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Chapter 1: Approach to the Patient with Renal Disease

A 67-year-old male patient is evaluated in the emergency department with a 3-day history of fever, chills, and malaise. He denies having any urinary symptoms, for example frequency, dysuria, and urgency. He states that 2 days ago he noticed watery stools, approximately 6–8 times a day accompanied by nausea and nonbilious vomiting. His medical history is remarkable for hypertension and osteoarthritis. His medications include lisinopril, metformin, furosemide, naproxen, and multivitamins.

On physical examination, his vital signs are stable: blood pressure is 132/88 mm Hg, heart rate is 92 beats/min, respiratory rate is 17 breaths/min, and he is afebrile. The rest of the physical examination is unremarkable.

Chest X-ray and electrocardiogram (ECG) are within normal limits.

Laboratory data:

  • Blood urea nitrogen (BUN) 70 mg/dL

  • Serum creatinine 2.8 mg/dL

  • Serum potassium 4.5 mEq/dL

  • Serum sodium 146 mEq/dL

  • Fractional excretion of sodium: 1.7%

  • Fractional excretion of urea: 7.5%

What is the most likely diagnosis?

A. Prerenal acute kidney injury

B. Interstitial nephritis

C. Obstructive nephropathy

D. Acute tubular necrosis

The answer is A. The most likely cause of acute kidney injury in this patient is volume depletion causing prerenal acute kidney injury. The fractional excretion of sodium (FENa) is a useful tool to differentiate whether a cause of acute kidney injury is due to prerenal or intrinsic causes. It is calculated as follows: (Urine sodium × plasma creatinine)/(urine creatinine × plasma sodium) ×100. A FENa below 1% suggests a prerenal cause; however, in the setting of diuretics, it may have limited value, as urine sodium may not be accurately reflected. When this situation is encountered, a fractional excretion of urea can be performed. It is calculated as (urine urea × plasma creatinine)/(urine creatinine × plasma urea) × 100. A value of less than 35%, suggest a prerenal cause. In this patient, with a FE urea of 7.5%, the most likely cause of acute kidney injury is volume depletion as he has gastrointestinal fluid losses.

Acute interstitial nephritis (AIN) has a multifactorial cause and could be related to medications most of the time, however, based on the patient’s history, volume depletion is the most common cause. Diagnosis of AIN is usually made with kidney biopsy.

Patient did not have any symptoms or signs of obstruction such as lower abdominal discomfort, urinary retention, or history of BPH. This clinical scenario is, therefore, less likely in this patient.

A 27-year-old female is being evaluated in the inpatient psychiatry unit due to elevated creatinine. The patient is currently hospitalized because of new-onset severe depression and suicidal ...

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