Transcript Analytical Mass Spectrometry TECHNIQUES, APPLICATION

Plasma creatinine and the estimation of glomerular
filtration rate (GFR)
R Neil Dalton
WellChild Laboratory
King’s College London/ Guy’s Hospital
ACB South West & Wessex Region Scientific Meeting
Salisbury, 4th July 2006
Glomerular Filtration Rate
Why the sudden interest in estimating GFR from plasma
creatinine?
Final realisation that a plasma creatinine normal range is
meaningless
Conceptual difficulty relating plasma creatinine to GFR
Formulae attempting to fix the problem - eGFR
eGFR routine in paediatric nephrology for nearly 30y
The Glomerulus
Glomerular Filtration Rate
fundamental to:
diagnosis of kidney disease
early detection
stratification
monitoring the progression of kidney disease
prognosis
efficacy of treatment
facilitate timing of therapeutic interventions
drug dosage
elimination of drugs/drug metabolites by the kidney
Glomerular Filtration Rate
Glomerular filtration rate (GFR) is the clearance, by
the kidney, of a marker in plasma, expressed as the
volume of plasma completely cleared of the
marker per unit time
UV ml/min
P
.
Requires accurately timed urine collection!
Glomerular Filtration Rate
The ideal marker is endogenous, freely filtered by
the glomerulus, neither reabsorbed nor secreted by
the kidney tubule, and eliminated only by the
kidney
No ideal marker described!
Defined using exogenous markers,
primarily inulin
Glomerular Filtration Rate
Hence, formal measurement of GFR rarely performed
Difficult!
Reliable?
Logistics of test performance
Radiochemical/biochemical tracer analysis
Failure to appreciate the clinical importance
Glomerular Filtration Rate
Clinical importance
Detection of kidney disease
Cardiovascular risk
Glomerular Filtration Rate
Compromise:
24h creatinine clearance
Ucr * V
Pcr
All the hassle and responsibility on the patient
As a result unreliable
Problem of tubular secretion of creatinine
Glomerular Filtration Rate
Measurement of GFR in children with type 1 diabetes
Clearance
Inulin
Creatinine
51Cr-EDTA
Diabetics (n=11)
ml/min/1.73m2
126 + 34
172 + 45 (137%)
116 + 30 (92%)
Controls (n=12)
ml/min/1.73m2
112 + 13
145 + 16 (129%)
104 + 13 (93%)
Diabetics, median age 13.9y (5.5-19.3)
Controls, median age 21.0y (16.2-34.0)
Glomerular Filtration Rate
Further compromise:
24h creatinine clearance
Ucr * V
Pcr
Therefore,
creatinine clearance  1/Pcr
Only need to measure plasma creatinine!
Glomerular Filtration Rate
Basics
As GFR declines, the elimination of a metabolite that
relies on clearance by the kidney, e.g. creatinine, is
maintained by increases in its plasma concentration
Glomerular Filtration Rate
As kidney function declines urine creatinine excretion
remains the same
i.e. creatinine excretion is independent of kidney function
creatinine clearance = (Ucr x V)/Pcr ml/min
Implies: creatinine clearance x Pcr = Uc rx V
Ccr
Pcr
Ucr x V
ml/min
µmol/l
µmol/min
Subject A
120
70
8.4
Subject B
60
140
8.4
Subject C
30
280
8.4
Serum creatinine v inulin clearance
Shemesh O et al, Kidney International, 1985
predicted creatinine
Glomerular Filtration Rate
Plasma creatinine determined by:
GFR
secretion by kidney tubules
production rate
children the worst case
– increasing production rate with age and anabolic
growth spurts
Valuable demonstration of estimating GFR from plasma
creatinine
Glomerular Filtration Rate
Plasma creatinine v Inutest GFR in children
Chart Title
900.0
plasma creatinine (µmol/l)
800.0
700.0
600.0
500.0
y = 4740.6x-0.9934
R2 = 0.7732
400.0
300.0
200.0
100.0
0.0
0
20
40
60
80
GFR (ml/min/1.73m2)
100
120
140
Glomerular Filtration Rate
creatinine clearance  1/Pcr
Glomerular Filtration Rate
1/Pcr v Inutest GFR in children
3.00
y = 0.0224x - 0.006
2
R = 0.6557
1/creatinine (l/µmol)
2.50
2.00
1.50
1.00
0.50
0.00
0
20
40
60
80
100
GFR (ml/min/1.73m2)
120
140
160
Glomerular Filtration Rate
creatinine clearance  1/Pcr
creatinine clearance = k/Pcr
