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Purpose and principlesIndications, selection and cautionsMethod and interpretationClinical next stepsRisks, monitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

ANCA, complement and extended autoantibodies

Rheumatology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

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Time-critical presentation

Pulmonary haemorrhage, rapidly declining kidney function, active urine sediment, mononeuritis multiplex, mesenteric ischaemia, myocarditis or threatened vision requires immediate specialist assessment. Draw relevant tests promptly, but do not delay organ-protective treatment or biopsy planning for antibody results.

Purpose and principles

Use ANCA, complement and extended autoantibodies to refine a clearly defined systemic phenotype, while prioritising tissue, urine and organ assessment and avoiding false reassurance or harm from indiscriminate panels.

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.

Indications, selection and cautions

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.

Method and interpretation

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Clinical next steps

01Organ-threatening syndromeTest and treat in parallelFirst stepPulmonary haemorrhage, glomerulonephritis, digital ischaemia, neuropathy, myocarditis or another rapidly progressive organ lesion.+
  1. 1Stabilise physiology, obtain blood, urine, imaging and infection samples, and contact rheumatology plus the organ specialty immediately.
  2. 2Send PR3/MPO, complement and phenotype-directed serology while arranging biopsy; do not wait for the full panel when delay risks irreversible injury.
  3. 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.+
  1. 1

Risks, monitoring and follow-up

  • 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.

Special situations

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.

Common pitfalls

  1. 01

    Ordering ANCA for nonspecific pain and treating a weak positive as vasculitis.

  2. 02

    Waiting for antibodies before escalating pulmonary haemorrhage or glomerulonephritis.

  3. 03

    Calling low complement diagnostic of lupus without considering infection or liver disease.

  4. 04

    Diagnosing antiphospholipid syndrome from one antibody result without a qualifying clinical event.

  5. 05

    Starting major immunosuppression without parallel infection assessment and planned tissue diagnosis when feasible.

Practice

Two practice questions

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Question 1 of 2
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RheumatologyOriginal SBA

Pulmonary–renal emergency

A patient has haemoptysis, falling haemoglobin, bilateral lung opacities, haematuria and rapidly rising creatinine. PR3 and MPO results will not return until tomorrow. What is the best next principle?

Choose one answer, then submit
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Sources and review status4 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • British Society for Rheumatology guidelinesSpecialist guidance for ANCA-associated vasculitis and systemic autoimmune disease.
  • NICE NG203 chronic kidney diseaseUrinalysis, proteinuria and urgent renal assessment.
  • NHS antiphospholipid syndrome diagnosisClinical events and repeat laboratory testing required to establish APS.
  • KDIGO glomerular diseases guidelineInternational authoritative guidance for glomerulonephritis diagnosis and organ management.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom

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