EFFECT OF CLASSIFICATION ON THE INCIDENCE OF POLYARTERITIS-NODOSA ANDMICROSCOPIC POLYANGIITIS

Objective, Polyarteritis nodosa (PAN) has been used as a generic term for systemic vasculitis. The distinction between classic PAN and micro scopic polyangiitis (MPA) has not always been made, The aims of this s tudy were to compare the American College of Rheumatology (ACR) criter ia for PAN with the Chapel Bill Consensus Conference (CHCC) definition s of classic PAN and MPA, and to estimate the annual incidence of PAN and MPA. Methods. The 1990 ACR criteria and CHCC definitions for syste mic vasculitis were applied to an unselected cohort of 130 patients wi th systemic vasculitis attending a single district hospital in the UK between February 1, 1988 and January 31, 1994. Results. Eight patients who met the ACR criteria for PAN and who also met the CHCC definition of MPA but not classic PAN were identified, A further 5 patients met the CHCC definition of MPA but not the ACR criteria for any other type of systemic vasculitis, No patient who met the CHCC definition of cla ssic PAN was identified, The annual incidence of MPA was calculated to be 3.6/million (95% confidence interval 1.7-6.9), and the annual inci dence of PAN (ACR criteria) was 2.4/million (95% confidence interval 0 .9-5.3). Conclusion. Classic PAN as defined by the CHCC is rare, becau se small vessel involvement is excluded from this definition.