Diagnosis: Acute Tubular Necrosis (ATN)
This patient has acute tubular necrosis (ATN), not pre-renal azotemia, based on the urine sodium of 60 mEq/L and inappropriately dilute urine osmolality of 220 mOsm/kg despite clinical dehydration from diarrhea. 1
Diagnostic Reasoning
The laboratory values definitively point to ATN through the following key findings:
Urine sodium >20 mEq/L has >85% specificity for ATN and effectively rules out pre-renal causes - this patient's value of 60 mEq/L strongly indicates tubular damage with inability to conserve sodium despite the clinical picture of dehydration. 1, 2
The urine osmolality of 220 mOsm/kg is inappropriately dilute for a dehydrated patient, reflecting tubular damage and inability to concentrate urine, which is characteristic of ATN rather than pre-renal azotemia where you would expect urine osmolality >500 mOsm/kg. 1
The specific gravity of 900 (assuming this means 1.009) is also inappropriately low for someone with dehydration, further supporting tubular dysfunction. 2
The fractional excretion of sodium (FENa) would be >1% in this case (calculated from the high urine sodium), which is diagnostic of ATN, whereas FENa <1% would suggest pre-renal causes. 1
Critical Clinical Pitfall
Do not be misled by the clinical context of diarrhea and dehydration into assuming this must be pre-renal azotemia. 3 While the patient presented with volume depletion, the tubular injury has already occurred, and the kidneys have lost their ability to appropriately respond to hypovolemia by conserving sodium and concentrating urine. 4 In fact, severe volume depletion can develop as a superimposed complication in patients with established ATN, and a low FENa in that specific scenario would suggest the need for volume resuscitation. 4
Immediate Management Steps
Stop all nephrotoxic medications immediately including NSAIDs, aminoglycosides, ACE inhibitors, ARBs, and any contrast agents to prevent further kidney damage. 1, 5
Provide cautious volume resuscitation with crystalloids to correct the dehydration from diarrhea, as aggressive fluid resuscitation is indicated in cases of hypovolemia even with established ATN. 5
Consider 20% albumin at 1 g/kg (maximum 100 g) for two consecutive days if the patient has cirrhosis or severe hypovolemia not responding to crystalloids. 1, 5
Measure serum creatinine daily to assess AKI stage progression and monitor urine output daily, as oliguria indicates poor prognosis. 1, 5
Withdraw diuretics if the patient is receiving them, as they should not be used to improve kidney function in ATN. 1, 5