Complement testing (Complement studies (C3, C4, CH50/CH100, AP50))
Blood tests that measure complement proteins (such as C3 and C4) and how well the complement system works, used for certain recurrent infections, some autoimmune conditions and unexplained swelling.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- It measures complement proteins (such as C3 and C4) or how well the complement system works (CH50/AP50).
- It is used for specific reasons — recurrent infections, autoimmune disease such as lupus, or unexplained swelling — not as a general check.
- Activity tests are fragile and must reach the laboratory quickly, or they can read falsely low.
- A result is a clue, not a diagnosis: low complement has several possible causes and needs specialist interpretation.
A plain-English summary. The detail — including risks and recovery — is below.
At a glance
A general guide. Your specialist will give you advice for your situation.
Can help diagnose and monitor autoimmune conditions such as lupus, where complement tracks disease activity
It is not a general 'immune health' test and has no role without a specific question.
Blood is taken from a vein in a few minutes. Activity tests need prompt handling by the lab.
A clinician who interprets complement in the context of the specific question.
Blood is taken from a vein in a few minutes. Activity tests need prompt handling by the lab.
Press on the site briefly. Mild bruising is normal and you can carry on as usual.
C3 and C4 levels are usually available; activity-test results may take a little longer.
Individual complement components are measured to find the missing one, and the test may be repeated.
What is complement testing?
Complement is a group of more than 30 blood proteins that work with antibodies to fight infection, clear away damaged cells and help control inflammation. Complement testing measures either the amount of individual proteins — most often C3 and C4 — or how well the whole system works, using activity tests such as CH50 (also called CH100) for the classical pathway and AP50 for the alternative pathway.
These tests are used in a few specific situations: to investigate recurrent infections that might point to a complement deficiency, to help diagnose and monitor autoimmune conditions such as lupus (SLE), and to look into unexplained swelling (angioedema), where the C1-inhibitor protein may be involved.
What the result means depends entirely on the situation. In lupus, C3 and C4 often fall when the disease is active, so they are used to track it. A low activity test can point to a missing complement component, which can occasionally explain a tendency to certain serious infections, including meningococcal disease.
Complement levels do not, by themselves, tell you what is wrong. They are a clue that the immune system is involved, which a specialist interprets alongside your symptoms and other tests.
Types, options & approaches
There may be different ways to do this. The right approach depends on the clinical question and your circumstances.
Options at a glance
These are the main approaches described in this guide. The right option depends on the diagnosis, your goals and what your clinician thinks is safe.
C3 and C4
The most commonly measured individual proteins. They often fall when lupus is active, so they are used to help diagnose and monitor it, and can change in other conditions.
Total complement activity (CH50 / CH100)
Tests whether the whole classical pathway works. A low or absent result can indicate a missing component and prompts testing of individual proteins.
Alternative pathway activity (AP50)
Checks the alternative pathway. Used alongside CH50 to work out where in the system a deficiency might lie.
C1-inhibitor (level and function)
Measured when hereditary or acquired angioedema is suspected. Both the amount and how well it works can be checked, as either can be abnormal.
Preparing for your test
- Bring details of your symptoms: recurrent infections, episodes of swelling, or features of an autoimmune condition.
- If swelling is the concern, note where it happens, how long it lasts and any triggers.
- List your medicines and any family history of complement deficiency, lupus or angioedema.
- Mention any recent infection, which can temporarily change complement levels.
- Be aware that activity tests need the sample handled quickly, so they are usually done where the lab is close by.
- Bring previous complement or autoimmune results if you have them.
- No fasting is usually needed, but check any specific instructions.
What happens
A nurse or phlebotomist takes a blood sample from a vein in your arm. For protein levels such as C3 and C4, the sample is handled in the usual way and results come back within days.
Activity tests (such as CH50) are more delicate. The complement proteins can break down if the sample is delayed or not kept correctly, which can make the result read falsely low. For this reason, activity testing is usually done where the sample can reach the laboratory quickly.
Your clinician interprets the results in the context of why the test was done. In lupus, low C3 and C4 may reflect active disease. A genuinely low activity test prompts measurement of individual components to find the missing one. For swelling, C1-inhibitor level and function are assessed together.
Is this test right for me?
A good consultation should explore whether it's the right choice for you now — including reasons to wait or consider something else.
May not be suitable if…
- It is not a general 'immune health' test and has no role without a specific question.
