#511 A case of MPO-ANCA positive dense deposit disease (DDD) with normal C3 levels presenting with end-stage renal disease (ESRD) at admission

Abstract Background and Aims Dense Deposit Disease (DDD) is a type of C3 glomerulopathy, a rare form of glomerulonephritis caused by abnormal regulation of the alternative complement pathway. It is primarily diagnosed in children aged between 5 and 15 years and is typically characterized by hypocomplementemia, specifically reduced serum C3 levels, although cases with normal C3 levels comprise only about 10% of all patients. Additionally, the presence of myeloperoxidase anti-neutrophil cytoplasmic antibodies (MPO-ANCA) in DDD patients is extremely uncommon. We present a case of a DDD patient with normal C3 levels and positive MPO-ANCA, a combination that, to the best of our knowledge, has not been previously reported in either adults or children. Case We report the case of a 14-year-old girl who was admitted with severe renal failure requiring hemodialysis. In June 2024, the patient visited a local clinic with bilateral leg edema as the chief complaint, where blood tests revealed elevated BUN and serum creatinine levels of 59.0 mg/dL and 3.1 mg/dL, leading to her referral to our pediatric nephrology outpatient clinic in July 2024. She stated that she had never been informed of any urine abnormalities during previous school health screenings and that this was her first time undergoing a blood test. Laboratory tests performed at our institution revealed elevated blood urea nitrogen (BUN) of 73.3 mg/dL, serum creatinine of 4.84 mg/dL, potassium of 6.0 mmol/L, and hemoglobin of 8.5 g/dL, along with high blood pressure measuring 130/89 mmHg. Initial immunological tests performed after admission revealed normal complement levels, including C3 and C4, but positive MPO-ANCA, raising suspicion for ANCA-associated vasculitis or thrombotic microangiopathy (TMA). However, there were no other organ findings suggestive of ANCA vasculitis, and no hemolytic signs indicated TMA, so hemodialysis was initiated, and a kidney biopsy was performed. The biopsy results confirmed a rare condition known as DDD, characterized by dominant C3 deposits and dark transformation of the glomerular basement membrane (GBM). She was later confirmed to have end-stage renal disease (ESRD) and has been continuing hemodialysis since then. Subsequent follow-up immunologic testing revealed persistent MPO-ANCA positivity, and she is being monitored in collaboration with the pediatric immunology. Conclusion This case highlights a rare instance of DDD, where initial C3 levels were normal and MPO-ANCA was positive. In cases of acute renal failure with unknown causes, a kidney biopsy may be necessary for definitive diagnosis. Additionally, the persistence of positive MPO-ANCA raises concerns about the potential for poor renal prognosis and rapid progression, warranting further consideration. On the other hand, it also underscores the necessity of ongoing monitoring, including the potential development of symptoms associated with ANCA vasculitis.