So, this is one of those few calculation problems that you might see on Step 1.
The vignette points to 3-day history of vomiting/diarrhea, holding nothing down, and little urine output. Also, from the BP readings, this guy is orthostatic pointing towards volume depletion.
However, looking at the Serum labs, he has azotemia with a BUN of 50mg/dL. In this case, AKI (ATN in the answers) and volume depletion should be your top two.
To differentiate, you could calculate FENa, the fraction of Na excreted.
FENa = ( [Na in the Urine] * Flow Rate of the Urine / [Na in the Plasma] ) / ([Cr in the Urine] * Flow Rate of the Urine / [Cr in the Plasma]
Simplifying to cancel out the Flow Rate of the Urine leaves us with:
FENa = ([Na in the Urine] * [Cr in the Plasma]) / ([Na in the Plasma] * [Cr in the Urine])
For this problem: FENa = [10mEq/L] * [2.2mg/dL] / ([146mEq/dL]*[19mg/dL]) = 0.007 or 0.7%
That points definitively to prerenal AKI and volume depletion.
ibpstepprepmore specifically , a specific gravity of greater than or equal to 1.030 = frank dehydration = volume depletion. I don't think you have to calculate the FeNa, knowing urine sodium is less than 20 indicates prerenal failure ( FA 19 590). The key to this problem is the specific gravity +4
cheesetouchI didn't do any calculation, just went off patient's history, tried to think of how the Na is barely high (146 when NBME normal is up to 145), what the heck would make uric acid high in that history, plus no RBCs in urine.+1
sexymexican888FA 2020 P 601: You don't have to calculate FENa, you can just look at the sodium 10 mEq/L. In pre-renal azotemia urine Na is <20 mEq/L meaning the tubules are patent and Na is being reabsorbed properly, even more than usual since its sensing volume depletion. IN CONTRAST intra-renal failure will have a urine Na >40 mEq/L bc the renal tubules are damaged (ATN) and cannot reabsorb Na properly. +1
sexymexican888FENa is the same concept
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pre-renal azotemia ->FENa <1% -> Na is being reabsorbed -> Kidney is healthy and working
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Intrinsic renal failure -> FENa >2% -> Na is NOT being reabsorbed properly -> Kidney is NOT healthy +
sexymexican888also BUN/Cr ratio: 50/2.2 ~ 25 in pre-renal azotemia BUN/Cr >20 (kidney's capacity to reabsorb is intact)
if it was ATN BUN/Cr ratio would be <15 (kidney is fucked up cant reabsorb urea properly)+4
ddaddyCalculation is a waste of time. Hx of vomiting/diarrhea is all that is needed+1
an1Tip: if a patient comes in with MI and urine function is cool but then declines after 2 days, suspect pre-renal AKI (sketchy path). Calculate the BUN/Cr ration. >20 is Pre-renal. <20 is intrarenal. and Post renal can be >20 in early stages or <20 in last stages+
submitted by โacerj(13)
So, this is one of those few calculation problems that you might see on Step 1.
The vignette points to 3-day history of vomiting/diarrhea, holding nothing down, and little urine output. Also, from the BP readings, this guy is orthostatic pointing towards volume depletion.
However, looking at the Serum labs, he has azotemia with a BUN of 50mg/dL. In this case, AKI (ATN in the answers) and volume depletion should be your top two.
To differentiate, you could calculate FENa, the fraction of Na excreted.
FENa = ( [Na in the Urine] * Flow Rate of the Urine / [Na in the Plasma] ) / ([Cr in the Urine] * Flow Rate of the Urine / [Cr in the Plasma]
Simplifying to cancel out the Flow Rate of the Urine leaves us with:
FENa = ([Na in the Urine] * [Cr in the Plasma]) / ([Na in the Plasma] * [Cr in the Urine])
For this problem: FENa = [10mEq/L] * [2.2mg/dL] / ([146mEq/dL]*[19mg/dL]) = 0.007 or 0.7%
That points definitively to prerenal AKI and volume depletion.