Discontinuing omeprazole is appropriate in this patient who may have interstitial nephritis as a cause of chronic tubulointerstitial disease. In contrast to acute kidney injury, chronic tubulointerstitial disease develops over months to years and is a cause of slowly declining kidney function. Chronic tubulointerstitial disease most commonly results from previous injury due to an episode of acute interstitial nephritis, but it can also result from ongoing, subacute interstitial nephritis and other glomerular, vascular, or obstructive diseases that may cause irreversible injury to the tubules and interstitium, even with treatment or resolution of the initial disease process. Symptoms and physical examination findings in patients with chronic tubulointerstitial disease can be minimal or absent unless an active associated disease is present; therefore, the diagnosis is often discovered by abnormalities detected on laboratory testing done for other purposes. This patient was noted to have progressively worsening kidney function, with the only abnormality being mild leukocytosis on urinalysis and subnephrotic-range proteinuria. Evaluation of chronic tubulointerstitial disease is focused on identification of potentially treatable causes. Because proton pump inhibitors are associated with interstitial nephritis, discontinuation of omeprazole is appropriate in this patient.

Although any drug may cause interstitial nephritis, there are no case reports of tamsulosin-induced interstitial nephritis. Stopping tamsulosin may exacerbate the benign prostatic hyperplasia, which could lead to postrenal acute kidney injury.

A 24-hour urine collection may be useful for measuring kidney function and quantifying proteinuria but would not be helpful in establishing the cause of this patient's declining kidney function.

Duplex ultrasonography can be useful for diagnosing renal artery stenosis, but this patient's clinical features of pyuria, mild proteinuria, normal blood pressure, and symmetric kidney size are not consistent with renal artery disease, making this study unnecessary.