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

Abstract

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&#xb0;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&#x2009;=&#x2009;.99) and precision (coefficient of variation <10%), with a laboratory RI between 8.0 and 14.0&#x2009;&#x3bc;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) &#x3bc;g/dL using ELISA. Both methods significantly differed with a mean difference of 2.2&#x2009;&#x3bc;g/dL. The RI for older dogs for LC-MS/MS was 4.4-15.0&#x2009;&#x3bc;g/dL, and for ELISA was 6.4-17.4&#x2009;&#x3bc;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.