- A delayed or mishandled activity sample cannot be reliably interpreted.
- On its own it cannot diagnose lupus or any single condition.
- If symptoms point clearly elsewhere, complement testing may add little.
- Testing during a major intercurrent illness may give results that only reflect that illness.
Delay or rearrange if…
- The sample cannot reach the laboratory quickly enough for an activity test.
- You currently have an active infection that may temporarily alter levels.
- The clinical question has not yet been defined with a specialist.
- Previous results are awaited that could guide testing.
- An acute illness may distort the picture and a more stable time would be better.
Alternatives to discuss
- No testing if there is no recurrent infection, autoimmune or swelling concern.
- Autoantibody testing if the main question is autoimmune disease.
- Antibody-level and vaccine-response testing if recurrent infection is the issue.
- A standard NHS pathway via your GP or specialist referral.
- Specific allergy or other assessment if swelling has another likely cause.
Before you decide
Use this as a shared-decision checklist. The aim is not just “can this be done?”, but whether it is right for you, now, with the risks and alternatives clearly understood.
What matters most to me?
Think about symptoms, daily life, work, caring responsibilities, sport, fertility, travel, appearance and anxiety — the right choice depends on your priorities, not just the medical facts.
What are all my options?
Ask about waiting, monitoring, medicines, rehabilitation, a smaller or larger procedure, a different test, NHS referral, or a second opinion where that would help.
What would make me pause?
Active infection, pregnancy, unstable medical problems, smoking, medicines that increase bleeding, poor support at home, or feeling pressured are all reasons to slow down and get tailored advice.
What happens if I do nothing today?
For some problems, waiting is safe; for others, delay can make treatment harder. A good consultation should explain the trade-off in plain English.
Benefits
- Can help diagnose and monitor autoimmune conditions such as lupus, where complement tracks disease activity
- Can identify a complement deficiency behind certain recurrent or serious infections
- Can help diagnose hereditary or acquired angioedema as a cause of unexplained swelling
- Guides preventive steps such as vaccination against meningococcal infection where a deficiency is found
- A normal, well-handled result can help rule out complement problems in the relevant setting
Risks & complications
- Minor bruising or soreness where blood was taken
- A falsely low activity result if the sample is delayed or mishandled, needing a repeat
- A borderline result that needs interpreting in context
- Anxiety while waiting for results
- Feeling faint during the blood test
- A low result with several possible causes that needs further testing to explain
- Repeat testing because levels change with disease activity or infection
- Infection at the needle site
- A misleading result that delays the right diagnosis if acted on without specialist input
The most practical pitfall is the fragility of activity tests: a delayed or warm sample can read falsely low, so handling matters. Beyond that, low complement has several causes — it can be consumed in active autoimmune disease, lost through the kidneys, or inherited — so the number alone does not give a diagnosis. Ask why the test is being done and what a low result would change.
Published figures to discuss
Complement testing is a low-risk blood test with no procedural complication rates. The clinically important uncertainties are about interpretation and sample quality: activity tests can read falsely low if delayed, low complement has several causes, and individual inherited deficiencies are rare and vary in how they present. Because reliable figures depend heavily on the population and condition, no single percentage is quoted here.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Low C4 in hereditary or acquired C1-inhibitor deficiency | Common during active bradykinin angioedema disorders | C4 is a screening clue, but C1-inhibitor level and function are needed for diagnosis. | Guide sourcesClinical context |
| Terminal complement deficiency and meningococcal infection | Rare deficiency, but markedly increased meningococcal risk | Vaccination and urgent fever advice are essential if a terminal complement defect is confirmed. | Interpretation of serological complement biomarkers in disease (PMC review)pmc.ncbi.nlm.nih.govSource-linked context |
| Low complement from active immune-complex disease | Common in active lupus nephritis and some vasculitides | Complement results should be interpreted with urine, kidney function, antibodies and symptoms. | Guide sourcesClinical context |
| Sample handling affecting complement results | Recognised pre-analytical issue | Unexpected results may need repeat testing with correct handling before major decisions. | Guide sourcesClinical context |
These are literature figures, not a personalised prediction. Your own risks and likely benefits depend on your circumstances, your health, and how your care is carried out and followed up.
What happens afterwards
There is no physical recovery from having blood taken. 'Afterwards' is about waiting for results and understanding what a low or normal complement result means in your situation.
