Low eGFR but normal urine output?

[SqEdSaint]

I have encountered several patients with eGFR below 15cc/min/173cmcubed (some even less) with high BUN and creatitine but still putting out fair urine amount (>0.5cc/kg/hr). Can someone plz explain the physiology behind this phenomenon? Thx!

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[drfunktacular]

https://www.uptodate.com/contents/urine-output-and-residual-kidney-function-in-kidney-failure

Although the glomerular filtration rate (GFR) is very low in patients with end-stage kidney disease (ESKD), the urine output is variable, ranging from oliguria to normal or even above normal levels. These findings are related to the fact that the urine output is determined not by the GFR alone, but also by the difference between the GFR and the rate of tubular reabsorption. If, for example, a patient with advanced acute or chronic kidney failure has a GFR of 5 L/day (versus the normal of 140 to 180 L/day), the daily urine output will still be 1.5 L if only 3.5 L of the filtrate is reabsorbed.

 

[SqEdSaint]

Sorry for my poor understanding of renal physiology. In this case, serum BUN/Cr are elevated because they are not being filtered through the glomeruli possibly due to scarring or reduced renal artery blood flow, then where's all that water in the tubules coming from if there is less blood filtering through the glomeruli? Please correct me if I'm getting this wrong.

 

[lilsebastian]

Sorry for my poor understanding of renal physiology. In this case, serum BUN/Cr are elevated because they are not being filtered through the glomeruli possibly due to scarring or reduced renal artery blood flow, then where's all that water in the tubules coming from if there is less blood filtering through the glomeruli? Please correct me if I'm getting this wrong.

Pay attention to the units. Remember the units of GFR are measured in ml per MINUTE. Glomeruli in normal kidneys filter over a hundred liters of blood a day. But, because of the way the nephron works, is reabsorbs almost all of that in the distal nephron losing only hundreds of ml to a couple liters of urine per day. It's remarkably efficient at doing at reabsorbing filtrate. Urea and creatinine are filtered (urea also reabsorbed, creatinine also secreted in addition to filtration) like everything else. Relative to normal kidneys, they're being filtered out less because there are fewer nephrons, but you are still filtering liters of blood a day, and therefore potentially able to produce liters of urine. If your gfr is 10 ml/min (ckd5), your glomeruli are still filtering 600 ml in an hour and 14.4 liters a day.