Saturday, 10 November 2018
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How to determine etiology of AKI based on serum and urine electrolytes?

For example:
56yF presents to hospital with 5 days of diarrhea. She has not taken any recent medications. Her labs were as follows:

Na 146, K 5.2, Cl 98, Cr 170 (Baseline 68)

Urinalysis demonstrates 1+ protein. Microscopy demonstrates abundant hyaline casts.
Urine Na: 26, Urine Cr 26

What is the cause of her AKI? (Pre-Renal vs ATN vs GN vs Obstructive Uropathy)

Urinary electrolytes are somewhat helpful in assessing AKI. If you decided a patient was volume contracted, then the appropriate response of the kidney is to conserve sodium - even down to 0 mmol/L (or fractional excretion <1%). However, if sodium is not low when the kidney should be conserving it, this is consistent with ATN as the tubules are unable to behave as they should. In terms of post-renal failure, you might find that hyperkalemia is present out of keeping with the degree of renal failure (so hyperkalemia at lower serum creatinine) - but this is not a hard and fast rule. Electrolytes do not help much in diagnosing GN.

There is a nice Medscape article on this linked below.

Dr. Jordan Weinstein
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