P59-PI03 STANDARDISING ESTIMATED GLOMERULAR FILTRATION RATES (EGFR): DO LABORATORY METHODS REALLY MATTER IN PRACTICE?
M. Quinn*1, A. Rainey2, K. J. Cairns2, A. H. Marshall2, F. Kee3, G. Savage3, D. G. Fogarty1
1Department of Nephrology, 2Centre for Statistical Science and Operational Research, 3Department of Epidemiology, Queen's University, Belfast, United Kingdom
Chronic kidney disease (CKD) guidelines have focused on the utility of
the modified 4-variable MDRD equation (traceable by isotope dilution
mass spectrometry IDMS). This formula accounts for variance in
creatinine measured by an analyser different to that used when the
original MDRD equation was devised.
To assess theoretically and in practice the effect sizes of IDMS
correction over the 4-variable MDRD equation in eGFR calculation with a
range of UK laboratories and the subsequent impact of this on CKD
MATLAB generated a range of creatinine data (30-300umol/l) for male and
female patients aged 20-100 years. The maximum differences between the
IDMS and MDRD equations for all 14 UK laboratory techniques were
explored with an averaged (IDMS + MDRD)-eGFR less than 60mls/min and
30mls/min. Similar procedures were applied to 712,540 samples
(reflecting 5 methods in Northern Ireland), belonging to patients 18
years+, to explore graphically maximum differences in techniques. CKD
prevalence using both estimation equations was compared.
Simulated data indicates that the majority of laboratories in the UK
demonstrate small differences between the IDMS and MDRD methods in
stages 4 and 5 CKD (where the averaged maximum difference for all
laboratory methods was 1.27mls/min for females and 1.59mls/min for
males).MDRD deviated furthest from the IDMS results for the Endpoint
Jaffe method: the maximum difference of 9.93mls/min for females and
5.42mls/min for males occurred at extreme ages and in those with eGFR
reflecting stage 3 or higher disease. The real data graphically agreed
with these theoretical results.
Existing data for 93,870 patients
yielded a first MDRD eGFR<60mls/min in 2001. 66,429 (71%) had a
second test >3months later of which 47,093 (71%) continued to have
an eGFR<60mls/min. Estimated crude prevalence was 3.97% for
laboratory detected CKD in adults using the MDRD equation which reduced
to 3.69% when applying the IDMS equation. Over 95% of this difference
in prevalence was explained by older females with stage 3 CKD; yet
close to the stage 2 interface, reemphasising the need for further
research into the subcategorisation of stage 3 CKD.
Improved accuracy of eGFR is obtainable by using IDMS corrected eGFR
especially in early stage CKD; however our data suggests this will have
little practical impact on stages 4-5 considering the current referral