The calculator in question is the ACR/EULAR (American College of Rheumatology/European League Against Rheumatism) Classification Criteria for Microscopic Polyangiitis. This scoring system is used in the clinical setting to classify patients with suspected microscopic polyangiitis, a systemic vasculitis that primarily affects small blood vessels.

The ACR/EULAR criteria are applied to patients who have been identified as having vasculitis, and are used to differentiate microscopic polyangiitis from other forms of vasculitis. The criteria include clinical, serological, and pathological parameters such as organ involvement, ANCA (anti-neutrophil cytoplasmic antibody) status, and histopathological findings.

The clinical utility of this calculator is to aid in the diagnosis of microscopic polyangiitis, which can help guide appropriate treatment strategies. It is also used in research settings to ensure consistent classification of patients in clinical studies.

Exclusion criteria for the use of this calculator include patients with secondary vasculitis due to conditions such as infections, malignancies, or drug exposures, as well as patients with other primary vasculitides.

ACR/EULAR classification criteria for microscopic polyangiitis

# Criteria
 # Entry criteria
A. Evidence of small- or medium-vessel vasculitis on imaging (+20)
B. Alternative diagnoses mimicking vasculitis are excluded (+20)
# Clinical criteria
C. Nasal involvement: bloody discharge, ulcers, crusting, congestion, blockage, or septal defect/perforation (-3)
# Laboratory, imaging, and biopsy criteria
D. Positive test for perinuclear antineutrophil cytoplasmic antibodies or anti-myeloperoxidase antibodies (+6)
E. Fibrosis or interstitial lung disease on chest imaging (+3)
F. Pauci-immune glomerulonephritis on biopsy (+3)
G. Positive test for cytoplasmic antineutrophil cytoplasmic antibodies or anti-proteinase 3 antibodies (-1)
H. Blood eosinophil count >= 1×10**9/L (-4)

# Conditions
- score >= 45
- true
- score == 0