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

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 4 Jaffe v Enzymatic Creatinine Enzymatic Creatinine (mmol/L) Jaffe Creatinine (mmol/L) Interferences

Creatinine Measurement Day of admission [Creatinine] (mmol/L) Jaffe Enzymatic 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-S110 Reviewed 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 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

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

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) x age x F x R F = if female; R = 1.2 if African American Revised IDMS aligned MDRD equation: eGFR = 175 x (creat/88.4) x age 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 Reporting to 90 mL/min – creatinine down to about 55 umol/L Reporting 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 99Tc GFR, mL/min/1.73m 2 MDRD eGFR, mL/min/m 2 “186” formula

Method Comparison

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!