Variations in assay protocol for the Dako cystatin C method may change patient results by 50% without changing the results for controls.
BACKGROUND
Cystatin C is increasingly used as a glomerular filtration marker, but so far only a few companies produce most of the cystatin C reagents suited for turbidimetry or nephelometry use in clinical laboratories.


METHODS
We studied different protocols for measuring cystatin C on an Architect ci8200 system using cystatin C reagents from Dako (Glostrup, Denmark). The results were compared with those obtained with Dade Behring reagents (Deerfield, IL, USA) on a BN ProSpec system.


RESULTS
Differences in assay protocol on the same instrument with the Dako reagent yielded an up to 50% difference in glomerular filtration rate calculated from the cystatin C results when analyzing patient samples, but had no effect on the results for controls. There were also significant differences regarding linearity and kinetics between samples and controls/calibrators.


CONCLUSIONS
The results indicate different reactivity of the Dako antibodies against calibrators and controls in comparison with patient samples, highlighting the importance of using controls and calibrators that do not differ from patient samples.
Flodin
Mats
M
Department of Medical Sciences, Clinical Chemistry, University Hospital, 751 85 Uppsala, Sweden.
Hansson
Lars-Olof
LO
Larsson
Anders
A
D002138:Calibration; D055316:Cystatin C; D015891:Cystatins / Q000097:blood*; D003950:Diagnostic Techniques, Urological / Q000592:standards*; D005919:Glomerular Filtration Rate; D006801:Humans; D007202:Indicators and Reagents / Q000737:chemistry / Q000592:standards; D009391:Nephelometry and Turbidimetry / Q000379:methods; D012015:Reference Standards; D015203:Reproducibility of Results
C525030:CST3 protein, human; D055316:Cystatin C; D015891:Cystatins; D007202:Indicators and Reagents