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The kidney biopsy is an invaluable tool that has become the gold standard for the diagnosis of pathologic kidney diseases since the early 1950s. Throughout the years, immunohistologic and ultrastructural microscopy techniques have improved and provide more information on the cause and classification of kidney diseases than that available from simple light microscopy alone. Kidney biopsy has become a preferred method to obtain critical information that can be used in conjunction with serologic, urinary, and genetic testing to diagnose a variety of kidney diseases, both acute and chronic. The kidney biopsy procedure carries relatively low risk and yields substantial information. Potential complications include bleeding requiring transfusion, gross hematuria, arteriovenous fistula formation, and perinephric hematoma, among others. Percutaneous kidney biopsies are typically performed using real-time ultrasound or computed tomographic imaging. This Core Curriculum briefly outlines the history of the kidney biopsy, then discusses indications, complications, and specific procedural aspects. Additional Readings ► Berns JS. A survey-based evaluation of selfperceived competency after nephrology fellowship training. Clin J Am Soc Nephrol. 2010;5(3):490-496. + ESSENTIAL READING ► Berns JS, O'Neill WC. Performance of procedures by nephrologists and nephrology fellows at U.S. nephrology training programs.

Autosomal-dominant polycystic kidney disease (ADPKD) is a systemic disease, marked by progressive increase of bilateral renal cysts, resulting in chronic kidney disease (CKD) and often leading to end-stage renal disease (ESRD). Apart from renal cysts, patients often have extra-renal disease, involving the liver, heart and vasculature. Other less common but equally important extra-renal manifestations of ADPKD include diverticular disease, hernias, male infertility and pain. Extra-renal disease burden is often asymptomatic, but may result in increased morbidity and mortality. If the disease burden is significant, screening may prove beneficial. We review the rationale for current screening recommendations and propose some guidelines for screening and management of ADPKD patients.

IntroductionAcute interstitial nephritis (AIN) is the cause of over 15% cases of acute loss of kidney function (1). Unlike many other causes of acute loss of kidney function, AIN is treatable. However, ongoing inflammation can lead to permanent kidney damage in AIN if it is not diagnosed and treated promptly. In fact, it is estimated that 40% to 60% of cases of AIN result in development of chronic kidney disease (CKD). Delay in diagnosis is one of the best predictors of incomplete recovery of kidney function. Recent studies estimated that at least 2% to 3% of cases of CKD could be from undiagnosed AIN from proton pump inhibitor use, a figure that is equivalent to 1 million US adults (2-5). Thus, a noninvasive biomarker for timely diagnosis of AIN could improve clinical care of patients suspected to have AIN and may reduce occurrence of CKD.A major challenge of AIN is distinguishing it from the other causes of acute rise in serum creatinine. When AIN was first described in association with β-lactam antibiotics, such as methicillin, it presented with typical features of an allergic reaction, such as onset of fever, rash, and eosinophilia, shortly after BACKGROUND. Clinical diagnosis of acute interstitial nephritis (AIN) is challenging because of lack of a diagnostic biomarker and requires a kidney biopsy. We hypothesized that AIN is mediated by specific T cell subsets such that specific T cell cytokine levels could serve as biomarkers to distinguish AIN from other causes of acute kidney disease (AKD). METHODS.We enrolled consecutive sampling participants who underwent a kidney biopsy for AKD evaluation at 2 centers between 2015 and 2018. Three pathologists independently established AIN diagnosis through review of kidney biopsies. Through univariable and multivariable analysis of 12 selected urine and plasma cytokines, we identified 2 that were diagnostic of AIN. RESULTS.Of the 218 participants, 32 (15%) were diagnosed with AIN by all 3 pathologists. Participants with AIN had consistently higher levels of urine TNF-α and IL-9 than those with other diagnoses, including acute tubular injury, glomerular diseases, and diabetic kidney disease, and those without any kidney disease. As compared with participants in the lowest quartile, we noted higher odds of AIN in participants in the highest quartiles of TNF-α levels (adjusted odds ratio, 10.9 [1.8, 65.9]) and IL-9 levels (7.5 [1.2, 45.7]) when controlling for blood eosinophils, leukocyturia, and proteinuria. Addition of biomarkers improved area under receiver operating characteristic curve over clinicians' prebiopsy diagnosis (0.84 [0.78, 0.91]) vs. 0.62 [(0.53, 0.71]) and a model of current tests (0.84 [0.76, 0.91] vs. 0.69 [0.58, 0.80]). CONCLUSIONS.Inclusion of urinary TNF-α and IL-9 improves discrimination over clinicians' prebiopsy diagnosis and currently available tests for AIN diagnosis. FUNDING.

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