Enrico Canuto, Former Faculty, Politecnico di Torino, Torino, Italy
Creatinine concentrations in blood serum and in urine [mg/dl] are intermediate indicators of kidney health. Creatinine is a by product of muscle metabolism that is excreted primarily and unchanged by kidneys . Creatinine production reflects lean body mass, which changing little day by day, makes production rate fairly constant, but declining with the age in line with decreasing muscle mass.
Creatinine clearance is the volume per unit time (volume flow) of blood plasma that is cleared of creatinine and is useful for deriving the standard clinically index of renal efficiency, the Glomerular Filtration Rate (GFR, see below). Given the urine concentration , the serum concentration and the urine volume flow , we write
Usually the clearance is scaled to a specific body by the body area ratio , where is the body surface area derived from the body mass and is a reference body area (see the Appendix). Thus (1) is rewritten for a body of mass as
For instance, given , , and , we obtain . The clinical lower limit is .
GFR experimental formulas
Glomerular filtration rate (GFR) corresponds to renal clearance rate (like that of creatinine in (1)), when the serum solute is freely filtered from glomerular capillaries into the so called Bowman’s capsules without being re-absorbed and secreted by other kidney capillaries, which latter occurs to creatinine.
As a result, the measured creatinine clearance in (1) overestimates GFR by 10% and more . Second, it does not account for body ageing. Third, it requires three measurements, two concentrations and urine flow. Several equations have been experimentally obtained from patient populations to account for age, gender, ethnicity and to just require the serum concentration . We just report the so called CKD-EPI formula (Chronic Kidney Disease Epidemiology Collaboration, 2009, ) :
where denotes gender, ethnicity and the age in years [a]. For instance for a Caucasian male with , and , we obtain .
The problem of formula (3) is the parameter uncertainty, which combines with the concentration measurement error.
Naively, creatinine is a creatine molecule that losing a water molecule forces carbon and nitrogen to close as in Figure 1
Figure 1 -
 A.O. Hosten, Chapter 193: BUN and Creatinin, in A.O. Hosten et al. eds, Clinical Methods: The History, Physical, and Laboratory Examinations, 3rd edition (Butterworths, Boston, 1990), pp. 874-878.
 J.R. Delanghe and M.M. Speeckaert, creatinine determination according to Jaffe- what does it stands for?, NDT Plus, Vol. 4, 2011, pp. 83-86.
 Wikipedia, Globular filtration rate.
Body surface area. A widely used formula is the Du Bois formula