While most often sent to help weigh in on whether a patient with acute kidney injury has acute interstitial nephritis, the differential for positive urine eosinophils is fairly broad. In addition to AIN, they may also be seen in atheroembolic disease, parasitic infection, pyelonephritis, or Schistosoma hematobium infection.
Eosinophils in the urine can be detected by applying a Hansel’s stain to the urine sediment. The presence of >1% eosinophils of the total urine WBCs constitute a positive test. In general, the same things which cause +urine eosinophils may also show elevated serum eosinophils, and in my limited experience it is the presence of SERUM eosinophilia which is more convincing that urine eosinophils.