Influence of Body Weight on the
Performance of Glomerular Filtration Rate
Estimators in Subjects With Type 2
Diabetes
RICHARD A. CHUDLEIGH, MRCP
1
GARETH DUNSEATH, MPHIL
1
RAJESH PETER, MRCP
1
JOHN N. HARVEY, MD, FRCP
2
RICHARD L. OLLERTON, PHD
3
STEVE LUZIO, MD
1
DAVID R. OWENS, MD, FRCP
1
T
he American Diabetes Association
recommends estimation of glomer-
ular filtration rate (GFR) (1) by ei-
ther the Cockcroft-Gault (2) or the
Modification of Diet in Renal Disease
(MDRD) (3) equation in all patients with
diabetes. The implication is that these
equations provide similar results. Body
weight is a numerator in the Cockcroft-
Gault equation; however, it is absent from
the MDRD equation. This may explain
some of the difference in the ability of
these equations to estimate GFR in pa-
tients with type 2 diabetes, over 80% of
whom are obese (4), and may lead to dis-
crepancies in reporting of chronic kidney
disease stage (5). Our study was designed
to identify whether body weight may ex-
plain variability in performance between
the Cockcroft-Gault and MDRD equa-
tions in patients newly diagnosed with
type 2 diabetes.
RESEARCH DESIGN AND
METHODS — The study population
consisted of 293 subjects newly diag-
nosed with type 2 diabetes; 96% were
Caucasian and the remainder of South
Asian origin. No African-American sub-
jects were included.
Following an overnight fast, anthro-
pometric and biochemical measurements
were made. Subjects were intravenously
cannulated, and 1 MBq
51
Cr-EDTA was
administered at 0 min, with further blood
sampling at 44, 120, 180, and 240 min.
The
51
Cr-EDTA plasma clearance
method for GFR measurement, corrected
for body surface area (BSA), has been vali-
dated previously (6). This allows estimation
of a two-compartment model. A close cor-
relation exists between total plasma clear-
ance of
51
Cr-EDTA and inulin clearance
determined by the classical technique (7).
Creatinine levels were determined us-
ing the OCD (Johnson & Johnson) dry
slide system on the Vitros 750 RC and
950 analyzer. The coefficients of variation
were 4.2% at a creatinine concentration of
103 mol/l and 1.92% at 16 mol/l.
Estimated GFR (eGFR) (in milliliters
per min per 1.73 meters squared) was cal-
culated by the Cockcroft-Gault formula,
corrected for BSA (2), and the MDRD for-
mula (3), both of which are shown below:
Cockcroft-Gault formula:
[140 age (years)] weight (kg) k c
serum creatinine (mol/l)
(eq. 1)
where k is 1.23 for men and 1.04 for fe-
males and c adjusts for BSA. c 1.73/BSA
with BSA calculated using the DuBois for-
mula (8):
BSA (m
2
) [weight (kg)]
0.425
[height (cm)]
0.725
0.007184
(eq. 2)
MDRD formula:
186 [serum creatinine (mol/l)/88.4]
1.154
[age (years)]
0.203
(0.742 if female)
(1.210 if African American) (eq. 3)
Statistical analysis
To compare formula performance over
different body weight ranges while main-
taining group sizes suitable to make the
calculations, subjects were grouped into
tertiles according to body weight. Other
comparisons were made using the full
ranges of the relevant data. eGFR results
derived by the Cockcroft-Gault and
MDRD formulae were compared with iso-
topic GFR by means of two-tailed paired
and unpaired t tests as appropriate (con-
firmed by nonparametric equivalents for
abnormal distributions) and
2
test for
proportions and linear regression. Statis-
tical test assumptions were checked
graphically and by use of suitable statis-
tics as required. All calculations were per-
formed using SPSS (version 12.0.1).
Results are presented as mean SD un-
less otherwise indicated. P 0.05 was
taken to indicate statistical significance.
RESULTS — Demographic character-
istics of study participants are summa-
rized in Table 1. Normoalbuminuric
subjects comprised 91% of participants.
A positive correlation between GFR and
body weight (r 0.194) was found and
was also seen across weight groups. Mean
fasting plasma glucose and A1C were sim-
ilar between groups.
Performance of Cockcroft-Gault– and
MDRD formulae– derived eGFRs accord-
ing to body weight is presented in Table 1.
Bias values show that eGFR significantly
underestimates isotopic GFR. However,
●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●
From the
1
Diabetes Research Unit, Llandough Hospital, Penarth, Cardiff, U.K.; the
2
Wrexham Maelor
Hospital, Wrexham, North Wales, U.K.; and the
3
School of Computing and Mathematics, University of
Western Sydney, Sydney, Australia.
Address correspondence and reprint requests to Richard Chudleigh, 20 Llewelyn Goch, Parc Rhydlafar,
St. Fagans, Cardiff, U.K. CF5 6HR. E-mail: rachudleigh@hotmail.com.
Received for publication 22 May 2007 and accepted in revised form 1 October 2007.
Published ahead of print at http://care.diabetesjournals.org on 12 October 2007. DOI: 10.2337/dc07-
1335.
Abbreviations: BSA, body surface area; eGFR, estimated glomerular filtration rate; GFR, glomerular
filtration rate; MDRD, Modification of Diet in Renal Disease.
A table elsewhere in this issue shows conventional and Syste `me International (SI) units and conversion
factors for many substances.
© 2008 by the American Diabetes Association.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby
marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Clinical Care/Education/Nutrition/Psychosocial Research
B R I E F R E P O R T
DIABETES CARE, VOLUME 31, NUMBER 1, JANUARY 2008 47