Microscopic Polyangiitis - MPA

Diagnosis

Indications for Testing

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

Laboratory Testing

  • Nonspecific testing – helpful in excluding other diagnoses or identifying organ dysfunction
    • BUN/creatinine – usually normal unless significant renal involvement
    • CBC – mild anemia; rule out infectious process
    • Urinalysis – hematuria, proteinuria, granular or red cell casts
    • Erythrocyte sedimentation rate (ESR)/C-reactive protein (CRP) – elevated >50% of the time
    • ANCA – antimyeloperoxidase predominant
      • Most effective test to demonstrate vasculitis
      • Positive in at least 40% of patients
        • Absence does not rule out MPA
  • Antiglomerular basement membrane antibodies – 5-10% of MPA patients are positive for this
    • Most useful to rule out antiglomerular basement membrane disease (anti-GBM disease)

Histology

  • Small and mid-size artery, venule, and capillary necrotizing vasculitis (nongranulomatous)
  • Pulmonary disease is often capillaritis – absence of immune complex deposition
  • Renal biopsy – focal segmental necrotizing, glomerulonephritis with crescents
    • Identical to biopsy in granulomatosis with polyangiitis and eosinophilic granulomatosis with polyangiitis

Imaging Studies

  • Chest x-ray – bilateral, patchy opacities suggest hemorrhage
  • CT – ground glass attenuation; eventual honeycombing when fibrosis occurs

Differential Diagnosis

Clinical Background

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

Epidemiology

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 – pulmonary hemorrhage, dyspnea, cough, chest pain
    • Pulmonary hemorrhage may be life threatening
  • Gastrointestinal – abdominal pain

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Urea Nitrogen, Serum or Plasma 0020023
Method: Quantitative Spectrophotometry

Assess renal function

   
Creatinine, Serum or Plasma 0020025
Method: Quantitative Enzymatic

Assess renal function

   
CBC with Platelet Count and Automated Differential 0040003
Method: Automated Cell Count/Differential

May help in ruling out infectious process

   
Urinalysis, Complete 0020350
Method: Reflectance Spectrophotometry/Microscopy

Screen for hematuria, proteinuria, and RBC casts

   
Sedimentation Rate, Westergren (ESR) 0040325
Method: Visual Identification

Initial evaluation for suspected vasculitis

   
C-Reactive Protein 0050180
Method: Quantitative Immunoturbidimetry

Initial evaluation for suspected vasculitis

   
Anti-Neutrophil Cytoplasmic Antibody with Reflex to Titer and MPO/PR-3 Antibodies 2002068
Method: Semi-Quantitative Indirect Fluorescent Antibody/Semi-Quantitative Multiplex Bead Assay

Initial evaluation for suspected ANCA (+) vasculitis

Components include anti-neutrophil cytoplasmic antibody, IgG; myeloperoxidase antibody; and serine proteinase 3 antibody

If screen is positive, titer and MPO/PR-3 antibodies testing will be added to aid in antibody determination

   
Renal Pathology Special Studies

Confirm type of vessel involvement or confirm renal glomerulonephritis

May not demonstrate disease due to skip lesions

 
Glomerular Basement Membrane Antibody, IgG by Multiplex Bead Assay and IFA 2008403
Method: Semi-Quantitative Multiplex Bead Assay/Qualitative Indirect Fluorescent Antibody

Rule out anti-GBM disease

   
Additional Tests Available
 
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Anti-Neutrophil Cytoplasmic Antibody, IgG 0050811
Method: Semi-Quantitative Indirect Fluorescent Antibody

If ANCA screen detects antibodies ≥1:20 dilution, titer to end point is added

Myeloperoxidase Antibody 0050526
Method: Semi-Quantitative Multiplex Bead Assay