Re: [FORGED] Allometric scaling of renal clearance with estimated glomerular filtration rate

Hi Rob, Thanks for bringing this up again. I don't think much has changed since I wrote this in 2013 ( http://cognigencorp.com/nonmem/current/2013-August/4697.html ) 1. Theory Based Allometry or Surface Area "Note that using surface area as a form of size standardization forglomerular filtration rate has no theoretical nor experimental support when compared to theory based allometry (Rhodin et al. 2009). So I donot agree with standardizing CLCR to 1.73 m^2. I know this is frequently done but in fact this is just based on tradition and an out of datetheory of scaling based on surface area (see Anderson & Holford 2008)." There is no biological or experimental support for using surface area to scale renal function markers such as GFR and CLcr. In contrast, there is strong biological based theory and experimental support for using theory based allometry (see Holford & Anderson 2017 for a recent review). 2. Mechanism Based Models for CLcr I also wrote in 2013: "The MDRD method of predicting glomerular filtration rate is astatistical absurdity which does not include any measurement of size for its prediction. I would certainly not recommend using it for anyscientific purpose. " This applies equally well to the CKD-EPI method. Let me explain why it is a absurdity generated by a naive statistician using CLcr as an example. CLcr can be calculated from the creatinine excretion rate (CER) and the serum creatiniine. This is based on the definition of clearance and is true without any assumptions. CLcr=CER/Scr If we then assume Scr is at steady state then CER will be equal to creatinine production rate (CPR) and we can use this: CLcr=CPR/Scr All rational models for predicting CLcr without measurement of CER use models to predict CPR e.g. CPR=(140-Age)*Weight/72 use Cockcroft & Gault to predict CPR in males then CLcr=CPR/Scr is Cockcroft & Gault CLcr ml/min Dividing CPR by Scr gives the CLcr. This can be written equivalently but less clearly: CLcr=CPR*Scr^-1 The empirical models such as MDRD and CKI-EPI (see below) involve the absurdity of estimating the known exponent for Scr of -1. These estimates must be wrong based on the theory I have outlined above (unless the estimate is exactly -1). The reported estimates are -1.209 for CKI-EPI and -1.154 for MDRD. In addition, and more importantly,they have no direct measure of body size which seriously limits the value outside the typical weight distribution and they are only applicable to adults. GFR can be described from premature neonates to adults using theory based allometry and maturation based on post-menstrual age so GFR predicttions should try to follow the concepts used there (Rhodin 2008). So what to do? First -- don't use MDRD or CKI-EPI unless you are sure you are applying them to a population similar to that used to develop these empirical predictions. You could add allometric scaling to the eGFR by assuming the 1.72m^2 value is equivalent to 70 kg with a fat free mass (FFM) of 56.1 kg. Then scaling the eGFR by (WT/70)^(3/4) or (FFM/56.1)^(3/4). I use the Schwartz (1992) equations for neonates, children and teenagers then the Matthews (2004) equation for adults. I am working on an integrated method for CPR prediction which was presented as a work in progress at PAGE this year. Watch this space... Best wishes, Nick MDRD eGFR =175 x (SCr)^-1.154 x (age)-0.203 x 0.742 [if female] x 1.212 [if Black] CKI-EPI eGFR = 141 x min(SCr/k, 1)^alpha x max(SCr /kappa, 1)^-1.209 x 0.993^Age x 1.018 [if female] x 1.159 [if Black] kappa = 0.7 (females) or 0.9 (males) alpha = -0.329 (females) or -0.411 (males) eGFR (estimated glomerular filtration rate) = mL/min/1.73 m2; SCr (standardized serum creatinine) = mg/dL Holford NHG, Anderson BJ. Allometric size: The scientific theory and extension to normal fat mass. Eur J Pharm Sci. 2017;109(Supplement):S59-S64. Rhodin MM, Anderson BJ, Peters AM, Coulthard MG, Wilkins B, Cole M, Chatelut E, Grubb A, Veal GJ, Keir MJ, Holford NH Human renal function maturation – a quantitative description using weight and postmenstrual age. Pediatr Nephrol. 2008 Schwartz GJ. Does kL/PCr estimate GFR, or does GFR determine k? Pediatr Nephrol. 1992;6(6):512-5. Matthews I, Kirkpatrick C, Holford N. Quantitative justification for target concentration intervention -- parameter variability and predictive performance using population pharmacokinetic models for aminoglycosides. Br J Clin Pharmacol. 2004;58(1):8-19. Holford N, Sherwin CM. Scaling renal function in neonates and infants to describe the pharmacodynamics of antibiotic nephrotoxicity. PAGE 26 Abstr 7208 [wwwpage-meetingorg/?abstract=7208]. 2017.