ANCA, complement and extended autoantibodies

01Purpose and principlesWhat the assessment is for and the core concepts behind it.

ANCA-associated vasculitis is a clinicopathological diagnosis. PR3 and MPO specificity can support granulomatosis with polyangiitis, microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis, but association is not identity. Test when the phenotype includes glomerulonephritis, pulmonary haemorrhage or nodules, destructive ENT disease, scleritis, purpura, mononeuritis multiplex or otherwise unexplained systemic inflammation with organ injury.

Complement proteins fall through consumption or reduced production. Low C3 and C4 can support active immune-complex lupus, cryoglobulinaemia or infection; isolated patterns may suggest different pathway activation. Always interpret with liver function, renal loss, immunoglobulins and the assay range. Complement values may track selected disease activity but cannot independently distinguish flare from infection.

Extended antibodies are chosen by phenotype. Anti-dsDNA and ENA refine an ANA-associated syndrome; myositis-specific antibodies can help classify inflammatory muscle and lung phenotypes; systemic-sclerosis antibodies inform subsets and surveillance; antiphospholipid tests investigate thrombosis or defined pregnancy morbidity. Positive results require awareness of assay limitations, cross-reactivity and the possibility that more than one disease or infection is present.

Immediate management follows the threatened organ. In pulmonary–renal syndrome obtain urinalysis, creatinine, FBC, coagulation, chest imaging and infection samples, and involve renal, respiratory and rheumatology teams. Seek tissue from kidney, skin, nerve, lung or another involved site when it will clarify diagnosis. Severe infection can mimic and trigger vasculitis; cultures and antimicrobial assessment must accompany immunosuppression planning.

Key points

ANCA testing is appropriate when small-vessel vasculitis is clinically plausible; it is not a general screen for fatigue, arthralgia or an isolated mildly raised CRP.
Interpret antigen-specific PR3-ANCA and MPO-ANCA with phenotype, urinalysis, renal function, chest findings, ENT disease, neuropathy and histology where feasible.
A positive ANCA can occur with infection, inflammatory bowel disease, autoimmune liver disease, medicines and other conditions, while ANCA-negative vasculitis remains possible.
Low C3 and C4 suggest complement consumption but occur in infection, immune-complex disease and other disorders; the pattern and clinical setting determine meaning.
Anti-dsDNA, ENA, antiphospholipid antibodies, myositis and systemic-sclerosis antibodies should follow compatible organ and examination findings rather than a blanket panel.
Urinalysis, urine protein quantification, creatinine trend and microscopy can reveal dangerous renal disease before the complete serology profile returns.
Antiphospholipid syndrome requires a qualifying clinical event and persistent laboratory positivity on tests at least 12 weeks apart; one result is not sufficient.
Biopsy can distinguish vasculitis, immune-complex disease, infection and mimics and should be coordinated early without delaying treatment in a life-threatening presentation.

02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.

Pulmonary–renal syndromeRed flag

Haemoptysis, falling haemoglobin, diffuse lung infiltrates, haematuria, proteinuria and rising creatinine indicate a medical emergency requiring parallel vasculitis and infection assessment.

ANCA-compatible systemic pattern

Destructive ENT disease, scleritis, purpura, nodules, mononeuritis multiplex or glomerulonephritis creates meaningful pre-test probability for PR3 and MPO testing.

Immune-complex pattern

Low complement with rash, arthritis, cytopenia, nephritis or cryoglobulinaemic features redirects work-up toward lupus, infection or another immune-complex disorder.

Antiphospholipid phenotype

Objectively confirmed thrombosis or defined pregnancy morbidity justifies lupus-anticoagulant, anticardiolipin and anti-beta-2-glycoprotein-I testing with planned confirmation.

Myositis or sclerosis phenotype

Objective proximal weakness, raised muscle enzymes, characteristic rash, interstitial lung disease, Raynaud phenomenon, skin thickening or digital ischaemia supports targeted antibody panels.

