Serum symmetric dimethylarginine in older dogs: Reference interval and comparison of a gold standard method with the ELISA

Background: Serum symmetric dimethylarginine (SDMA) is used to screen for renal dysfunction in dogs. The gold standard technique for measuring SDMA, liquid chromatography-tandem mass spectrometry (LC-MS/MS) is not widely available. Age-specific reference intervals for SDMA in older dogs are lacking.Objectives: Prospective study in older dogs to validate a commercially available LC-MS/MS method for SDMA, compare SDMA concentrations with concentrations measured using ELISA and obtain a reference interval (RI) for older dogs using both methods.Animals: Client-owned older dogs undergoing health screening.Methods: The LC-MS/MS method was analytically validated (limit of detection, precision, and linearity). Serum was sent cooled overnight for ELISA or was frozen at -80 degrees C until batch analysis using LC-MS/MS. Results of LC-MS/MS and ELISA were compared and RIs for older dogs were calculated according to international guidelines.Results: The LC-MS/MS method showed good linearity (r(2) = .99) and precision (coefficient of variation <10%), with a laboratory RI between 8.0 and 14.0 mu g/dL. Paired measurements were available from 118 different dogs. Median SDMA concentration were 9.4 (range, 5.0-21.2) using LC-MS/MS and 12.0 (range, 5.0-22.0) mu g/dL using ELISA. Both methods significantly differed with a mean difference of 2.2 mu g/dL. The RI for older dogs for LC-MS/MS was 4.4-15.0 mu g/dL, and for ELISA was 6.4-17.4 mu g/dL.Conclusions and Clinical Importance: The ELISA provided significantly higher SDMA concentrations compared to the validated LC-MS/MS method, indicating the need for device- or assay-specific RI. The obtained age-specific RI for SDMA is considerably higher in older dogs compared to the general laboratory RI.