Variation of serum creatinine, cystatin C, and creatinine clearance tests in persons with normal renal function.
BACKGROUND
To determine the potential sensitivity of several renal function tests for detecting early changes in renal function, we compared the within-individual (W-I) variation over 5 months of serum creatinine, serum cystatin C, and creatinine clearance.


METHODS
On 31 healthy subjects, blood and timed urine specimens were collected once each month to get 6 collections. Creatinine (enzymatic) in serum and urine and cystatin C (immunonephelometric) in serum were measured and glomerular filtration rate (GFR) by creatinine clearance and the Modification of Diet in Renal Disease (MDRD) equation were calculated. To compare W-I variations between different creatinine methods, we also measured creatinine by both enzymatic and kinetic alkaline picrate methods on 15 sets of frozen samples.


RESULTS
For the 31 volunteers, the mean W-I variations for serum creatinine (5.8%) and cystatin C (5.4%) were both much lower than the W-I variation of creatinine clearance (18.7%). As expected, the MDRD GFR had a similar W-I variation (6.7%) to that of serum creatinine and its values were markedly different than GFR by creatinine clearance. On the 15 sets of frozen samples, the W-I variation of creatinine measured by the enzymatic method (CV 5.2%) was slightly less than by the picrate method (CV 6.2%).


CONCLUSIONS
The low W-I variation of both serum cystatin C and serum creatinine suggests that serial measurements of either would detect a changes in renal function earlier than would GFR by creatinine clearance or MDRD equation, which allows reporting only for GFRs<60 ml/min/1.7 m(2). While we measured only creatinine clearance, the large variability, difficulty, and cost of all clearance measurements make them impractical for routine monitoring of patients.
Department of Pathology, Duke University Medical Center, Durham, NC, USA. toffa002@mc.duke.edu
John G
JG
Toffaletti
Elizabeth H
EH
McDonnell
D000328:Adult; D000368:Aged; D003404:Creatinine / Q000097:blood* / Q000378:metabolism* / Q000652:urine; D055316:Cystatin C; D015891:Cystatins / Q000097:blood* / Q000378:metabolism / Q000652:urine; D005260:Female; D005919:Glomerular Filtration Rate; D006801:Humans; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
C525030:CST3 protein, human; D055316:Cystatin C; D015891:Cystatins; D003404:Creatinine