To the Editor: I applaud the recent position statement of the Australasian Creatinine Consensus Working Group.1 It is in line with worldwide support for the Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines for chronic kidney disease2 expressed by the Renal Association of Great Britain and the International Society of Nephrologists as well as North American groups. However, I am concerned about universal reporting of the Modification of Diet in Renal Disease (MDRD) calculation of glomerular filtration rate (GFR) applied to pregnant women.

As described in the K/DOQI guidelines, the MDRD calculation has not been validated in pregnancy. Other formulas are available for calculating estimated GFR (eGFR) using serum creatinine level and several other variables, yet none have been accepted for use in pregnancy. A single study3 from 1994 followed 34 women with impaired renal function through 38 pregnancies and compared creatinine clearance derived from a 24-hour urine collection with the Cockroft–Gault eGFR during each trimester. While the study showed good correlation between this eGFR and creatinine clearance, I hesitate to recommend national usage based on a single study of 34 women. As recommended by the K/DOQI guidelines and commonly used by obstetricians, the accepted method for the determination of GFR in pregnancy is still the 24-hour urine collection with formal calculation of creatinine clearance.

Pregnancy can occur in women with all stages of renal impairment, including women on dialysis or after transplantation. Fertility is diminished and the risk of harm to the fetus is increased if a woman with renal impairment has a pre-pregnancy GFR of < 70 mL/min/1.73m2.4 Given that there is an almost 50% increase in GFR between early pregnancy and delivery,4 it is possible that even if the MDRD-based eGFR accurately and precisely estimated GFR, reporting high eGFR simply as “> 60 mL/min” could result in failure to detect renal impairment in some pregnant women.

While I strongly support the effort to improve our recognition of chronic kidney disease, I urge caution with regard to implementing a scheme in the specific population of pregnant women when this scheme has not been adequately studied. Perhaps a multi-centre cooperative study is in order.