0 to 4 or more; about a third of people carry at least one null allele, giving a modestly low C4 with normal C3 and CH50 and, usually, no disease. Complete C4 deficiency is very rare and predisposes to lupus.
Infective endocarditis and chronic infections
Immune complexes from persistent bacterial infection (endocarditis, infected shunts, chronic hepatitis B or C) lower C3 and C4 and can cause glomerulonephritis.
Severe liver disease and malnutrition
Cirrhosis and liver failure reduce synthesis of C3 and C4 along with albumin; low complement in cirrhosis is associated with a higher risk of bacterial infection.
Sepsis, DIC and atypical hemolytic uremic syndrome
Massive complement activation in severe sepsis and in complement-mediated thrombotic microangiopathy lowers C3; low C3 in aHUS points to a defect in alternative-pathway regulation.
Inherited deficiency of a single component
Complete C2, C3 or other component deficiencies are rare; C3 deficiency causes recurrent severe bacterial infections from infancy, and C2 deficiency raises the risk of lupus and pneumococcal infection. An undetectable CH50 is the clue.
Symptoms that can go with it
- Often none; low complement is a laboratory finding and the symptoms belong to the cause
- Joint pain and swelling, a butterfly rash across the cheeks, mouth ulcers, hair loss and sun-sensitive rashes with lupus
- Foamy urine, blood in the urine, ankle or facial swelling and rising blood pressure with glomerulonephritis
- Recurrent episodes of swelling of the face, hands, feet, abdomen or throat without itching or hives, lasting 2 to 5 days, with hereditary angioedema
- Purple spots on the lower legs, numbness or weakness in the feet, and joint pain with cryoglobulinemia
- Fever, night sweats, a new heart murmur and tiny red spots under the nails or in the eyes with endocarditis
- Repeated serious bacterial infections from early childhood with inherited C3 deficiency
- Pleuritic chest pain, breathlessness, extreme fatigue and unexplained fevers during a lupus flare
Read it alongside
- ANA — the screening antibody for lupus; low complement with a negative ANA makes lupus unlikely and points to the kidney, liver or inherited causes.
- Anti-dsDNA antibodies — rise as complement falls in active lupus; the pair together is the standard flare-monitoring combination.
- Urinalysis, urine protein and blood — glomerulonephritis is the organ complication most tightly linked to low complement, and the urine is where it shows first.
- Creatinine and eGFR — a rising creatinine with low complement is a reason for urgent nephrology review and often a biopsy.
- CBC and platelets — low white cells, lymphocytes or platelets and hemolytic anemia are common in active lupus and add to the classification score.
- CRP and ESR — ESR is usually high in active lupus while CRP stays modest; a high CRP with low complement suggests infection or serositis.
- C1 inhibitor level and function — the confirmatory tests when low C4 comes with recurrent angioedema.
- Cryoglobulins and hepatitis C antibody — for a very low C4 with purpura, neuropathy or kidney involvement.
What usually happens next
- If both C3 and C4 are low and you have joint pain, rashes, mouth ulcers or fatigue, expect ANA, anti-dsDNA, a urinalysis, creatinine and a CBC; a positive ANA with low complement is usually referred to rheumatology.
- If C3 is low after a recent sore throat or skin infection with blood in the urine or swelling, expect a repeat C3 in 6 to 8 weeks; recovery confirms post-streptococcal nephritis and persistence leads to nephrology referral.
- A low C4 on its own with no symptoms is most often inherited; a CH50, a repeat test when well and a review of any history of unexplained swelling are usually enough, and C1 inhibitor testing is added if angioedema has occurred.
- In known lupus, a drop in complement from your usual level, even within the normal range, is a reason to check urine, kidney function and anti-dsDNA sooner and to review symptoms; ask what your personal baseline is.
- A low CH50 with normal C3 and C4 is repeated with the sample separated and frozen promptly; if it is still undetectable, individual component levels are measured for inherited deficiency.
