page38
Renal function is measured either in terms of estimated
glomerular
filtration rate (eGFR)
calculated from a
formula derived from the Modification of Diet in Renal
Disease study (‘MDRD formula’
that uses serum creat-
inine,
age,
sex,
and race (for Afro-Caribbean patients))
or it
can be expressed as creatinine clearance (best
derived from a 24-hour urine collection but often calcu-
lated from the Cockcroft and Gault formula (CG).
Cockcroft and Gault formula
Estimated Creat-
inine Clearance in
mL/minute
=
(140 – Age) Weight Constant
Serum creatinine
Age in years
Weight in kilograms; use ideal body-weight
Serum creatinine in micromol/litre
Constant = 1.23 for men; 1.04 for women
The serum-creatinine concentration is sometimes used
instead as a measure of
renal
function but it is only a
rough guide to drug dosing.
Important
Renal
function
in
adults
is
increasingly
being
reported on the basis of estimated glomerular filtra-
tion rate (eGFR) normalised to a body surface area
of 1.73 m
2
and derived from the Modification of Diet
in Renal
Disease (MDRD)
formula.
However,
pub-
lished information on the effects of renal impairment
on drug elimination is usually stated in terms of
creatinine clearance as a surrogate for glomerular
filtration rate (GFR).
The information on dosage adjustment in the BNF is
expressed in terms of eGFR,
rather than creatinine
clearance,
for
most
drugs (see exceptions below:
Toxic Drugs and Patients at
Extremes of
Weight).
Although the two measures of renal function are not
interchangeable,
in practice,
for most drugs and for
most patients (over 18 years) of
average build and
height,
eGFR (MDRD ‘formula’)
can be used to
determine dosage adjustments in place of creatinine
clearance. An individual’s absolute glomerular filtra-
tion rate can be calculated from the eGFR as follows:
GFR
Absolute
= eGFR (individual’s
body surface
area/1.73)
Toxic drugs
For
potentially toxic drugs with a
small safety margin, creatinine clearance (calculated
from the Cockcroft
and Gault
formula)
should be
used to adjust drug dosages in addition to plasma-
drug concentration and clinical response.
Patients at extremes of weight
In patients at
both extremes of weight (BMI of
less than 18.5 kg/
m
2
or
greater
than 30 kg/m
2
)
the absolute glom-
erular filtration rate or creatinine clearance (calcu-
lated from the Cockcroft and Gault formula) should
be used to adjust drug dosages.
In the BNF,
values for eGFR,
creatinine clearance (for
toxic drugs),
or another measure of
renal
function are
included where possible.
However,
where such values
are not available, the BNF reflects the terms used in the
published information.
Chronic kidney disease in adults:
UK guidelines
for
identification, management and referral (March 2006)
define renal function as follows:
Degree of impairment
eGFR mL/minute/1.73 m
2
Normal - Stage 1
More than 90 (with other
evidence of kidney damage)
Mild - Stage 2
60–89 (with other evidence
of kidney damage)
Moderate
1
- Stage 3
30–59
Severe - Stage 4
15–29
Established renal failure -
Stage 5
Less than 15
1. NICE clinical guideline 73 (September 2008)—Chronic
kidney disease: Stage 3A eGFR 45–59, Stage 3B eGFR 30–
44
Dialysis
For prescribing in patients on continuous ambula-
tory peritoneal
dialysis (CAPD)
or
haemodialysis,
consult specialist literature.
Drug prescribing should be kept to the minimum in all
patients with severe renal disease.
If
even mild renal
impairment
is considered likely on
clinical
grounds,
renal
function should be
checked
before prescribing any drug which requires dose mod-
ification.
Where care is needed when prescribing in renal impair-
ment,
this is indicated under the relevant
drug in the
BNF.
18
Prescribing in renal impairment
BNF 61
Prescribing in renal impairment