Validation of commercially available ELISAs for the detection of circulating sclerostin in hemodialysis patients.

Background: Sclerostin is an endocrine regulator in chronic kidney disease - mineral and bone disorder (CKD-MBD). Validation of assay comparability and pre-analytical handling is mandatory for establishment of sclerostin as a biomarker.

Methods: Blood samples (serum, EDTA, heparin and citrate plasma) were obtained from 12 hemodialysis (HD) patients after the long dialysis interval. Passing-Bablok regression analysis and Bland-Altman difference plots were used to evaluate the agreement between sclerostin levels measured with two commercially available ELISAs from TECOmedical and Biomedica.

Results: Independent of the sample type, the agreement of the two assays was poor with a strong proportional but no systematic bias. Compared to the TECOmedical assay, the Biomedica test yielded almost 2-fold higher sclerostin values throughout all sample types. Spike recovery and linear dilution studies revealed a higher accuracy of the TECOmedical assay (97% and 96%) compared to the Biomedica assay (118% and 78%). Sclerostin levels were stable within 4 hours after sample collection, in particular when analyzed in plasma. In contrast to the Biomedica assay, the TECOmedical showed a systematic but no proportional bias between serum and plasma samples with higher values for plasma samples. Among the 3 different plasma samples no systematic error could be documented.

Conclusion: Careful consideration of the pre-analytical handling and comparative assay validation are necessary to facilitate a more differentiated interpretation of studies reporting circulating sclerostin levels. The presence of a proportional bias demonstrates that in HD patients the two ELISAs for measuring sclerostin should not be used interchangeably. Furthermore, caution is necessary when comparing sclerostin results obtained from different blood sample types.