Creatinine and the eGFR
Graham Jones
Department of Chemical Pathology
St Vincent’s Hospital, Sydney
AACB-AIMS Annual Scientific meeting Hobart 2006
Summary
• Creatinine measurement– Past– Present– Future
Creatinine measurement
• 1886: Jaffe devised Alkaline Picrate assay for creatinine
• Not specific for creatinine
• Measurand: “chemical reactivity with picrate in alkaline environment”
• Also responds to: protein, albumin, ketones, bilirubin, cephalosporins, ...
Jaffe assays
• Following Jaffe there has been over 100 years of modifications to the Jaffe reaction to improve specificity
• Read-frame adjustment
• Rate-blanking
• Reagent concentration optimisation
• Detergent addition (at 41 degrees)
• NOT ALL JAFFE assays are the same
Jaffe Reactions
• Effect of different reaction conditions
Jaffe Reactions
• Absorbance-time curves for various analytes in Jaffe reaction
Jaffe Creatinine Assays
0
2
4
6
8
0 2 4 6 8 10 12
Minutes
Ab
sorb
ance
Creatinine
Drug
Enzymatic assays
• Brought into routine use
• Only method on Vitros analysers
• Not widely used on other platforms in Australia– Investigate unexpected high creatinine– Specific patients
• In common use in some countries
• Some interferences (trinder reaction)
Case 4Jaffe v Enzymatic Creatinine
0
0.1
0.2
0.3
0.4
0 0.1 0.2 0.3 0.4
Enzymatic Creatinine (mmol/L)
Jaffe Creatinine (mmol/L)
Interferences
Creatinine Measurement
Day of admission
[Cre
atin
ine]
(m
mol
/L)
Jaffe
Enzymatic
00.020.040.060.080.1
0.120.140.160.180.2
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Hydralazine Methyldopa
Interferences
Other assays
• Fullers earth
• HPLC
• IDMS
• Manufacturers have chosen various methods as accuracy base for their assays.
GFR
• Glomerular Filtration Rate
• Single most important parameter of renal function
• Key factor in definition of CKD
• “CKD” - Chronic Kidney Disease
Measuring GFR
• Exogenous substances– Inulin, Cr-EDTA, Iothalamate– Time consuming, expensive, radioactivity
• Endogenous substances– Creatinine (cystatin-C)– Over 40 GFR estimation equations based on serum
creatinine
Cockcroft and Gault
• Developed in 1976 from 249 people (96% male)– Subsequently validated in at least 58 studies
• A measure of creatinine clearance• Estimate urine creatinine based on age, weight and
sex of patient.• False elevation of serum creatinine assays (in 1976)
gave lower results, serendipitously approximating the GFR
• Newer (better) creatinine assays give falsely elevated GFR estimates (approx 15%)
MDRD
• 1990s: Modification of Diet in Renal Disease study.
• Can progression of renal disease be slowed by dietary modification?
• Multiple ancillary studies to validate tools
• Devised and published the MDRD formulae for GFR estimation
• Used Beckman-Coulter CX3 assay at Cleveland Clinic Laboratory
MDRD - Levy 1999
> +/- 30% scatter using results from one instrument
MDRD - Opportunity
• MDRD formula adopted by the NKDEP
• National Kidney Disease Education Program
• Aim to highlight moderate to severe renal disease more simply than can be done from serum creatinine alone.
• Renal disease is bad for patients and health systems
• Earlier detection (pre-symptomatic) allows better chance for intervention
Clin Biochem Reviews 2003
Guidelines Review
The National Kidney Foundation Guideline on Estimation of the Glomerular Filtration Rate: a critical appraisal.
Regarding: Am J Kidney Dis 2002; 39(S1): S76-S110Reviewed by Graham RD Jones and Ee-Mun Lim.
“However, for the reasons stated above, we do not believe that the MDRD equation is yet ready for routine use.”
Australia: RCPA,AACB,KHA
“Lab pack”
What about creatinine?
JCTLM
• 2004 Formation of JCTLM
• Joint Committee for Traceability in Laboratory Medicine
• “Joint” of: BIPM, IFCC, ILAC
• The Role of the BIPM is “worldwide uniformity of measurement”
Creatinine
• NKDEP Lab working group
• Sets total error criteria for serum creatinine measurement at +/- 15%
• Decides to adopt Reference Method from JCTLM
• Isotope Dilution Mass Spectrometry
WOW!
NKDEP Lab Group
• Established need for standardisation
• Agreed on accuracy base
• Met with industry
• Industry has agreed to revise assay calibration to align with IDMS
• Will occur over next 6 - 18 months
WOW!
NKDEP Working Group
• Working with NIST (USA) to produce and validate commutable reference material
• On-going review of assay performance
NIST LN25 – May 2005
www.nkdep.nih.gov
Commutability Testing
IFCC WG-GFRA
• Working Group - GFR Assessment
• 1st Meeting July 2005
• One of many IFCC WG
WG-GFRA - October 2006
• IFCC website
• Specific creatinine assays
• Reference Lab network
• Global introduction of revised equation
• Terminology
• Reporting >60 mL/min/1.73m2
www.ifcc.org
Specific Creatinine Assays
• Should more specific creatinine assays be recommended
• Brief literature review– No current data
• Aim to study magnitude and significance of assay non-specificity
The revised eGFR formula
Original “4 variable” MDRD equation:eGFR = 186 x (creat/88.4)-1.154 x age-0.203 x F x R
F = 0.742 if female; R = 1.2 if African American
Revised IDMS aligned MDRD equation:eGFR = 175 x (creat/88.4)-1.154 x age-0.203 x F
Gives same answer if IDMS creatinine assays 0.95 x original Cleveland Clinic Beckman CX3 method(Ann Int Med 2006)
Serum Creatinine and Reporting Limits
0
20
40
60
80
100
120
140
20 40 60 80 100
Age (years)
Cre
atin
ine
(um
ol/
L)
M - 60 mL/min
M - 90 mL/min
F - 60 mL/min
F - 90 mL/min
Reporting to 90 mL/min – creatinine down to about 55 umol/LReporting to 60 mL/min – creatinine down to about 80 umol/L “175” equation, IDMS aligned assays
RCPA-AACB-KHA
• Meeting planned December 2006
• Current status of Australian assays
• Introduction of “175” formula
• Reporting up to 90 mL/min/1.73m2
• Age-related decision points
• Effects of race
• Drug dosing decisions
• Common reference intervals
MDRD DATA: Christchurch
n=601y = 3.1023 + 1.0047x
0 50 100 150 200 250
250
200
150
100
50
0
99Tc GFR, mL/min/1.73m2
MD
RD
eG
FR
, mL
/min
/m2
“186” formula
Method Comparison
0.000
0.200
0.400
0.600
0.800
1.000
1.200
1.400
1.600
1.800
2.000
0.000 0.500 1.000 1.500 2.000
LC-MS-MS
Lab 1 Hitachi 917 Jaffe
Lab 3 Beckman CX4 Jaffe
Lab 3 Beckman CX4 RocheEnzymatic
Lab 4 Dimension RXL Jaffe
Lab 5 Dimension AR Jaf fe
Lab 6 Vitros 950 Dry ChemEnzymatic
0%
2%
4%
6%
8%
10%
12%
0 100 200 300 400
CV
RCPA-QAP 2006 data calculated from within and between precision.
Creatinine Assays - Precision
Conclusion
• We are seeing an example of co-operation– Clinicians– Laboratories– International Organisations (NKDEP, IFCC,
JCTLM)– Local Organisations (AACB, RCPA, KHA)
• With the aim of improving patient care!