Equations to estimate creatinine excretion rate: the CKD epidemiology collaboration.
OBJECTIVE
Creatinine excretion rate (CER) indicates timed urine collection accuracy. Although equations to estimate CER exist, their bias and precision are untested and none simultaneously include age, sex, race, and weight.


METHODS
Participants (n = 2466) from three kidney disease trials were randomly allocated into equation development (2/3) and internal validation (1/3) data sets. CER served as the dependent variable in linear regression to develop new equations. Their stability was assessed within the internal validation data set. Among 987 individuals from three additional studies the equations were externally validated and compared with existing equations.


RESULTS
Mean age was 46 years, 42% were women, and 9% were black. Age, sex, race, weight, and serum phosphorus improved model fit. Two equations were developed, with or without serum phosphorus. In external validation, the new equations showed little bias (mean difference [measured - estimated CER] -0.7% [95% confidence interval -2.5% to 1.0%] and 0.3% [95% confidence interval -2.6% to 3.1%], respectively) and moderate precision (estimated CER within 30% of measured CER among 79% [76% to 81%] and 81% [77% to 85%], respectively). Corresponding numbers within 15% were 51% [48% to 54%] and 54% [50% to 59%]). Compared with existing equations, the new equations had similar accuracy but showed less bias in individuals with high measured CER.


CONCLUSIONS
CER can be estimated with commonly available variables with little bias and moderate precision, which may facilitate assessment of accuracy of timed urine collections.
Ix
Joachim H
JH
Division of Nephrology and Hypertension, Department of Medicine, University of California San Diego, and San Diego VA Healthcare System, 3350 La Jolla Village Drive, Mail Code 111-H, San Diego, CA 92161, USA. joeix@ucsd.edu
Wassel
Christina L
CL
Stevens
Lesley A
LA
Beck
Gerald J
GJ
Froissart
Marc
M
Navis
Gerjan
G
Rodby
Roger
R
Torres
Vicente E
VE
Zhang
Yaping Lucy
YL
Greene
Tom
T
Levey
Andrew S
AS
D000328:Adult; D000368:Aged; D002908:Chronic Disease; D003299:Cooperative Behavior; D003404:Creatinine / Q000652:urine*; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D007674:Kidney Diseases / Q000503:physiopathology; D008297:Male; D008875:Middle Aged
D003404:Creatinine