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Historically, the presence of urine eosinophils has been used as a biomarker for acute interstitial nephritis (AIN). Support for eosinophiluria as a marker for AIN originally came from two studies, only nine patients each, that found urine eosinophils present in patients with biopsy-confirmed AIN (1,2). However, subsequent studies with more robust populations have not supported eosinophiluria as a sensitive or specific marker for AIN (3,4).
These findings are best illustrated in a 2013 retroactive study by Muriithi et al. of 566 patients that found urine eosinophils demonstrated only 30% sensitivity and 68% specificity for AIN compared to all other diagnoses – with only 15.6% positive and 85.7% negative predictive values (5), concluding that eosinophiluria exists in many kidney diseases and is no better at distinguishing AIN from acute tubular necrosis or other kidney diseases (5). Per Muriithi et al, most diagnoses of AIN are determined by clinical history, physical examination and laboratory findings. Renal biopsy is the gold standard for atypical cases or those where definitive confirmation is required for therapy.
In 2021, the Association for Diagnostics and Laboratory Medicine published a guidance document for laboratory investigation of acute kidney injury (AKI) recommending diagnostic thresholds, the role of new biomarkers, and discontinuation of wasteful testing (eg, urine eosinophils) (6). The guidance recommends against using urinary eosinophils to confirm or exclude AIN nor should it be considered in the evaluation of AKI (6).