AN ATYPICAL CASE OF POLYARTERITIS NODOSA. B.N. Batson 1 , J.W. Blackston 1 , C. Subramony 1 ; 1 University of Mississippi Medical Center, Jackson, MS (Tracking ID #75156) LEARNING OBJECTIVES: Recognize the clinical and lab findings of polyarteritis nodosa. Perform an appropriate workup when polyarteritis nodosa is suspected. CASE INFORMATION: A 46 y/o BM was transferred complaining of abdominal pain for 4 days that had not improved with conservative treatment. Outside x-rays demonstrated dilated loops of bowel without obstruction. He denied fever, chills, nausea, and vomiting but reported fatigue, anorexia, and a 40-pound weight loss over the past 6 months. He also reported progressive weakness and numbness of both legs which limited ambulation. Physical was significant for BP of 178/114 and a diffusely tender abdomen without guarding, rebound or mass. He had faint bowel sounds and was heme positive on rectal. He had 2/5 strength in bilateral hip, knee, and ankle flexors and extensors. Sensation was diminished in both feet up to mid-shin. Reflexes and Babinski's were normal. CXR was negatvie except for blebs, but KUB showed multiple dilated loops of bowel with air-fluid levels. Lab included WBC 14.1, Hct 27, and plts 417. BUN was 23 and creatinine 1.2. Total protein was 6.7 with albumin 2.3. Hep BsAg was positive; p-ANCA was negative. On HD#2, CT abdomen showed a 9x5cm right lower quadrant mass, moderate ascites, and patchy kidney enhancement with wedged-shaped hypodensities. Renal ultrasound showed normal kidney size with inhomogeneous architecture. On HD#3, sural nerve biopsy was suspicious for mononeuritis multiplex. Later on HD#3, a mesenteric arteriogram was normal. By HD#4, the patient had decreased abdominal pain with stable hematocrit, but creatinine had risen to 2.7. On HD#5 colonoscopy was normal. He then had an MRI-guided biopsy of the RLQ mass which yielded only red blood cells. On HD#8 surgical exploration found bloody ascites and an 8x10cm mass of the ileocolonic junction requiring hemicolectomy. Ascitic cytology was negative for malignancy. Pathology of the mass showed extensive polyarteritis nodosa involving the small-and medium-sized submucosal and mesenteric arteries. IMPLICATIONS/DISCUSSION: While this patient exhibited many classic symptoms of polyarteritis nodosa, his workup of that suspected diagnosis was confounded by an abdominal mass and false-negative mesenteric arteriogram. One must remember that the diagnosis of PAN is best made with tissue biopsy of an involved organ system. While a positive p-ANCA is suggestive of PAN, and a mesenteric arteriogram showing pseudo-aneurysms is sufficient for diagnosis, both of these tests are positive in less than 50% of patients with known polyarteritis nodosa.