Health condition
Complement testing in aHUS
Complement testing in suspected aHUS spans functional blood tests, autoantibody assays and a genetic panel of complement regulators; results refine the diagnosis, relapse risk, treatment duration and family implications, though normal results do not exclude the disease.
Why trust this guide
- 01
Editorial team
Written by our editorial team from the UK sources listed on each page.
- 02
Sourced from guidance
Checked against current NHS and UK specialist society guidance at the time of review.
- 03
Current for 2026
Reviewed on 2026-09-27 with the next review scheduled for 2027-09-27.
Key facts
The essentials at a glance.
The symptoms to recognise, when to seek help, which specialist to see, and what treatment usually involves.
-
A gene variant is found in roughly half of aHUS patients - a negative panel does not rule it out
-
Key genes include CFH, CFI, CD46 (MCP), C3, CFB and THBD, plus anti-factor H antibodies
-
Samples for functional tests are best drawn before plasma therapy or transfusion
-
Results influence how long complement inhibition continues and inform kidney transplant planning
Understanding the condition
What it is and what happens next.
What causes it, how it is diagnosed, and the treatment routes available in the UK.
What it is and how it feels
Testing operates on three levels. Functional assays - C3, C4, factor H and I levels, CH50 - offer quick but imperfect clues at the bedside. Anti-factor H antibody testing matters especially in younger patients, because antibody-driven disease may respond to immunosuppression. The definitive layer is genetic sequencing of the complement regulatory genes, which takes weeks and should never delay treatment.
Diagnosis and treatment routes
Interpretation needs expertise: variants of uncertain significance are common, CD46-associated disease relapses less and may allow earlier treatment withdrawal, while factor H variants carry higher recurrence risk, shaping decisions about stopping therapy and about live-related kidney donation - relatives being considered as donors need testing first. UK centres with complement laboratories coordinate this workup. In many parts of the UK, routine NHS referrals for this kind of problem can take months, whereas an initial appointment with a private consultant is typically available within around two weeks.
Seeing a specialist
Who to see, and how quickly you can be seen.
The specialist for this condition is usually a consultant nephrologist. Your GP can refer you on the NHS, or you can arrange a private consultation directly.
A quiet reminder
This guide is for information, not medical advice.
Your GP knows your history and can tell you which parts apply to you. If in doubt, see them - and seek urgent care for any of the emergency symptoms described above.
Frequently asked
Common questions about Complement testing in aHUS.
Quick, plain-English answers to what people ask us most.
-
What is complement testing in aHUS?
Complement testing in atypical haemolytic uraemic syndrome (aHUS) involves specific blood tests to assess the activity and genetic basis of the complement system, a part of the immune system that becomes abnormally overactive in this rare, serious kidney condition. Identifying specific complement abnormalities helps confirm the diagnosis, guide treatment choice, and inform prognosis. This detailed testing is an important part of comprehensive management for this condition.
-
Why is complement testing important in aHUS?
aHUS is caused by abnormal, excessive activation of the complement system, often due to specific genetic mutations affecting complement regulation, leading to damage to small blood vessels, particularly in the kidneys. Complement testing helps confirm this underlying mechanism, distinguishes aHUS from other similar-looking conditions with different causes and treatments, and can identify the specific genetic abnormality present in many cases. This detailed understanding directly informs the most effective treatment approach for aHUS.
-
What does complement testing involve?
Testing typically includes blood tests measuring levels and activity of various complement proteins, and genetic testing to look for specific mutations in genes that regulate the complement system, which are found in a significant proportion of aHUS cases. Testing may need to be repeated over time, since complement activity can change, particularly around the time of an acute aHUS episode. A specialist nephrology or complement-related disease centre typically coordinates this detailed testing.
-
How do complement test results guide treatment?
Complement testing results help confirm the aHUS diagnosis and can guide decisions about starting and, importantly, the potential duration of specific complement-blocking treatments (such as eculizumab or ravulizumab), which have transformed outcomes for this condition. Certain genetic findings may also help predict the likelihood of aHUS recurrence, which is relevant for decisions such as kidney transplantation. This detailed genetic and functional information increasingly allows more personalised treatment decisions.
-
Should family members be tested?
This depends on the specific genetic findings — if a specific complement-regulating gene mutation is identified, genetic counselling can help determine whether testing family members is appropriate and what the implications would be, since not everyone carrying a relevant mutation develops aHUS (a second trigger is often needed). A genetic counsellor experienced in this specific area can provide detailed guidance tailored to your specific family situation. This is a nuanced area given the incomplete penetrance of these genetic changes.
-
When should I see a specialist?
If aHUS is suspected or diagnosed, prompt specialist nephrology assessment including complement testing is important, as this is often a serious, sometimes rapidly progressive condition. Given the seriousness of aHUS, this typically requires urgent specialist input; contact your treating hospital team promptly, with a private nephrology consultation available within about 2 weeks if additional specialist input or a second opinion is wanted.