Always Consider a Repeat Kidney Biopsy: Acute Interstitial Nephritis Soon After Membranous Nephropathy

Introduction The heterogeneity of membranous nephropathy is well described in the literature, and its clinical course and response to treatment vary. Similarly, acute interstitial nephritis (AIN) can present in unexpected and unusual ways and should always be considered within the differential diagnosis of worsening renal function. This case study describes the cross‐section of these two disease entities. Clinical Case A 68‐year‐old male with a history of hypertension, chronic obstructive pulmonary disease (COPD), hyperlipidaemia, gout and treated prostate cancer presented with bilateral lower limb swelling and progressive renal dysfunction. Initial laboratory findings demonstrated nephrotic syndrome with impaired renal function. A positive phospholipase A2 receptor (PLA2R) antibody confirmed primary membranous nephropathy. Renal biopsy showed typical findings of membranous nephropathy. Treatment with prednisolone and cyclophosphamide improved renal function initially. However, as prednisolone was tapered, creatinine levels rapidly worsened, leading to a second biopsy. The second biopsy demonstrated AIN superimposed on chronic membranous nephropathy. A drug‐induced aetiology was suspected, with pantoprazole, trimethoprim–sulfamethoxazole and frusemide identified as the most likely contributors. Following withdrawal of these agents and reinitiation of high‐dose corticosteroids, the patient’s renal function improved markedly, obviating the need for dialysis. Conclusion This case highlights the importance of considering AIN and the need for timely repeat renal biopsy in cases of deteriorating renal function.