Urinary eosinophils in AIN: farewell to an old biomarker?

Urinary biomarkers of kidney injury are an area of ongoing research in nephrology. For years, urinary eosinophils have been used as biomarkers for acute interstitialnephritis (AIN).Thecharacteristics thatwould make eosinophiluria a useful biomarker for AIN include that (1) eosinophils are consistently present in the urine of patients with AIN (i.e., AIN is characterized by an eosinophilic interstitial infiltrate), (2) eosinophils are absent in the setting of another etiology of AKI (i.e., other renal and extrarenal diseases are not associated with eosinophiluria), and (3) a predetermined percentage of eosinophils is easily visualized in the urine when present (i.e., excellent stain performance). As early as 1967, urinary eosinophils were reported in rejection episodes after kidney transplantation, suggesting that they were associated with “kidney inflammation” (1). The earliest description of this test as a biomarker for AIN was by Galpin et al. (2) in 1978. Using the Wright stain, nine of nine cases of methicillin-associated AIN had urinary eosinophils. Six of nine cases had biopsy-proven AIN, with eosinophils within the renal interstitium and occasionally, tubular lumens; 0 of 43 patients with AKI from another diagnosis had eosinophiluria. These data implied that eosinophiluria was a sensitive and specific test for AIN. Linton et al. (3) subsequently described eosinophiluria in six of nine patients with drug-induced AIN. Eight patients had biopsy-proven AIN, with five patients having prominent interstitial eosinophils. Overall, these two small studies suggested that eosinophiluria might be a reasonably good biomarker for AIN. Additional work on this subject by Corwin et al. (4) examined the clinical correlates of eosinophiluria using theWright stain in 65 patients. AINwas diagnosed in 9 of 65 patients, with eosinophiluria noted in 8 of 9 of these cases, replicating previous data. However, urinary eosinophils were also shown in 27 of 56 patients with diagnoses other than AIN (Table 1), suggesting that the test was rather nonspecific. Corwin et al. (4) concluded that using a .5% eosinophil cutoff might identify a group with higher probability of having AIN. However, 6 of 56 non-AIN patients and only 4 of 9 AIN patients met this cutoff, which (not unexpectedly) improved specificity but reduced sensitivity. The lack of sensitivity of urinary eosinophils was further emphasized in a review paper based on data from 10 studies examining drug-induced AIN showing that eosinophiluria was present in only 59% (19/32) of biopsy-proven AIN cases (5). Because lowsensitivitywas themajorconcern,Nolan et al. (6) renewed interest in eosinophiluria as an AIN biomarker with aNew England Journal of Medicine publication that hailed the Hansel stain as a better method to visualize urinary eosinophils. This stain was chosen based on its use in identifying eosinophils in nasal, bronchial, and ocular secretions of patients with allergy-related diseases; 92 patients with an active urine sediment were identified by the nephrology consult service, of which 11 patients had AIN (proven by biopsy in 2 patients). Hansel stain was positive in 10 of 11 AIN cases, whereas Wright stain was positive in only 2 of 11 cases. Hansel stainwas also positive in other diagnoses but negative in all 30 acute tubular necrosis cases (Table 1). Overall, the Hansel stain provided a sensitivity of 91% and positive predictive value of 45%. Nolan et al. (6) concluded that the Hansel stain improved the sensitivity of eosinophiluria in AIN but that the testwas not specific.Nolan et al. (6) emphasized that itwas usefulwhen the differential diagnosis ofAKI boiled down to AIN versus acute tubular necrosis. Additionally, Nolan et al. (6) argued that a positive test in the setting of high pretest probability for AIN tends to confirm the diagnosis, whereas negative eosinophiluria in patients with low pretest probability can reasonably exclude AIN. However, lack of biopsy confirmation of AIN was a major flaw of the paper. In the face of these data, a study was undertaken to morecompletelydefinethesignificanceofeosinophiluria (7). Urine for eosinophilswas examined in 183 patients assigned a clinical diagnosis using blinded chart review, with AIN based on biopsy or clinical diagnosis. Eosinophiluria was documented in 20 patients, with Hansel stain positive in 18 of 20 patients and Wright stain positive in 8 of 20 patients. Hansel and Wright stains were positive in five of eight and two of eight AIN cases, respectively. Clearly, Hansel stain improved urinary eosinophil detection compared with Wright stain, confirming the data in the work by Nolan et al. (6). Although Hansel stain was also positive in other diagnoses, it still had a very good specificity and negative predictive value of 98% (Table 1). Corwin et al. (7) concluded that Hansel stain improved detection of urinary eosinophils, but its use for diagnosis of AIN remained unclear. *Section of Nephrology, Yale University School of Medicine, New Haven, Connecticut; and Division of Nephrology, Columbia University Medical Center, New York, New York