ARTHRITIS & RHEUMATISM Vol. 43, No. 3, March 2000, pp 584–592 © 2000, American College of Rheumatology ERYTHEMA NODOSUM Etiologic and Predictive Factors in a Defined Population CARLOS GARCI ´ A-PORRU ´ A, MIGUEL A. GONZA ´ LEZ-GAY, MANUEL VA ´ ZQUEZ-CARUNCHO, LUIS LO ´ PEZ-LAZARO, MERCEDES LUEIRO, MARIA L. FERNA ´ NDEZ, JAVIER ALVAREZ-FERREIRA, and RAMO ´ N M. PUJOL Objective. To examine the frequency and features of erythema nodosum (EN), establish disease associa- tions, and identify the optimal set of predictors for the occurrence of secondary EN. Methods. We performed a retrospective study of an unselected population of patients 14 years and older with biopsy-proven EN diagnosed at a referral hospital between 1988 and 1997. Patients were classified as having either idiopathic EN or EN secondary to other diseases if the skin nodules occurred in the context of a well-defined disease, or if there was a precipitating event in close temporal proximity to the onset of EN. Results. One hundred six patients (82 women) were diagnosed as having biopsy-proven EN. At the time of diagnosis, no precipitating events or underlying dis- eases were identified in 36.8% of patients. Sarcoidosis and nonstreptococcal upper respiratory tract infections (URI) were the most common conditions associated with secondary EN. Only 1 of 35 patients with an initial diagnosis of idiopathic EN and a followup of at least 1 year was finally diagnosed as having secondary EN. The best predictive model of secondary EN included an abnormal results on a chest radiograph, a previous history of nonstreptococcal URI, and a significant change in antistreptolysin O (ASO) titer in 2 consecu- tive determinations performed in a 2–4-week interval. Also, the presence of peripheral synovitis, a positive tuberculin skin test, and a history of diarrhea suggested the presence of secondary EN. This model showed high sensitivity and specificity. Conclusion. Idiopathic EN is common. A basic procedure including careful medical history-taking, a physical examination for peripheral synovitis, 2 consec- utive ASO determinations, a tuberculin skin test, and chest radiography may be sufficient to diagnose EN. Erythema nodosum (EN) is the most common cause of inflammatory nodules and usually affects the lower extremities. It is a self-limiting disease character- ized by lesions that evolve from inflammatory, painful and tender, nonulcerating, nonscarring, cutaneous and subcutaneous red nodules, which are typically multiple and bilateral, becoming bruised in appearance, and, finally, resolving completely. EN may be associated with a wide spectrum of systemic diseases, infections, or various drugs. The prognosis with regard to the skin lesions is usually excellent, but the presence of a possible underlying condition needs to be excluded (1–5). The purpose of this study was to examine the frequency, features, and outcome of EN, establish dis- ease associations, and identify the optimal set of predic- tors for the diagnosis of either idiopathic or secondary EN. We reviewed the clinical symptoms, laboratory findings, and followup of all patients 14 years and older with biopsy-proven EN diagnosed between 1988 and 1997 in a defined area of northwestern Spain. PATIENTS AND METHODS We studied retrospectively the case records of all patients diagnosed with biopsy-proven EN in the department of medicine of Hospital Xeral-Calde from January 1988 through December 1997. Since the pediatric population was not studied in our department, we examined the spectrum of Carlos Garcı ´a-Porru ´a, MD, PhD, Miguel A. Gonza ´lez-Gay, MD, PhD, Manuel Va ´zquez-Caruncho, MD, Luis Lo ´pez-Lazaro, MD, PhD, Mercedes Lueiro, MD, Maria L. Ferna ´ndez, MD, Javier Alvarez- Ferreira, MD: Hospital Xeral-Calde, Lugo, Spain; Ramo ´n M. Pujol, MD, PhD: Hospital Santa Creu i San Pau, Barcelona, Spain. Address reprint requests to Miguel A. Gonza ´lez-Gay, MD, PhD, Rheumatology Division, Hospital Xeral-Calde, c) Dr. Ochoa s/n, 27004 Lugo, Spain. Submitted for publication June 7, 1999; accepted in revised form November 19, 1999. 584