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15 Cards in this Set
- Front
- Back
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Know the differences btwn creatinine clearance and true GFR:
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Measurements of creatinine in the urine can aid in estimating the glomerular filtration rate and renal fx. Creatine clearance OVER estimates true GFR.
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How do Urea clearance and GFR compare?
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Urea clearance has a systematic bias towards underestimation true GFR.
'UU': Urea Underestimates |
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What is the relationship between serum creatinine and creatine clearance?
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Every time the serum creatinine doubles, the creatinine clearance halves. Thus in the beginning, small changes in serum creatinine correspond to big changes in clearance (serum creatinine measurements are insensitive to renal fx decline), and as serum creatinine levels get higher, around 6-12 mg/dl, the measurements are overly sensitive to renal fx decline.
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The pitfalls of using serum creatinine:
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There are normal range issues.
Non-linear relationship mkes interpretation more difficult. small changes from normal can indicate a big change in clearance. This means that some GPs wait too long to refer pts to nephrology. Difficulties in interpretation related to age, gender, fitness-related differences in muscle mass and creatine production. (A 70 yo, 30 kg female's curve is shifted down and to the left compared with a 30 yo 100 kg male. -- the 70 yo appears to be in the normal range for longer than she really is -- easy to underestimate the severity of her disease) |
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cockroft and gault equation:
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Corrects for the underestimations of creat clearance due to age/gender/muscle mass differences.
For males: (140-age) * weight(kg) -------------------------------- 72 * serum creat (mg/dl) For women: (140-age) * weight(kg) -------------------------------- *0.85 72 * serum creat (mg/dl) Women have less creat clearance and more calculations needed |
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cockroft and gault over estimates or underestimates GFR?
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OVERestimates GFR.
This is to be expected.. the Cock and gault formula was made to estimate creat clearance, and creat clearance overestimates GFR. *Body builders over-do it, and they also have a lot of creatine compared to nl. |
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What formula(s) do(es) aim to estimate true GFR?
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MDMR
CDK-EPI Serum Cystatin C (And obviously the tried and true Ithalimate, Inulin, Cr-EDTA and Tc-DTPA) |
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What's good about measuring serum cystatin c to estimate GFR? What's bad about it?
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Pros:
-Serum cystatin c is not affected by race, muscle or gender. -There's a constant rate of production btwn age .25-70, when ppl are healthy (less variations in the nl range). -Freely filtered, completely catabolized by tubules. -Negligible amnt of cystatin c in urine. No extrarenal route of excretion. Cons: -More expensive -Still an inverse relationship btwn serum cystatin c and GFR |
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What formula is approp for kiddies?
What does it estimate? |
Pediatric creat clearance should be estimated w the Schwartz formula.
creat clearance = (k * height) ----------------- (serum creat) |
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When is eGFR (MDRD) potentially erroneous?
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-fluctuating renal fx (acute renal failure)
-morbid obesity -volume overload (CHF, ascites) -HEALTHY PTS in general (GFR> 60 ml/min/1.73m2)... remember, this formula was developed on ppl who were already sick. -advanced liver disease -extreme serum albumin values -organ transplant recipients -lil babes and the geriatric population |
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to estimate GFR in healthy pts, use the ___ formula.
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the CDK-EPI
Learning from the MDRD trial, CDK-EPI included a more comprehensive sample group, too. Women were better represented, blacks were better represented (though not hispanics and asians). Diabetics were also better represented in CKD-EPI. |
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Does ckd-epi under or over estimate actual gfr, on avg?
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on avg, it underestimates a little bit. *MDRD underestimates gfr a lot more, on avg.
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why do we even bother having mdrd around?
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well, it's not as good as cdk-epi, that's true, but cdk-epi came from it, so it's an important and relevant part of understanding these tools
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what determines GFR?
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(1) # of fx-ing nephrons
(2) filtration rate of single nephrons glomerular UF coefficient (k) mean transcapillary P diff (hydrostatic - oncotic P) (3) "nl" is 100-120 ml/min/1.73m2 |
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GFR estimation formula by determined substance:
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urine concentration times volume of urine, all divided by the plasma concentration
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