In reply: Jose and Lawton identify a concern about the wording in the position statement relating to one of the caveats on the use of estimated glomerular filtration rate (eGFR).

The intention of the Australasian Creatinine Consensus Working Group was to highlight that, in certain populations, the MDRD (Modification of Diet in Renal Disease) formula for calculating GFR has not been validated, and that, in such populations, caution should be exercised in applying the formula. This conservative approach was deemed appropriate, as significant population differences in muscle mass (such as have been documented between whites and African-Americans and between men and women) could lead to a formula correction factor being applied in these groups.

It was not the intention of the Working Group to deny the advantages of automatic reporting of eGFR to any section of the community. The sparse evidence that exists suggests that the amount of fat-free mass found in Indigenous Australians does not differ from that found in non-Indigenous Australians.1 In retrospect, it may have been more appropriate for the position statement not to have singled out Indigenous Australians for special mention.

The real need is for the eGFR formula to be validated in Aboriginal and Torres Strait Islander populations so that a firm basis for its use can be established and a correction factor applied if that is found to be required. Until this evidence is available, it appears clinically appropriate for the eGFR to be calculated and used prudently in Indigenous Australians.

Jones is mistaken in stating the body surface area formula we quoted is incorrect. The formula is expressed with the correction factor to normal body size incorporated, and is correct in the context in which it was used. We agree with his recommendation to continue using the Cockcroft–Gault formula for estimating doses of drugs cleared by the kidney, at least until evidence for this purpose is accumulated using the MDRD formula.

Davey raises the possibility that significant errors may occur as a result of poor standardisation of serum creatinine assays. There is indeed room for improvement in routine assays for serum creatinine, but we believe, based on available data, that the current assays from most major suppliers are suitable for the purpose. Further details on acceptable assays have been provided by the Working Group for all Australian laboratories.2 Davey notes the presence of two reference standards for serum creatinine in the position statement: the CX3 method (which was used in the original MDRD article) and the international reference method (IRM). We believe that assays aligned with either of these standards meet the specifications, as the IRM gives lower results than the CX3 method and there are no current commercial assays giving results lower than the IRM. Thus, any current assays within 15% of the IRM are higher than the IRM and within 15% of the CX3 method. Furthermore, the claim of a positive bias of 16% for the current CX3 assay is taken at a single creatinine concentration that is below those that may be used for eGFR calculations. As this bias is absolute rather than proportional, it is not as marked at higher creatinine concentrations. Other studies do not support this degree of difference.

We agree that improvements in creatinine assay standardisation will be of benefit, and this issue is actively being pursued by working groups of the International Federation of Clinical Chemistry and Laboratory Medicine and the National Kidney Disease Education Program (NKDEP). Indeed, the attention given to serum creatinine assays as part of the deliberations of these groups is of benefit to any clinical assessment based on serum creatinine results.

The upper limit of reporting eGFR, recommended at 60 mL/min/1.73m2, is in keeping with the position statement of the NKDEP in the United States,3 although not with the position of the Renal Association in the United Kingdom, which recommends reporting up to 90 mL/min/1.73m2.4 At this stage, we believe that assay variability at lower creatinine concentrations and lack of validation of the MDRD formula at higher values for eGFR make 60 mL/min/1.73m2 a suitable starting point. We hope that, with improvements in both these factors, a higher limit may be acceptable.