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This approach feels a little uncomfortable to me. The whole point of using the correction factors was to attempt to generate an eGFR figure that would have some "traceability" back to to the evidence base, and to limit the possibility of misclassification of patients into CKD stages by modulating the impact of analytical differences in creatinine at lower concentrations close to the upper reference limits. Whether the creatinine is in the reference range or not is irrelevant. You are using the eGFR to define the populations not the creatinine reference intervals. The determination of age and sex related reference intervals should be undertaken using the IFCC approved approach surely. Using such a back extrapolation approach involves calculation from an estimate of GFR that has wide confidence limits at the upper limits reference limits. The consequences of this are that the derived upper reference limits for that population will lack precision and are being forced to be equivalent to that of the MDRD population. The reason for the latter is that the formula was derived from a particular population that may differ in terms of lean body mass, lifestyles etc etc. These factors we are extrapolating to our populations by adopting a formula containg derived factors that are functions of the population characteristics. We are therefore extrapolating the evidence from MDRD to populations that may not be exactly equivalent, is it appropriate to force the MDRD creatinine reference intervals to our populations also by utilising such back extrapolation techniques? Again I would reiterate it does not matter what your creatinine reference interval is, given the current approach, it is the egger that defines the action (or Of points) either in absolute terms or rate of change. Our GPs seem to have grasped the concept once explained. Regards Bill Bartlett ________________________________ From: Clinical biochemistry discussion list [mailto:[log in to unmask]] On Behalf Of Everitt, Tony Sent: 27 June 2006 15:07 To: [log in to unmask] Subject: eGFR again! Following introduction of eGFR calculated using 175 formula adjusted for local creatinine method, it became obvious that our age/sex related reference range for creatinine was out of step with eGFR. This caused some 'GP confusion' with reports of 'abnormal' eGFR (i.e. <59 ml/min) with 'normal' creatinine (and vice-versa). Following discussion with the renal physicians we have back-calculated our upper creatinine range level using the MDRD formula so that abnormal eGFR = abnormal creatinine. What about the lower limit? We did quote 60 umol/L for females and 78umol/L for males. Should we now not have a lower limit and quote the reference range for creatinine as 'less than xxx'. Two out of 5 renal physicians agree with this approach - the other three abstaining! Rgds Tony Tony Everitt, Consultant Biochemist Basildon and Thurrock University Hospitals NHS Foundation Trust 01268 593014 ************************************************************************ ******** The information contained in this email may be subject to public disclosure under the Freedom of Information Act 2000. Unless the information is legally exempt from disclosure, the confidentiality of this email, and your reply, cannot be guaranteed. It is intended solely for the addressee. Please notify the sender immediately if you are not the intended recipient. Access to this email by anyone else is unauthorised. 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