BUN:Cr ratio and predicting azotemia

[toothless rufus]

---

Members don't see this ad.

Ok, so according to goljan, >15 is prerenal AND postrenal <=15 is renal AND normal. Is this right? If so, how can you rely on the ratio to determine whats going on since the measures can mean two things?

 

[Doctor4Life1769]

Ok, so according to goljan, >15 is prerenal AND postrenal <=15 is renal AND normal. Is this right? If so, how can you rely on the ratio to determine whats going on since the measures can mean two things?

I was under the impression, pre-renal was > 20, renal > 15, post-renal < 15.

 

[markglt]

azotemia is a myth

 

[toothless rufus]

I was under the impression, pre-renal was > 20, renal > 15, post-renal < 15.

These values are from RR. I think on the audio he used 20 as the cutoff.

 

[Captopril]

I was under the impression, pre-renal was > 20, renal > 15, post-renal < 15.

These are the numbers I've always used as well

 

[Lonely Sol]

Can anyone confirm the exact number we should go by? I feel that the each source has their own numbers.

 

[ensuii]

Can anyone confirm the exact number we should go by? I feel that the each source has their own numbers.

I think it's more important to look at the ratio in context to FeNa and symptoms to identify prerenal/renal/postrenal

 

[shan564]

I think it's more important to look at the ratio in context to FeNa and symptoms to identify prerenal/renal/postrenal

+1. You can't diagnose prerenal/renal/postrenal azotemia based purely on the BUN:Creatinine ratio... especially in patients who have mixed disease. If the ratio is borderline, you can usually use the clinical context to figure it out.

 

[docray]

Not to add to the confusion, but I've always known this :

Normal is 10
In Prerenal Azotemia, the BUN: Cr ratio is > 20 (some say 15😕)

In Intrarenal, the BUN: Cr ratio is may be normal (10) cos both BUN and Cr are elevated but may also increase though still less than 15

In post renal ratio is also >20 but may reduce to intrinsic range if secondary renal damage occurs due to chronic obstruction. i.e it becomes intrarenal.

+ as someone already said, also consider the FeNa http://countdown2ck.blogspot.com/2012/05/remember_18.html

 

[OveractiveBrain]

Ok, so according to goljan, >15 is prerenal AND postrenal <=15 is renal AND normal. Is this right? If so, how can you rely on the ratio to determine whats going on since the measures can mean two things?

Stop. Entirely. Erase everything you learned from Goljan regarding this issue. Reset to 0.

IF you are considering an isolated elevation of creatinine, then the first clue you have is a BUN:Cr ratio. Its a clue. Thats it. If it is >20, then it is probably prerenal. If it is >30 it is likely prerenal. If its >40 its almost certainly prerenal. This all assumes you had a normal creatinine to begin with (and this is usually the case on Step 2).

Given that clue, FeNa (no diuretics) and FeUrea (diuretics) can be used to confirm suspicion. Here, cutoffs actually matter. FeNa < 1 Prerenal, FeNa > 4 Post, anything in between intrarenal (of course presuming there is only one derrangement and no preexisting kidney disease, normal is usually the case on Step 2).

That being said, if they have a good reason to be volume down (fever, tachypnea, burns, N/V, no po) then you can presume vol down and give fluids. If they are obviously fluid overloaded (CHF with fluid to their nipples) then you can assume prerenal from 3rd spacing and give lasix. Then you reassess with a Cr to see whats changed.

You cannot use the BUN:Cr ratio to accurately diagnose the condition, and, if you try, the only useful data point is a BUN:Cr ratio > 20 for the diagnosis of prerenal (Prerenal vs NOT prerenal, thats it).