Review of the Acute Interstitial Nephritis (AIN) Risk calculator

The AIN risk calculator was developed to improve non-invasive diagnostic accuracy for acute interstitial nephritis, an asymptomatic and often poorly recognized but potentially reversible cause of acute kidney injury. The equation was developed in a cohort of adult patients who underwent a kidney biopsy for acute kidney injury and has been validated in multiple kidney biopsy cohorts. The four variable equation requires blood urea nitrogen (BUN), serum creatinine, and two dipstick urinalysis components—urine specific gravity and urine protein grade. This tool has a low positive predictive value and a high negative predictive value. Hence, a low probability score can effectively rule out AIN. The calculator is therefore intended to be used for patients in whom kidney biopsy is being considered but it cannot diagnose AIN or be used to prescribe treatment without biopsy evidence. A kidney biopsy is required to diagnose AIN for those who are at higher than low risk. Urinary biomarkers have been shown to improve the diagnostic accuracy of the calculator and, once clinically available, could further enhance its performance and facilitate non-invasive diagnosis.