- A small bruise or tender spot where blood was taken
- No change in how you feel — the test does not treat anything
- Being asked to repeat an activity test if the sample was delayed
- Levels that move up and down with disease activity in autoimmune conditions
- A normal result that still needs reading alongside your symptoms
Aftercare
- Keep the dressing on for an hour or so if the site is sore.
- If an activity test may have been delayed, ask whether it should be repeated.
- Note how and when you will receive your results.
- Do not change treatment based on a result until a clinician advises.
- Bring your symptom history and any previous results to the discussion.
- If a deficiency is found, ask about protective vaccinations and what to watch for.
- Keep copies of results, especially if complement is being used to monitor a condition.
- Symptom history written down (infections, swelling, autoimmune features)
- Details of any swelling episodes and triggers
- Medicine list and family history noted
- Previous complement or autoimmune results gathered
- Awareness that activity tests need prompt handling
- Clear note of how results will be communicated
- Questions about what a low result would change
⚠ Get urgent help if…
- A high temperature with a stiff neck, severe headache, sensitivity to light or a rash that does not fade — this could be meningitis; call emergency services
- Feeling very unwell with a high temperature, shivering or confusion — seek urgent help
- Swelling of the lips, tongue, throat, or difficulty breathing or swallowing — call emergency services
- Severe abdominal pain with swelling episodes (possible angioedema attack)
- A serious infection that is spreading or not responding to treatment
- Feeling faint or unwell after the blood test itself
Who to contact: your clinician, clinic or test provider first (keep their number to hand). For urgent advice when you can't reach them, call NHS 111. In an emergency, call 999.
General guidance — it doesn't replace the specific advice your specialist gives you.
Results & realistic expectations
What a complement result means depends on why it was done. In lupus, low C3 and C4 often reflect active disease and are used to monitor it, so a fall may prompt a review rather than alarm in itself. A genuinely low activity test (CH50 or AP50) can indicate a missing complement component, which occasionally explains a tendency to certain serious infections such as meningococcal disease.
A low result is not a single diagnosis — complement can be consumed in active disease, lost through the kidneys, or be inherited, and a falsely low activity test can simply reflect a delayed sample. A normal result is reassuring in the relevant setting but still read alongside your symptoms. The most useful result is one that changes your care, so it is fair to ask your specialist what your result actually means.
Complement levels are not fixed. In autoimmune conditions they rise and fall with disease activity, which is exactly why they are used for monitoring. They can also change with infection and kidney function. An inherited deficiency, by contrast, is lifelong and may mean ongoing protective measures. A result is therefore interpreted as a snapshot or a trend, depending on the situation.
Related tests, treatments or support
Complement tests are often done alongside autoantibody tests (such as ANA and anti-dsDNA) when lupus is suspected, and with antibody-level testing when recurrent infections are being investigated. When swelling is the concern, C1-inhibitor level and function and C4 are assessed together. Your clinician chooses the combination based on the question.
Follow-up & long-term care
Follow-up depends on the reason for testing. In autoimmune disease, complement may be checked repeatedly to monitor activity. A confirmed deficiency usually leads to specialist immunology review, vaccination advice and a plan for managing infection risk. Angioedema findings lead to a tailored management plan. A good service makes clear who interprets results and arranges next steps.
- Repeat testing to monitor disease activity in autoimmune conditions
- Protective vaccinations (for example against meningococcal infection) if a deficiency is found
- A clear action plan for infections or swelling episodes where relevant
- Ongoing specialist review for inherited complement deficiencies
Repeat, follow-on and what comes next
- A falsely low activity result from a delayed sample often needs repeating.
- A genuinely low activity test leads on to testing individual components.
- Levels change with disease activity, so monitoring may show them rise and fall.
- A normal result does not exclude every complement-related condition.
Ask what happens if the result is unclear or needs repeating, and what is included if further tests or follow-up are needed.
What good aftercare looks like
- A clinician who interprets complement in the context of the specific question.
- Reliable, prompt sample handling for activity tests, with repeat if needed.
- Vaccination advice and an infection action plan if a deficiency is confirmed.
- A clear monitoring plan where complement is tracking disease activity.
- Onward referral to specialist immunology where appropriate.
