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Published on: May 10, 2013
The impact of interlaboratory differences in cystatin C assay measurement on glomerular filtration rate estimation
Christine A White1, Andrew D Rule, Christine P Collier
1Division of Nephrology, Department of Medicine, Queen’s University, Kingston, Ontario, Canada. cw38@queensu.ca
Insights
Laboratory differences in Cystatin C (CysC) measurements significantly impact estimated glomerular filtration rate (eGFR) calculations. This variability, even with identical assays, necessitates careful interpretation of CysC and eGFR results.
Area of Science:
- Clinical Chemistry
- Nephrology
- Laboratory Medicine
Background:
- Cystatin C (CysC) is a key biomarker for estimating glomerular filtration rate (GFR).
- Existing equations for GFR estimation from CysC show performance variability in validation studies.
- This variability persists even when using the same CysC assay from a single manufacturer.
Purpose of the Study:
- To investigate discrepancies in CysC measurements and estimated GFR (eGFR) between two laboratories.
- To evaluate the performance of the Mayo Clinic equation for eGFR using Siemens' nephelometric immunoassay.
- To assess the predictive accuracy of eGFR for measured GFR.
Main Methods:
- Ninety-seven split patient samples were analyzed.
- Samples were processed at the Children's Hospital of Eastern Ontario (CHEO) and the Mayo Clinic.
- Siemens' nephelometric immunoassay and the Mayo Clinic equation were employed.
Main Results:
- CHEO CysC measurements were, on average, 0.17 mg/L (10%) lower than Mayo Clinic values.
- The Mayo Clinic equation yielded a mean eGFR difference of 7.2 ml/min/1.73 m² (15%) between laboratories.
- Agreement within 10% was observed in 36% of results, while 13% showed discordance exceeding 30%.
Conclusions:
- Significant interlaboratory variability in CysC measurements exists, even with identical assays and manufacturers.
- This variability leads to clinically relevant differences in eGFR, impacting patient assessment.
- Awareness of interlaboratory CysC and eGFR variability is crucial for accurate interpretation and comparison of results.
Background And Objectives:
Cystatin C (CysC) is a promising marker of GFR. Several equations have been derived to estimate GFR from its serum concentration. Heterogeneity in the performance of these equations exists in validation studies even when the same CysC assay from the same manufacturer is utilized. This study was designed to examine the differences in CysC and GFR estimation (eGFR) using Siemens' nephelometric immunoassay and the Mayo Clinic equation. The ability of the eGFRs to predict measured GFR was also examined.
Design, Setting, Participants, & Measurements:
Ninety-seven split samples were sent to laboratories at Children's Hospital of Eastern Ontario (CHEO) in Ottawa, Canada, and at the Mayo Clinic in Rochester, Minnesota.
Results:
The mean CHEO CysC was 0.17 mg/L (10%) lower than the mean Mayo Clinic CysC. Using the Mayo Clinic equation, the mean eGFR difference was 7.2 ml/min per 1.73 m(2) (15%). Approximately 36% of the results agreed within 10%, while 13% were discordant by greater than 30%. Larger absolute differences in mean eGFR between the two laboratories were found in the subgroup with CysC less than 1.41 mg/L as compared with the subgroup greater than 1.41 mg/L (9.5 versus 5.0 ml/min per 1.73 m(2)). Correction of CHEO values to the Mayo Clinic did not improve GFR estimation.
Conclusions:
Significant differences in CysC measurement exist between laboratories using the same assay by the same manufacturer and these lead to clinically relevant differences in GFR estimation. This interlaboratory variability needs to be recognized when interpreting and comparing CysC and eGFR results.
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