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Test ID: MPO    
Myeloperoxidase Antibodies, IgG, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Evaluating patients suspected of having immune-mediated vasculitis, especially microscopic polyangiitis (MPA), when used in conjunction with other autoantibody tests (see Cautions)

 

May be useful to follow treatment response or to monitor disease activity in patients with MPA

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Myeloperoxidase (MPO) enzyme is found in neutrophil primary granules and monocyte lysosomes. MPO catalyzes the conversion of hydrogen peroxide to hypochlorite and hypochlorous acid. MPO is encoded by a single gene that undergoes posttranslational modification to produce the active enzyme found in leukocytes.

 

Autoantibodies to MPO (MPO antineutrophil cytoplasmic antibodies: ANCA) occur in several diseases and may be involved in the pathogenesis of vascular inflammation in patients with microscopic polyangiitis (MPA).(1,2) Patients with MPA often develop MPO ANCA and may present with azotemia secondary to glomerulonephritis (pauci-immune necrotizing glomerulonephritis). MPO ANCA are not specific for MPA, and also may be detected in patients with systemic lupus erythematosus with or without lupus nephritis, Goodpasture syndrome and Churg-Strauss syndrome. Lupus nephritis and Goodpasture syndrome, as well as Wegener granulomatosis may present with azotemia and progressive renal failure. It is not possible to distinguish among these diseases on the basis of clinical signs and symptoms; autoantibody testing may be helpful.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

<0.4 U (negative)

0.4-0.9 U (equivocal)

> or =1.0 U (positive)

Reference values apply to all ages.

Interpretation Provides information to assist in interpretation of the test results

A positive result has a high predictive value for microscopic polyangiitis (MPA) in patients with negative test results for systemic lupus erythematosus (antinuclear antibodies) and Goodpasture syndrome (glomerular basement membrane antibody). A negative result significantly diminishes the likelihood that a patient has MPA.(3)

 

While myeloperoxidase levels often decline following successful treatment of MPA, specific guidelines for this clinical purpose are not available.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Since it is not possible to distinguish between microscopic polyangiitis (MPA) and other causes of progressive renal failure or systemic illness (eg, Wegener granulomatosis, lupus nephritis, Goodpasture syndrome), this test should be employed in conjunction with other diagnostic tests in the initial evaluation of such patients. The recommended test in this setting is VASC / Antineutrophil Cytoplasmic Antibodies Vasculitis Panel, Serum, which includes myeloperoxidase (MPO) antibodies, proteinase 3 (PR3) antibodies and, if indicated, antineutrophil cytoplasmic antibodies (ANCA). The test for ANCA identifies 2 types of antibodies-cytoplasmic (cANCA), which are specific for PR3 and perinuclear (pANCA), which are specific for MPO.

 

The presence of MPO is quite specific for MPA (diagnostic specificity approaches 95%); but, it is recommended that positive results obtained by EIA be confirmed by another testing method. This is best accomplished by testing for pANCA, which confirms the positive MPO result and increases the diagnostic specificity for MPA to 97%.(3) Nevertheless, positive results for MPO have been reported in patients with systemic lupus erythematosus, Goodpasture syndrome, and Churg-Strauss syndrome. Therefore, clinicians must rule out these diagnoses to maximize the specificity and positive predictive value of the MPO test result.

 

While sequential measurements of MPO may be used to follow treatment response or to monitor disease activity in patients with MPA, results should not be exclusively relied upon to assess response to treatment or disease activity.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Falk RJ, Jennette JC: Anti-neutrophil cytoplasmic autoantibodies with specificity for myeloperoxidase in patients with systemic vasculitis and idiopathic necrotizing and crescentic glomerulonephritis. N Engl J Med 1988 Jun 23;318(25):1651-1657

2. Stone JH, Hellman DB: Small and medium-vessel vasculitis. In Clinical Immunology Principles and Practice. Third edition. Edited by RR Rich, TA Fleisher, WT Shearer, et al. Mosby/Elsevier, 2007, pp 859-884

3. Russell KA, Wiegert E, Schroeder DR, et al: Detection of antineutrophil cytoplasmic antibodies under actual clinical testing conditions. Clin Immunol 2002 May;103(2):196-203