Microscopic Polyangiitis - MPA

Microscopic polyangiitis (MPA) is a necrotizing vasculitis without granulomatous inflammation. MPA is considered a small-vessel antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (Chapel Hill, 2012).

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Diagnosis

Indications for Testing

Unexplained systemic illness with multiple system involvement (renal, neurologic, dermatologic)

Laboratory Testing

  • Antineutrophil cytoplasmic antibodies (ANCA) – antimyeloperoxidase predominant
    • Most effective test to demonstrate vasculitis
    • Positive in at least 40% of patients
    • Absence does not rule out microscopic polyangiitis (MPA)
  • Nonspecific testing – helpful in excluding other diagnoses or identifying organ dysfunction
    • Blood urea nitrogen (BUN)/creatinine or basic metabolic panel – usually normal unless significant renal involvement
    • CBC – mild anemia; rule out infectious process
    • Urinalysis – hematuria, proteinuria, granular or red cell casts
    • C-reactive protein (CRP)
  • Antiglomerular basement membrane antibodies
    • 5-10% of MPA patients test positive
    • Most useful to rule out antiglomerular basement membrane disease (anti-GBM disease)

Histology

Biopsy and pathologist examination remain gold standard for diagnosis (Yates, European League Against Rheumatism [EULAR], 2016)​

Imaging Studies

  • Chest x-ray – bilateral, patchy opacities suggest hemorrhage
  • Computed tomography (CT)
    • Ground glass attenuation
    • Eventual honeycombing when fibrosis occurs

Differential Diagnosis

Background

Epidemiology

  • Incidence – 2.4-10/million in Europe (Yates, European League Against Rheumatism [EULAR], 2016)
  • Age – peak onset is 50-60 years
  • Sex – M>F, 1.8:1
  • Ethnicity – higher prevalence in Europeans (Southern>Northern descent)

Pathophysiology

  • Necrotizing vasculitis of the microscopic vessels (predominantly arterioles, capillaries, or venules) in sites other than kidney
  • Necrotizing crescentic glomerulonephritis on renal biopsy
  • Differentiated from granulomatosis with polyangiitis (GPA) by absence of granulomatous inflammation

Clinical Presentation

  • Constitutional – weight loss, fever, myalgias
  • Otorhinolaryngological – oral ulcers, epistaxis, sinusitis
  • Renal – rapidly progressing glomerulonephritis
  • Cardiovascular – pericarditis, endocarditis
  • Neurological – mononeuritis multiplex, sensorimotor polyneuropathy
  • Dermatological – palpable purpura, papules, ulcers
  • Ophthalmological – scleritis, uveitis, episcleritis
  • Pulmonary – dyspnea, cough, chest pain, pulmonary hemorrhage (may be life threatening)
  • Gastrointestinal – abdominal pain

ARUP Laboratory Tests

Preferred first-line reflex panel for the evaluation of ANCA-associated vasculitis

Components: ANCA, IgG; MPO, IgG; PR3, IgG

Panel includes calcium, carbon dioxide, chloride, creatinine, glucose, potassium, sodium, and urea nitrogen

Preferred test to detect acute phase inflammation (eg, autoimmune diseases, connective tissue disease, rheumatoid arthritis, infection, or sepsis)

Confirm type of vessel involvement or confirm renal glomerulonephritis

May not demonstrate disease due to skip lesions

Renal Pathology Special Studies

Detect glomerular basement membrane (GBM) antibodies in suspected or established anti-GBM disease

If positive, may be useful for monitoring treatment response

Positive result should be confirmed by renal biopsy

Panel includes GBM antibody, IgG by multiplex bead assay, and indirect fluorescent antibody (IFA)

Related Tests

Screening test to evaluate kidney function

Screening test to evaluate kidney function

Assay interference (negative) may be observed when high concentrations of N-acetylcysteine (NAC) are present

Negative interference has also been reported with NAPQI (an acetaminophen metabolite) but only when concentrations are at or above those expected during acetaminophen overdose

Comprehensive panel for the evaluation of ANCA-associated vasculitis

When used in conjunction with other autoantibody tests (ANCA, PR3), may assist in evaluating suspected immune-mediated vasculitis, especially microscopic polyangiitis (MPA)

May be useful when monitoring MPA disease and/or treatment response

Panel tests are available for the workup of suspected vasculitis; refer to ANCA-associated vasculitis profile (ANCA/MPO/PR3) with reflex to ANCA titer, and for patients with a history of vasculitis, refer to ANCA reflex to titer and MPO/PR3 antibodies

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