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PATIENT 
Mrs. F is a 63-year-old woman with a history of diastolic dysfunction, hypertension, and knee and hip osteoarthritis. Her usual medications are atenolol, lisinopril, and acetaminophen, and her baseline serum creatinine is 1.1 mg/dL. Four weeks ago, she came to your office with severe pain, erythema, and swelling of her right first metatarsophalangeal joint.
You diagnosed gout and prescribed indomethacin 25 mg 3 times daily until the symptoms resolved. She returned for a follow-up yesterday, reporting that the gout had resolved in a few days, but that she kept taking the indomethacin because it also relieved her chronic knee and hip pain. Despite your reservations, you agree to refill the prescription because she clearly feels so much better than usual, cautioning her to use the medication only when she needs it. Today you receive the results of the blood tests you ordered during the visit: Na, 141 mEq/L; K, 5.0 mEq/L; Cl, 100 mEq/L; HCO3, 20 mEq/L; BUN, 32 mg/dL; creatinine, 2.5 mg/dL.
At this point, what is the leading hypothesis, what are the active alternatives, and is there a must not miss diagnosis? Given this differential diagnosis, what tests should be ordered?
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RANKING THE DIFFERENTIAL DIAGNOSIS
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At this point, the differential for her AKI is quite broad, but it is logical to focus on the pivotal point, in this case, the recent use of indomethacin. Through prostaglandin inhibition, NSAIDs can cause decreased renal blood flow, leading to a prerenal state. NSAIDs are also 1 of the classes of drugs most commonly associated with an intrarenal disease, interstitial nephritis. Although obstruction must always be considered, she is having no urinary symptoms and has no risk factors. Table 28-7 lists the differential diagnosis.
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Mrs. F’s urine Na is 35 mEq/L, and the FENa is 1.5%. Urinalysis shows 1+ protein, 3 RBCs/hpf, 5–10 WBCs/hpf, and no casts. Renal ultrasound is normal.
Is the clinical information sufficient to make a diagnosis? If not, what other information do you need?
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You call Mrs. F and tell her to stop taking the indomethacin; because of her abnormal urinalysis, you also order urine eosinophils. One week later, her creatinine is still 2.5 mg/dL. Urine eosinophils are negative.
Have you crossed a diagnostic threshold for the leading hypothesis, NSAID-induced renal hypoperfusion? Have you ruled out the active alternatives? Do other tests ...