Diet, lifestyle and natural levers with evidence
What can move C3/C4 once the cause is known. Nothing here replaces finding the cause.
Make sure the sample and timing are right
Moderate evidenceNo diet, supplement or herb raises a complement level that is being consumed; the only way to restore C3 and C4 is to treat the disease using them up. What you can influence is the quality of the measurement. Complement degrades in unseparated blood at room temperature, so CH50 in particular should be drawn where the lab can separate and freeze serum within one to two hours, and results from different laboratories are not directly comparable. Because infection raises C3 and C4 and can mask a fall, a test drawn during an acute illness is best repeated once you are well, and trends in the same lab are more informative than single values.
Weight and metabolic health for a high C3
Preliminary evidenceC3 is produced by fat tissue as well as the liver, and it correlates with BMI, waist circumference, triglycerides and insulin resistance in population studies. Observational studies after bariatric surgery report C3 falling by around 15 to 25 percent over the following year, and smaller reductions with diet-induced weight loss of 5 to 10 percent. This is of no clinical consequence for the complement result itself, but a high C3 in someone who is otherwise well is a reasonable prompt to look at waist size, blood pressure, triglycerides and glucose.
For people with lupus: smoking, sun and vitamin D
Preliminary evidenceLifestyle does not move C3 and C4 directly, but some factors influence the disease activity that consumes them. Smokers with lupus have higher disease activity scores, more skin disease and a poorer response to hydroxychloroquine in observational studies, and quitting is recommended by EULAR. Ultraviolet light triggers flares in photosensitive patients, so broad-spectrum sunscreen and sun avoidance are standard advice. Small randomised trials of vitamin D in vitamin D-deficient lupus patients found modest reductions in disease activity and fatigue, but no consistent change in complement levels.
Echinacea, alfalfa sprouts and other supplements marketed to stimulate immunity have been linked to lupus flares in case reports and are best avoided; discuss any supplement with your rheumatologist.
When to see a clinician promptly
- Swelling of the lips, tongue or throat, trouble swallowing or breathing, with a history of low C4 or unexplained swelling: possible hereditary angioedema airway attack, call emergency services; antihistamines and epinephrine do not work for it.
- Blood in the urine, foamy urine, rapidly swelling legs or face, or a sudden drop in urine output with low complement: possible active glomerulonephritis, needs same-week kidney assessment.
- Chest pain worse on breathing, breathlessness, high fever or confusion in someone with lupus: serositis, infection or a serious flare, needs urgent review.
- New purple spots on the legs, numbness or weakness in the feet, or joint pain with a very low C4, which may be cryoglobulinemic vasculitis.
- Persistent fever, night sweats and a new heart murmur, especially with an artificial valve or recent dental work: possible endocarditis.
- Low C3 and C4 with a positive ANA and symptoms such as joint pain, rash, mouth ulcers or hair loss, for rheumatology referral; low C3 that has not recovered 8 to 12 weeks after post-streptococcal nephritis, for nephrology referral.
Frequently asked questions
What does low C3 and C4 mean?
That the complement system is being used up faster than the liver replaces it, most often by immune complexes. Active systemic lupus, especially lupus nephritis, is the classic cause, followed by membranoproliferative glomerulonephritis, cryoglobulinemia, endocarditis and serum sickness; severe liver disease lowers both by reducing production. Low complement with a positive ANA and symptoms such as joint pain or rash is usually referred to a rheumatologist, and the urine and kidney function are checked promptly.
What is a normal C3 complement level?
Roughly 90 to 180 mg/dL in adults, though laboratories quote ranges as varied as 82 to 185 and 88 to 201 mg/dL, so use the one on your report. C4 is normally about 10 to 40 mg/dL. Both are lower in newborns, higher in pregnancy and during infections, and C4 varies widely between healthy people because of differences in gene copy number.
Does low complement mean I have lupus?
