DDx
Diagnostic Considerations
In addition to the conditions listed in the differential diagnosis, causes of elevated blood urea nitrogen (BUN) or creatinine levels that are unrelated to kidney function should be considered, such as the following:
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Gastrointestinal hemorrhage
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Large protein meal
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Total parenteral nutrition
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Steroids
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Ketoacidosis
Therapy with medications such as trimethoprim, cimetidine, cefoxitin, and flucytosine should be considered; these agents impair creatinine excretion.
Differential Diagnoses
Media Gallery
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Graph shows relation of glomerular filtration rate (GFR) to steady-state serum creatinine and blood urea nitrogen (BUN) levels. In early renal disease, substantial decline in GFR may lead to only slight elevation in serum creatinine. Elevation in serum creatinine is apparent only when GFR falls to about 70 mL/min.
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Diagnostic indices in azotemia. Although such indices are helpful, it is not necessary to perform all these tests on every patient. Comparison should always be made with patients' baseline values to identify trends consistent with increase or decrease in effective circulating volume. Use of some of these indices may be limited in certain clinical conditions, such as anemia (hematocrit), hypocalcemia (serum calcium), decreased muscle mass (serum creatinine), liver disease (blood urea nitrogen [BUN], total protein, and albumin), poor nutritional state (BUN, total protein, and albumin), and use of diuretics (urine sodium). Fractional excretion of urea and fractional excretion of trace lithium appear to be superior for assessing prerenal status in patients on diuretics.
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