Glomerular Filtration Rate
creatinine clearance  1/Pcr
creatinine clearance = k/Pcr
Schwartz et al, 1976
analysis of 1/Pcr, ht/PCr, & SA/Pcr v CrCl
best fit with height (length)
Glomerular Filtration Rate
creatinine clearance v 0.55*ht(cm)/Pcr(mg/dl)
in children (Schwartz et al, 1976)
Glomerular Filtration Rate
k
Schwartz et al,1976
0.55 (Pcr mg/dl)
48.6 (Pcr µmol/l)
creatinine clearance ml/min/1.73m2
Method: end-point Jaffe
Glomerular Filtration Rate
k
Counahan et al, 1976
0.43 (Pcr mg/dl)
38.0 (Pcr µmol/l)
51Cr-EDTA plasma clearance ml/min/1.73m2
Method: ion exchange absorption, end-point Jaffe
Glomerular Filtration Rate
k
Morris et al, 1982
40.0 (Pcr µmol/l)
51Cr-EDTA plasma clearance ml/min/1.73m2
Method: automated kinetic Jaffe
Glomerular Filtration Rate
35*ht/Pcr (MSMS) v Inutest plasma clearance ml/min/1.73m2
in children
Chart Title
180.0
35*ht/plasma creatinine (l*cm/µmol)
160.0
140.0
y = 1.0011x - 0.3701
120.0
2
R = 0.8745
100.0
80.0
60.0
40.0
20.0
0.0
0
20
40
60
80
GFR (ml/min/1.73m2)
100
120
140
Glomerular Filtration Rate
Difference plot
35*ht/Pcr (MSMS) – Inutest plasma clearance v Inutest
plasma clearance ml/min/1.73m2 in children
SDMA v Inutest GFR
35*ht/plasma creatinine-GFR (ml/min/1.73m2)
50.0
40.0
30.0
20.0
10.0
0.0
0
20
40
60
80
-10.0
-20.0
-30.0
-40.0
GFR (ml/min/1.73m2)
100
120
140
Glomerular Filtration Rate
Lessons learned from paediatrics





Can apply a simple formula to estimate a GFR from plasma
creatinine, even in children, where creatinine production rate is
increasing
Accuracy totally dependent on method for measuring plasma
creatinine
Need to understand the measure of GFR any formula derived
from
Does the formula correct for BSA, i.e. ml/min or
ml/min/1.73m2?
Despite good correlation for a population it is important to
appreciate that there are wide limits of agreement
Glomerular Filtration Rate
Application of a formula to estimate GFR from plasma
creatinine in adults
We know plasma creatinine is a poor marker of
glomerular filtration rate
Glomerular Filtration Rate
Serum creatinine (MSMS) v formal GFR
MSMS creatinine (µmol/l)
1200.0
1000.0
-0.8465
y = 3058.4x
800.0
2
R = 0.8701
600.0
400.0
200.0
0.0
0.00
50.00
100.00
150.00
GFR (ml/min/1.73m2)
200.00
Serum creatinine v inulin clearance
Shemesh O et al, Kidney International, 1985
predicted creatinine
Glomerular Filtration Rate
The early decline in GFR results in a relatively small
increase in plasma creatinine
A population normal range is inappropriate
Biological Variation of Serum Creatinine
Gowans & Fraser 1988, Ann. Clin. Biochem. 25:259-263
Glomerular Filtration Rate
GFR
PCr
ml/min/1.73m2 µmol/l
120
70
115
73
110
76
105
80
100
84
95
88
90
93
85
99
80
105
75
112
70
120
65
129
60
140
GFR
PCr
ml/min/1.73m2
µmol/l
30
280
25
336
20
420
15
560
10
840
5
1680
Glomerular Filtration Rate
GFR
PCr
ml/min/1.73m2 µmol/l
120
50
115
52
110
55
105
57
100
60
95
63
90
67
85
71
80
75
75
80
70
86
65
92
60
100
GFR
PCr
ml/min/1.73m2
µmol/l
30
200
25
240
20
300
15
400
10
600
5
1200
Glomerular Filtration Rate
Application of a formula to estimate GFR from plasma
creatinine in adults
Assumes individual production rates are predictable from
demographics
Some reasonable measure of GFR essential for
rationalisation of services for kidney disease
Glomerular Filtration Rate
16 March 1999 Volume 130 Number 6
Annals of Internal Medicine
A More Accurate Method To Estimate Glomerular
Filtration Rate from Serum Creatinine: A New
Prediction Equation
Andrew S. Levey, MD; Juan P. Bosch, MD; Julia Breyer
Lewis, MD; Tom Greene, PhD; Nancy Rogers, MS; and
David Roth, MD, for the Modification of Diet in Renal
Disease Study Group*
Glomerular Filtration Rate
National Kidney Foundation (NKF)
Kidney Disease Outcomes Quality Initiative (K/DOQI)
Glomerular Filtration Rate
NKF-K/DOQI guidelines
GUIDELINE 4. ESTIMATION OF GFR
Estimates of GFR are the best overall indices of the level of kidney function.
