Re: Schwartz formulae

Mahesh, Samer, The Schwartz formulae are based on theoretical allometric principles. The allometric theory is based on the assumption that creatinine production rate (CPR) is proportional to body surface area. Other possibilities exist e.g. that CPR is directly proportional to body mass. It is usually assumed that CPR is proportional to muscle mass. Theoretical allometry (West et al. 1999) would suggest that a structural feature such as muscle mass would be best described by an allometric coefficient of 1. Putting these two assumptions together would indicate that CPR and body mass should be related linearly. This is the assumption made in the Cockcroft & Gault formula. Part of the need to change 'k' for different sizes of children may be due to choosing the wrong allometric model. The Schwartz formula coefficient 'k' has various empirically determined values depending on age and sex. This means that the original allometric theory has been discarded in favour of empirical allometry. Theoretical allometry is primarily concerned with the relationship between body size and body structure or function. Other things such as age and sex can of course be important to describe how muscle mass and thus CPR may differ between people. But these other factors should be considered in addition to the fundamental allometric method and not used to distort it. The changing values of 'k' in the various Schwartz formulae are a consequence of this ad hoc approach. It should also be noted that the Schwartz formulae are designed to predict creatinine clearance (CLcr). In children with more or less normal renal function the Schwartz predicted CLcr is 30% greater than concurrently measured inulin clearance (a 'gold standard' GFR estimation method) (see Hellerstein et al 1992). This is different from adults in whom GFR and CLcr are very similar. Nick Hellerstein S, Alon U, Warady BA. Creatinine for estimation of glomerular filtration rate. Pediatr Nephrol. 1992;6:507-11. West GB, Brown JH, Enquist BJ. The fourth dimension of life: fractal geometry and allometric scaling of organisms. Science. 1999;284(5420):1677-9. Mouksassi Mohamad-Samer wrote: > Dear Mahesh, > > As any model the original equation of Schwartz is evolving over the years and > as more data is gathered. > > The original paper had only one k : > > Schwartz GJ, Haycock GB, Edelmann CM Jr, Spitzer A. A simple estimate of > glomerular filtration rate in children derived from body length and plasma > creatinine. Pediatrics 1976; 58:259–263. > > Then it was extended to other age groups to account for the maturational > changes: > > Schwartz GJ, Brion LP, Spitzer A. The use of plasma creatinine concentration > for estimating glomerular filtration rate in infants, children, and > adolescents. Pediatr Clin North Am 1987; 34:571–590. > > Then other papers suggested different adjustment to the original formula... and > to make it applicable for other age groups. > > Please also note that the original k are applicable only for the old/standard > Jaffe method to measure serum creatinine. > Newer methods require some adjustments. > > Newer formulas are now published using cystatin and other markers. > > Zappitelli M, Parvex P, Joseph L, et al. Derivation and validation of cystatin > C-based prediction equations for GFR in children. Am J Kidney Dis 2006; > 48:221–230 > > (standard Schwartz formula may be misleading when we have renal impairment) > New equations to estimate GFR in children with CKD. > Schwartz GJ, Muñoz A, Schneider MF, Mak RH, Kaskel F, Warady BA, Furth SL. > J Am Soc Nephrol. 2009 Mar;20(3):629-37. Epub 2009 Jan 21. > > Estimating and measuring glomerular filtration rate in children. > Work DF, Schwartz GJ. > Curr Opin Nephrol Hypertens. 2008 May;17(3):320-5. Review. > > The right formula will depend on the nature of your pediatric population you > want to calculate CRCL for , the method used for serum creatinine and whether > you have some renally impaired children or not. > > Hope this helps > > Samer >