Susceptibility of glomerular filtration rate estimations to variations in creatinine methodology: a study in older patients.
BACKGROUND
It is recommended that measurement of serum creatinine should be supplemented with a creatinine-based estimation of glomerular filtration rate (GFR). The influence of creatinine methodology on these estimates is not always appreciated. We have studied differences in creatinine methods and their influence on GFR estimation specifically in older people.


METHODS
In all, 46 older patients (mean age 80 y, range 69-92 y) with predominantly mild or moderate kidney disease were studied. Serum creatinine was measured using a rate Jaffe method and two different enzymatic methods. Isotope dilution mass spectrometry served as the reference creatinine method. GFR was estimated using both the Modification of Diet in Renal Disease (MDRD) and Cockcroft and Gault formulae: a 51Cr-EDTA GFR estimation served as the reference GFR method.


RESULTS
Both enzymatic methods produced creatinine results that were significantly different (P<0.001) from the reference method. The Jaffe method over- and underestimated creatinine at low and high concentrations, respectively. The most likely explanation for these differences relates to standardization of the assays. Irrespective of creatinine method, the Cockroft and Gault formula tended to underestimate GFR, and the MDRD formula to overestimate GFR. Use of the differing creatinine methods to estimate GFR produced predictable biases of the estimate, with mean GFR estimates varying by 14% across the creatinine methods.


CONCLUSIONS
Estimates of GFR depend critically upon the accuracy and precision of the creatinine measurement used in their calculation.
Lamb
Edmund J
EJ
Department of Clinical Biochemistry, East Kent Hospitals NHS Trust, Kent and Canterbury Hospital, Canterbury, Kent, UK. edmund.lamb@ekht.nhs.uk
Wood
Joanna
J
Stowe
Helen J
HJ
O'Riordan
Shelagh E
SE
Webb
Michelle C
MC
Dalton
R Neil
RN
D000368:Aged; D000369:Aged, 80 and over; D002860:Chromium Radioisotopes / Q000493:pharmacokinetics; D003404:Creatinine / Q000097:blood*; D004032:Diet; D004492:Edetic Acid / Q000378:metabolism; D005260:Female; D005919:Glomerular Filtration Rate / Q000502:physiology*; D006801:Humans; D007674:Kidney Diseases / Q000097:blood / Q000175:diagnosis*; D008297:Male; D013058:Mass Spectrometry; D008657:Metabolic Clearance Rate; D012680:Sensitivity and Specificity; D013223:Statistics as Topic / Q000379:methods
D002860:Chromium Radioisotopes; D004492:Edetic Acid; D003404:Creatinine