03Method and interpretationA systematic approach to the test and its findings.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
PR3-ANCA and MPO-ANCAFirst step
Why
Support classification of suspected ANCA-associated vasculitis within a compatible organ phenotype.
Interpretation and limitations
Strength and antigen specificity alter probability but are not diagnostic alone. Negative results cannot exclude all vasculitis, especially limited or treated disease.
02
C3 and C4 complement
Why
Detect complement consumption that supports selected immune-complex, infectious or complement-mediated processes.
Interpretation and limitations
Use the pattern, trend, liver function and protein loss. Low levels are not specific for lupus and normal values do not exclude organ-threatening autoimmune disease.
03
Urinalysis, protein quantification and renal profile
Why
Detect glomerular injury and define urgency before serology is complete.
Interpretation and limitations
Blood and protein with a rising creatinine or active sediment requires urgent renal assessment; bland urine makes active glomerulonephritis less likely but not impossible.
04
Phenotype-directed extended antibodies
Why
Refine suspected lupus, antiphospholipid, myositis, systemic-sclerosis or overlap disease.
Interpretation and limitations
Order the smallest panel that answers the syndrome, then interpret individual specificity, titre, repeat requirements and assay limitations with specialist support.
05
Biopsy of an involved organ
Why
Demonstrate vasculitis, immune-complex deposition, tissue injury or an alternative such as infection and malignancy.
Interpretation and limitations
Choose the safest high-yield site and coordinate timing with treatment; negative sampling can reflect patchy disease and must be reconciled with the complete phenotype.

04Clinical next stepsHow the result changes management or prompts escalation.

01Organ-threatening syndromeTest and treat in parallelFirst stepPulmonary haemorrhage, glomerulonephritis, digital ischaemia, neuropathy, myocarditis or another rapidly progressive organ lesion.

1Stabilise physiology, obtain blood, urine, imaging and infection samples, and contact rheumatology plus the organ specialty immediately.
2Send PR3/MPO, complement and phenotype-directed serology while arranging biopsy; do not wait for the full panel when delay risks irreversible injury.
3Agree immunosuppression and antimicrobial coverage with specialists using illness severity, infection probability and tissue findings, then monitor organ response closely.

02Positive ANCA without clear vasculitisChallenge the pre-test contextPR3 or MPO antibody is reported in a patient lacking a convincing small-vessel-vasculitis syndrome.

1Reassess ENT, lung, kidney, skin, nerve and systemic features and review urinalysis, renal function and imaging for objective organ involvement.
2Consider infection, inflammatory bowel or liver disease and medicine-related positivity, and avoid labelling vasculitis from the test alone.
3Discuss persistent high-specificity results with rheumatology and plan observation or targeted tissue assessment rather than empirical toxic treatment.

03Thrombotic or pregnancy phenotypeConfirm persistent antiphospholipid antibodiesThere is objectively documented thrombosis or defined pregnancy morbidity with no immediate bleeding emergency.

1Send lupus anticoagulant, anticardiolipin and anti-beta-2-glycoprotein-I tests, recording anticoagulants and acute illness that may interfere.
2Manage the clinical event according to thrombosis or obstetric pathways without declaring antiphospholipid syndrome from one sample.
3Arrange repeat testing at least 12 weeks after the first sample and specialist interpretation, integrating antibody profile and recurrence risk into long-term treatment.

05Risks, monitoring and follow-upComplications, safety checks and further assessment.

Trend creatinine, urine protein, sediment, haemoglobin and oxygenation closely in pulmonary–renal disease.
Monitor the clinically involved organ rather than serial antibody titres alone.
Repeat antiphospholipid antibodies at least 12 weeks after the first sample before confirming persistent positivity.
Review cultures and infection markers during immunosuppression because flare and infection can coexist.
Record assay, antigen specificity and laboratory range when comparing results over time or across centres.

06Special situationsVariants, exceptions and circumstances that change the usual approach.

ANCA supports a syndrome

The antibody changes probability after the organ pattern is defined; it cannot create vasculitis in a patient with no compatible clinical evidence.

Urine is time critical

Dipstick blood and protein can expose glomerular injury immediately and should not wait for specialised serology.

Complement is contextual

Consumption, liver synthesis, renal loss and inherited factors all influence concentration, so isolated low values require explanation.

Persistence matters in APS

Acute illness and anticoagulants can affect testing, and classification requires a clinical event plus appropriately confirmed laboratory positivity.

Tissue can prevent toxic error

Biopsy may reveal infection, malignancy or a different immune mechanism and thereby substantially change treatment intensity.

07Common pitfallsFrequent interpretation and management errors.

01
Ordering ANCA for nonspecific pain and treating a weak positive as vasculitis.
02
Waiting for antibodies before escalating pulmonary haemorrhage or glomerulonephritis.
03
Calling low complement diagnostic of lupus without considering infection or liver disease.
04
Diagnosing antiphospholipid syndrome from one antibody result without a qualifying clinical event.
05
Starting major immunosuppression without parallel infection assessment and planned tissue diagnosis when feasible.