A 61 YEAR DIABETIC MAN WITH METHICILLIN‐SENSITIVE STAPHYLOCOCCUS AUREUS SEPTIC ARTHRITIS AND ACUTE RENAL FAILURE: A CASE OF IGA‐DOMINANT POSTINFECTIOUS GLOMERULONEPHRITIS

A 61 year-old male patient presented to this hospital because of acute worsening of a chronic left knee swelling and fever. Past medical history was significant for longstanding diabetes mellitus, hypertension, benign prostatic hypertrophy, coronary artery disease, metastatic melanoma and pulmonary embolism. Physical examination was significant for a left knee swelling, warmth and severe tenderness, with moderate-sized effusion. Blood pressure was normal and there were no skin rashes. Serum creatinine and blood urea nitrogen (BUN) levels were 0.9 mg/dL and 16 mg/dL at admission (Table S1). Aspiration of the knee joint revealed grossly purulent fluid, with greater than 250 000 leukocytes/ mm, 99% neutrophils; cultures were both positive for methicillin-sensitive Staphylococcus aureus. The patient was initially started on i.v. vancomycin, which was switched to nafcillin after 4 days as sensitivity of the isolate became available. On the fifth day, BUN and creatinine levels increased to 21 mg/dL and 1.8 mg/dL, respectively. At this point, the antibiotic was switched to ceftriaxone, and lisinopril was discontinued. However, renal function continued to deteriorate rapidly despite interventions, with BUN and creatinine levels rising to 50 mg/dL and 5.3 mg/dL in the subsequent 4 days. The patient became anuric despite receiving hydration with normal saline. Urinalyses revealed proteinuria with active urinary sediments containing coarse granular casts, many leukocytes and red blood cells. Urine sodium level was 52 mmol/L. Urine was positive for eosinophils, without peripheral eosinophilia. Antinuclear antibody was negative. Serum complement levels were normal. Viral hepatitis panel was negative. Retroperitoneal ultrasound was non-revealing. Renal biopsy, performed 4 days after beginning of creatinine elevation, yielded 31 glomeruli for pathological analysis, which revealed the presence of mesangial and focal endocapillary proliferative glomerulonephritis (GN) with focally increased circulating neutrophils and IgApredominant mesangial and rare glomerular basement membrane subepithelial hump-like immune complex deposits (Fig. 1). There was also patchy, moderately severe, mixed interstitial inflammation involving lymphocytes, plasma cells and eosinophils. In addition, there was evidence of evolving diabetic nephropathy and parenchymal chronicity. Overall, the findings were indicative of IgA-dominant postinfectious glomerulonephritis (PIGN) with concurrent acute interstitial nephritis (AIN) and underling chronic renal disease. Postinfectious glomerulonephritis is an important consideration in this patient given his recent knee joint infection with methicillin-sensitive Staphylococcus aureus and urine studies suggestive of presence of nephritic syndrome. The classic subtypes of poststaphylococcal GN are IgGor IgMmediated, resemble poststreptococcal GN, and are associated with low complement levels, hence unlikely in this patient who had normal complement levels. However, a newly recognized form of poststaphylococcal GN is a predominantly IgA-mediated disease, which may be associated with normal