The level of GFR should be estimated from prediction equations that take
into account the serum creatinine concentration and some or all of the
following variables: age, gender, race, and body size.
The following equations provide useful estimates of GFR:
In adults, the MDRD Study and Cockcroft-Gault equations.
In children, the Schwartz and Counahan-Barratt equations.
Glomerular Filtration Rate
NKF-K/DOQI guidelines
The serum creatinine concentration alone should not be used to
assess the level of kidney function.
Clinical laboratories should report an estimate of GFR using a
prediction equation, in addition to reporting the serum creatinine
measurement.
Autoanalyzer manufacturers and clinical laboratories should
calibrate serum creatinine assays using an international standard.
Measurement of creatinine clearance using timed (for example, 24hour) urine collections does not improve the estimate of GFR over
that provided by prediction equations.
Glomerular Filtration Rate
NKF-K/DOQI guidelines
A 24-hour urine sample provides useful information for:
Estimation of GFR in individuals with exceptional dietary intake
(vegetarian diet, creatine supplements) or muscle mass
(amputation, malnutrition, muscle wasting);
Assessment of diet and nutritional status;
Need to start dialysis.
Glomerular Filtration Rate
National Service Framework (NSF) for Renal Services
Step three: Testing kidney function
Local health organisations can work with pathology services and
networks to develop protocols for measuring kidney function by
serum creatinine concentration together with a formula-based
estimation of glomerular filtration rate (estimated GFR), calculated
and reported automatically by all clinical biochemistry
laboratories.
Glomerular Filtration Rate
National Service Framework (NSF) for Renal Services
QUALITY REQUIREMENT ONE: People at increased risk of developing or having undiagnosed
chronic kidney disease, especially people with diabetes or hypertension, are identified, assessed
and their condition managed to preserve their kidney function.
Markers of good practice
All people at increased risk of CKD are identified, and given appropriate advice, treatment and
support (which is sensitive to the differing needs of culturally diverse groups) to preserve their
kidney function.
People
identified as having an increased risk of CKD have their
kidney function assessed and appropriately monitored, using
estimated GFR.
Implementation of the NICE clinical guideline on the management of Type 1 diabetes.
Implementation of the NICE clinical guidelines on the management of Type 2 diabetes: renal
disease; blood glucose; blood pressure and blood lipids.
Implementation of the NICE clinical guideline on the management of hypertension in adults in
primary care.
For children and young people with potential urinary tract infection, accurate diagnosis and prompt
antibiotic treatment, and investigation sufficient to identify structural renal defects and to prevent
Glomerular Filtration Rate
Lessons learned from paediatrics





Can apply a formula to estimate a GFR from plasma creatinine
even in children where creatinine production rate is increasing
Accuracy totally dependent on method for measuring plasma
creatinine
Need to understand the measure of GFR any formula derived
from
Does the formula correct for BSA, i.e. ml/min or
ml/min/1.73m2?
