A CASE OF SCLERODERMA RENAL CRISIS REQUIRED DIFFERENTIAL DIAGNOSIS FROM ANTI-GBM ANTIBODY-ASSOCIATED RAPIDLY PROGRESSIVE GLOMERULONEPHRITIS.

Scleroderma renal crisis (SRC) is a life-threatening complication of systemic sclerosis characterized by abrupt onset of severe hypertension and rapidly progressive renal failure, often associated with thrombotic microangiopathy (TMA). Although the detection of anti-glomerular basement membrane (anti-GBM) antibodies is highly specific for anti-GBM disease, low-level positivity may occasionally occur in other settings, complicating the differential diagnosis. We report a case of SRC with TMA in which anti-GBM antibody positivity initially suggested rapidly progressive glomerulonephritis (RPGN), but renal biopsy confirmed isolated TMA.

A 62-year-old woman with Sjögren’s syndrome and interstitial pneumonia had received steroid pulse therapy in year X-3 and was maintained on prednisolone 5 mg/day and tacrolimus 3 mg/day. She was diagnosed with diffuse cutaneous systemic sclerosis in year X-1. In mid-March of year X, she developed exertional dyspnea and dull back pain. At a local clinic on April 2, her blood pressure was 180/110 mmHg and serum creatinine (Cr) was 3.04 mg/dL, prompting referral to our hospital the following day for suspected SRC.

　On admission, her blood pressure was 187/117 mmHg, hemoglobin 10.8 g/dL, platelets 98,000/μL, LDH 788 U/mL, and schistocyte were present. Urinalysis showed proteinuria and hematuria. Anti-GBM antibody was slightly positive (7.8 U/mL). Imaging showed pre-existing interstitial pneumonia and pericardial effusion. Laboratory tests revealed low haptoglobin and elevated renin activity. TMA and SRC were suspected, so we started captopril immediately.

　Although blood pressure gradually decreased, renal function worsened, with Cr reaching 5.96 mg/dL on day 5, requiring hemodialysis. Because RPGN could not be excluded due to anti-GBM antibody positivity, plasmapheresis and renal biopsy were performed. However, light microscopy revealed only findings consistent with TMA, with no crescent observed. Immunofluorescence was negative for IgG, and electron microscopy showed no specific findings. Pathology confirmed TMA without evidence of anti-GBM disease, and plasmapheresis was discontinued.

　Although TMA activity improved with ACE inhibition, renal function did not recover. On day 42, she developed acute respiratory distress syndrome (ARDS), which temporarily improved with antibiotics and steroid escalation but recurred on day 58. She died on day 62.

This case illustrates diagnostic challenges when anti-GBM antibodies are detected in SRC. Anti-GBM testing has high sensitivity and specificity for Goodpasture’s syndrome, but low-level positivity has been reported in asymptomatic individuals prior to clinical onset. In this patient, pre-existing interstitial pneumonia may have exposed alveolar basement membrane antigens, leading to low-level antibody production without clinical anti-GBM disease. The absence of crescentic glomerulonephritis, negative IgG deposition, and typical TMA pathology were key to the correct diagnosis.

We experienced a case of SRC with TMA and low-level anti-GBM antibody positivity that initially led to suspicion of RPGN. This case highlights the importance of integrating clinical, serological, and pathological findings when evaluating acute kidney injury with atypical serological profiles, to avoid misdiagnosis and unnecessary treatment.