Not on its own. Low C3 and C4 occur in only about half to three-quarters of people with active lupus and also in several other conditions, including post-infectious kidney disease, hepatitis C-related cryoglobulinemia and liver disease, while a modestly low C4 alone is often simply inherited. Lupus is classified using a combination of a positive ANA with clinical features and laboratory findings, of which low complement is one; the 2019 criteria give it 3 to 4 of the 10 points needed.
What does low C4 with a normal C3 mean?
The most common explanation is an inherited low number of working C4 gene copies, found in about a third of people and harmless in nearly all of them. The important conditions to consider are hereditary angioedema, where C4 is low in over 95 percent of patients and is confirmed by C1 inhibitor testing, cryoglobulinemia (often with hepatitis C), and early or mild lupus. A repeat when well, a CH50 and a careful history of swelling episodes usually sort it out.
What does high C3 mean?
Usually that your liver is producing more complement in response to inflammation: an infection, an inflammatory condition such as rheumatoid arthritis or inflammatory bowel disease, recent surgery, pregnancy, obesity or cancer. It does not cause symptoms and does not need treatment. Its main relevance is in people with lupus, where a raised baseline can hide a fall during a flare, so trends matter more than a single value.
What is a CH50 test?
CH50 (total hemolytic complement) measures the ability of your serum to burst antibody-coated red cells, which requires every component of the classical pathway (C1 through C9) to be present and functioning. It is low when complement is consumed and essentially zero when one component is completely missing, which is how inherited deficiencies such as C2 deficiency are found. Because the proteins degrade quickly, the sample must be processed and frozen promptly, and a low CH50 with normal C3 and C4 is often a handling artefact.
Can low complement levels go back to normal?
Yes, when the cause is treated or resolves. After post-streptococcal glomerulonephritis, C3 returns to normal within 6 to 8 weeks on its own. In lupus, C3 and C4 typically rise back into range as treatment brings a flare under control, and their recovery is one sign of response. Inherited partial C4 deficiency does not change, and persistent low C3 beyond three months after a kidney illness needs further investigation.
Why do doctors check C3 and C4 in lupus?
Because complement consumption tracks disease activity. Falling C3 and C4, particularly together with a rising anti-dsDNA, often precede a flare by weeks and are a signal to check urine, kidney function and blood counts more closely; levels recovering on treatment show it is working. Rheumatologists usually check them every three to six months when the disease is stable and more often during flares, in pregnancy, and when treatment is being reduced.
References
- Complement blood test. MedlinePlus, US National Library of Medicine (2023)View source
- Complement system. Merck Manual Professional Version (2024)
- Walport MJ. Complement. First of two parts. New England Journal of Medicine (2001)
- Aringer M, Costenbader K, Daikh D, et al.. 2019 European League Against Rheumatism/American College of Rheumatology classification criteria for systemic lupus erythematosus. Arthritis & Rheumatology (2019)
- Fanouriakis A, Kostopoulou M, Andersen J, et al.. EULAR recommendations for the management of systemic lupus erythematosus: 2023 update. Annals of the Rheumatic Diseases (2024)
- Weinstein A, Alexander RV, Zack DJ. A review of complement activation in SLE. Current Rheumatology Reports (2021)
- Birmingham DJ, Hebert LA. The complement system in lupus nephritis. Seminars in Nephrology (2015)
- Sethi S, Fervenza FC. Membranoproliferative glomerulonephritis: a new look at an old entity. New England Journal of Medicine (2012)
- Busse PJ, Christiansen SC. Hereditary angioedema. New England Journal of Medicine (2020)
- Ricklin D, Reis ES, Lambris JD. Complement in disease: a defence system turning offensive. Nature Reviews Nephrology (2016)
- Hertle E, van Greevenbroek MM, Stehouwer CD. Complement C3: an emerging risk factor in cardiometabolic disease. Diabetologia (2012)
- Diagnosis and classification of poststreptococcal glomerulonephritis. Merck Manual Professional Version (2024)
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