Despite good correlation for a population it is important to
appreciate that there are wide limits of agreement
Glomerular Filtration Rate
Use of plasma creatinine for the estimation of GFR
Need to understand the factors on which a plasma
creatinine depends
Need to appreciate the importance of the creatinine
measurement
Need to understand the limitations of any formula derived
eGFR
Glomerular Filtration Rate
Factors affecting plasma creatinine
GFR, tubular secretion, production rate
Calculation of eGFR assumes that the rate of production
is related to a series of demographics,
e.g. height, weight, sex, ethnic origin, age
Statistically may be true for a population but not
necessarily for the individual
Limitations of plasma creatinine
determination and eGFR
Age
y
Subject1 40
Subject2 40
Wt
kg
Pcr
µmol/l
GFR C&G
GFR inulin
ml/min/1.73m2
80
80
68
120
144
82
Effect of 50% loss of renal function
Subject1 40
80
136
73
Subject2 40
80
240
41
Normal range for creatinine 55-120µmol/l
116
118
58
59
Glomerular Filtration Rate
Plasma creatinine measurement is critical
Accuracy very poor
Assays vary in standardisation, linearity, and relative
interferences between and within supplier
Glomerular Filtration Rate
Measurement of plasma creatinine
Comparison of various routine methods with isotopedilution electrospray mass spectrometry-mass
spectrometry
Fully validated method using a NIST traceable standard
and EC certified reference materials
Between assay CV 2%
Plasma creatinine - isotope-dilution MSMS
Difference betw een m ethods
(Ortho enzym atic - ID-MS, um ol/L)
method comparison
20
10
0
Zero bias
-10
-20
-30
40
90
140
190
240
Serum creatinine (ID-MS, um ol/L)
Plasma creatinine - isotope-dilution MSMS
Difference betw een m ethods
(Roche enzym atic - ID-MS,
um ol/L)
method comparison
20
10
0
Zero bias
-10
-20
-30
40
90
140
190
240
Serum creatinine (ID-MS, um ol/L)
Plasma creatinine - isotope-dilution MSMS
Difference betw een m ethods
(Jaffe - ID-MS, um ol/L)
method comparison
20
10
0
Zero bias
-10
-20
-30
40
90
140
190
240
Serum creatinine (ID-MS, um ol/L)
Glomerular Filtration Rate
Difference plot
Plasma creatinine (µmol/l) Jaffe v MSMS
creatinine (µmol/l) Jaffe - MSMS
50
0
0.0
200.0
400.0
600.0
800.0
1000.0
-50
-100
-150
-200
creatinine (µm ol/l) MSMS
1200.0
1400.0
1600.0
Glomerular Filtration Rate
Difference plot
Plasma creatinine (µmol/l) compensated Jaffe v MSMS
creatinine (µmol/l) compensated Jaffe MSMS
100.0
50.0
0.0
0.0
200.0
400.0
600.0
800.0
1000.0
-50.0
-100.0
-150.0
-200.0
creatinine (µm ol/l) MSMS
1200.0
1400.0
1600.0
Glomerular Filtration Rate
Difference plot
Plasma creatinine (µmol/l) enzymatic v MSMS
100.0
creatinine (µmol/l) enzymatic - MSMS
50.0
0.0
0.0
200.0
400.0
600.0
800.0
1000.0
-50.0
-100.0
-150.0
-200.0
-250.0
-300.0
creatinine (µm ol/l) MSMS
1200.0
1400.0
1600.0
Glomerular Filtration Rate
Difference plot
Plasma creatinine (µmol/l) Cleveland Jaffe v MSMS
100.0
creatinine (µmol/l) Cleveland Jaffe - MSMS
80.0
60.0
40.0
20.0
0.0
0.0
200.0
400.0
600.0
800.0
1000.0
-20.0
-40.0
-60.0
-80.0
creatinine (µm ol/l) MSMS
1200.0
1400.0
1600.0
Measurement of GFR
Glomerular Filtration Rate
Plasma creatinine methods synonymous with lack of
uniformity
Alignment with the MDRD formula laboratory would
improve uniformity, but is wrong
Plasma creatinine: the importance of being consistently
wrong
Finally, alignment with isotope dilution MS
Glomerular Filtration Rate
Analytical variation in plasma creatinine will have a
significant impact on estimated GFR (eGFR), e.g. Lamb
et al. Susceptibility of glomerular filtration rate estimations to
variations in creatinine methodology: a study in older patients.
Ann Clin Biochem 2005
Glomerular Filtration Rate
Alignment of plasma creatinine standardisation and
methodology, preferably to a true reference standard,
could significantly improve the situation
However, interferences represent a major problem on a
patient by patient basis, e.g. the impact of underrecognised renal failure in liver disease
Use eGFR in this patient group?
