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!