What affects the cost
Costs vary a great deal between people and providers, and we don't publish prices. What matters is understanding what drives the cost and making sure your quote is complete. The main things that affect it:
- Whether individual proteins (C3, C4) or activity tests (CH50/AP50) or both are done
- Additional testing of individual components if activity is low
- C1-inhibitor level and function testing if angioedema is being investigated
- Logistics of getting fragile activity samples to the laboratory quickly
- The consultant's fee for interpreting results in context
- Any follow-up consultation or repeat monitoring
- Which complement tests are included (levels, activity, C1-inhibitor)
- What happens, and what it costs, if a low activity test needs repeating
- The laboratory fee and the consultant's interpretation fee, separately
- Whether follow-up monitoring is included if complement tracks a condition
- Whether further component testing is extra if a deficiency is suspected
- Who is responsible for interpreting results and arranging next steps
On the NHS? Complement testing is available on the NHS when clinically indicated; private testing is sometimes used for faster access, but activity tests need prompt sample handling and all results need specialist interpretation.
You're entitled to your total cost in writing — including reports, follow-up and what happens if the result is inconclusive — before you decide.
Consent traps and marketing red flags
These are not small details. They are often where patients lose choice, time to reflect, or realistic expectations.
Consent traps
- Not explaining that an activity test can read falsely low if mishandled.
- Treating a single low complement as a diagnosis without finding the cause.
- Not arranging protective vaccination advice when a deficiency is found.
- Failing to explain why the test is being done and what it would change.
- Not making clear who interprets results and follows them up.
Marketing red flags
- Selling complement testing as part of a broad consumer 'immune panel'.
- Implying a single result diagnoses lupus or another condition.
- Not addressing the prompt-handling requirement for activity tests.
- Suggesting complement testing explains general tiredness or minor infections.
- Overstating what a normal or abnormal result proves on its own.
Choosing a specialist safely
- Check the specialist is on the GMC Specialist Register for this area.
- Make sure they work at a CQC-registered service, and look for membership of the relevant Royal College or professional body.
- You're entitled to time to consider and to have your questions answered before you agree — the specialist who looks after you should explain it, not a salesperson.
- Be wary of pressure: time-limited offers or deposits taken before you've had time to think are red flags, not bargains.
- You're entitled to your total cost in writing — including any follow-up — before you decide.
Questions to ask your medical professional
Take this to your consultation. A good specialist will welcome every one of these.
- Why are you testing complement in my case, and what would a low result change?
- Was the activity test handled quickly enough, or should it be repeated?
- Could a low result reflect active disease rather than a deficiency?
- If I have a deficiency, which vaccinations and precautions do I need?
- If complement is monitoring my condition, how often will it be checked?
- What other tests do you need alongside this one?
- Are you on the GMC Specialist Register for this area, and which Royal College or professional body are you a member of?
- Will you be the specialist who carries out my test, and who looks after me afterwards?
- What are the risks for someone like me, and how often do your own patients have a problem or need it repeated or redone?
- What does a realistic result look like — and what can this test not achieve?
- What are my options, including waiting, doing nothing for now, or choosing a different approach?
- Can I have written information, results and aftercare instructions in a format I can use, including any accessibility or communication support I need?
- What is the total cost in writing, including any follow-ups, and how much time do I have to decide?
Frequently asked questions
Why has my doctor ordered a complement test?
What does a low complement level mean?
Why does the sample need to get to the lab quickly?
Does a complement deficiency mean I'll get serious infections?
How does complement help monitor lupus?
Can I have this test on the NHS?
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How we made this page
Medically reviewed by a GMC-registered consultant. Written in plain English, checked against NHS, NICE, GMC and relevant Royal College / specialist-society guidance, and kept under review. No clinic paid to appear on this page, and we publish no pricing. This is general information to help you prepare — it is not a substitute for advice from your own clinician. How we review our guides →
Source hierarchy: UK regulator and NHS/NICE guidance first, then relevant Royal College or specialist-society guidance, then peer-reviewed evidence for procedure-specific figures where available.
Sources & standards: Lab Tests Online UK — Complement Interpretation of serological complement biomarkers in disease (PMC review) North West London Pathology NHS — Complement C3 and C4 Immunodeficiency UK — Primary immunodeficiency information British Society for Immunology — Clinical immunology guidelines
Reviews reflect patients' experience of care, not clinical outcomes. For procedure volumes and outcome data see PHIN.
Last medically reviewed 2026-09-21. Spotted something wrong or out of date? Report an error in this guide.
Related guides: Immune system blood tests · Autoantibody / autoimmune testing · Immunoglobulin level testing · IgG subclass testing · Vaccine response testing