Glomerular Filtration Rate
eGFR
The formulae
Glomerular Filtration Rate
16 March 1999 Volume 130 Number 6
Annals of Internal Medicine
A More Accurate Method To Estimate Glomerular Filtration
Rate from Serum Creatinine: A New Prediction Equation
Andrew S. Levey, MD; Juan P. Bosch, MD; Julia Breyer Lewis,
MD; Tom Greene, PhD; Nancy Rogers, MS; and David Roth,
MD, for the Modification of Diet in Renal Disease Study Group*
Glomerular Filtration Rate
Levey et al, 1999
Comparison of a range, 7 in total, of eGFR formulae
True GFR measure
radioactive iothalamate clearance
Glomerular Filtration Rate
Equation 1: Serum creatinine
GFR (ml/min/1.73m2) = 0.69 * [100/Pcr]
Equation 2: Cockcroft–Gault formula
GFR (ml/min) = 0.84 * [(140-age) * wt]/(Pcr *72) note for females 85
Equation 3: Creatinine clearance
GFR (ml/min) = 0.81 * [Ccr]
Equation 4: Average of creatinine and urea clearance
GFR (ml/min/1.73m2) = 1.11 * [(Ccr + Curea)/2]
Equation 5: Creatinine clearance, urea clearance, and demographic
variables
GFR (ml/min/1.73m2) = 1.04 * [Ccr]0.751 * [Curea] 0.226 * [1.109 if patient is
black]
Glomerular Filtration Rate
Equation 6: Demographic, serum, and urine variables
GFR (ml/min/1.73m2) = 198 * [Pcr]-0.858 * [Age]-0.167 * [0.822 if patient is
female] * [1.178 if patient is black] * [SUN]-0.293 * [UUN]0.249
Equation 7: Demographic and serum variables only
GFR (ml/min/1.73m2) = 170 * [Pcr]-0.999 * [Age]-0.176 * [0.762 if patient is
female] * [1.180 if patient is black] * [SUN]-0.170 * [Alb]0.318
Reduced/practical MDRD formula:
GFR (ml/min/1.73m2) = 186 (175) * [Pcr/88.4]-1.154 * [Age]-0.203 * [0.742 if
patient is female] * [1.121 if patient is black]
Glomerular Filtration Rate
No bias
Equation 6 the most precise, R2=91.2%
Equation 7, R2=90.3%
Reduced formula only appeared in abstract form,
performance equivalent to equation 7
Glomerular Filtration Rate
Glomerular Filtration Rate
90th centile %age absolute errors
19.1 ml/min/1.73m2 (47.5%) for Cockcroft & Gault
formula
12.9 ml/min/1.73m2 (28.4%) for equation 7
A more accurate method to estimate GFR
Good enough?
Glomerular Filtration Rate
Glomerular Filtration Rate
Limits of agreement a problem
NB the study done in one laboratory with a particular
creatinine method
In practice, without equivalence of creatinine methods
eGFR not going to be clinically useful
MDRD with ECOS (evolving connectionist systems)?
Glomerular Filtration Rate
Limits of agreement a problem
Even with equivalence of creatinine methods eGFR
will not significantly improve early detection
eGFR of 80ml/min/1.73m2 could be anywhere from
56 to 104ml/min/1.73m2 – 90% of the time!
Only reporting values <60 ml/min/1.73m2 while
prudent does not improve early detection
Biological Variation of Serum Creatinine
Gowans & Fraser 1988, Ann. Clin. Biochem. 25:259-263
Glomerular Filtration Rate
The key to early detection of renal disease using
plasma creatinine is to provide an assay with
excellent between assay precision and monitor
change
True primary care medicine
Urinary albumin/creatinine ratio?
Glomerular Filtration Rate
Limits of agreement also a problem
for classification of disease stage
eGFR useful for monitoring progression of kidney
disease once baseline established using formal GFR
but so is plasma creatinine
Beware therapeutics affecting creatinine production,
e.g. fibrates, and/or tubular secretion, e.g. cimetidine
Glomerular Filtration Rate
Actions
Improve creatinine standardisation and methodology
Quote limits with every eGFR report
Develop a formula relevant to the UK demographic using a valid
renal clearance technique (i.e. collect some urine!)
Early detection of kidney disease requires a better plasma marker
Cystatin C any better?
Measurement of GFR
Early detection of kidney disease and appropriate
staging remains a challenge
eGFR is a significant start, provided all associated
professionals understand what it means
Kidney disease is a major public health problem and
a significant determinant of cardiovascular risk
that necessitates early detection and treatment
Measurement of GFR
Beware:
Deacon’s Challenge No:54
Complexity of eGFR calculation
MDRD eGFR 41ml/min/1.73m2
Creatinine clearance 29ml/min
Comments:
inaccuracy of timed urine collection
failure to correct Ccr for BSA
Ccr should always be corrected for BSA
creatinine is secreted by tubules so Ccr is always higher than GFR
the 2 values are actually within the limits of agreement of the 2
methods – that is the problem!
Acknowledgements
Charles Turner
Edmund Lamb and colleagues
Finlay McKenzie
Frederick van Lente
Carlo Donadio
The WellChild Trust
Guy’s & St Thomas’ Charity
Guy’s & St Thomas’ NHS Foundation Trust