Effect of storage temperature and time on measurement of serum symmetric dimethylarginine concentration using point‐of‐care and commercial laboratory analyzers in cats and dogs

BACKGROUND: Stability of serum symmetric dimethylarginine (sSDMA) during short‐ and long‐term storage has not been assessed for the immunoassay of the Point‐of‐Care IDEXX Catalyst DX (POC) analyzer and the Enzyme Multiplied Immunoassay Technique of IDEXX commercial laboratory (CL). Also, the agreement between both analyzers is questioned. OBJECTIVES: To determine (a) the effect of storage time and temperature on sSDMA measured by POC and CL; (b) the agreement between sSDMA measured by POC and CL; and (c) the imprecision of the POC. ANIMALS: Serum of cats (n = 17) and dogs (n = 18) with a range of SDMA concentrations (6 to >100 μg/dL). METHODS: Based on an equivalence trial with predefined equivalence range (−3.0 to +3.0 μg/dL) and using T0 as baseline, stability was evaluated after 24 hours at 22°C and 4°C (POC); after 7 days at 4°C (POC and CL) and after 10 and 24 months at −24°C and −80°C (CL). Bland‐Altman plots enabled method comparison. Imprecision of the POC was assessed by duplicate sSDMA measurements at T0. RESULTS: The POC analyzer produced equivalent sSDMA measurements if samples were stored for 24 hours at 4°C (95% confidence interval [CI]: −2.5‐2.0 μg/dL), but not when stored for 24 hours at room temperature (RT; 95% CI: −4.1 to 0.5 μg/dL) or after 7 days at 4°C (95% CI: −3.6‐1.0 μg/dL). The CL analyzer was less affected by preanalytical variation with clinically similar results obtained when samples were stored for 7 days at 4°C (95% CI: −2.2 to 2.4 μg/dL) and for at least 24 months at −24°C (95% CI: −1.7 to 2.9 μg/dL) and −80°C (95% CI: −1.5 to 3 μg/dL). A relevant mean difference of −2.3 μg/dL between both analyzers was found. Duplicate POC measurements were equivalent (95% CI: −2.6 to 2.0 μg/dL). CONCLUSIONS: Delayed analysis may significantly change sSDMA depending on storage and measurement conditions. Interchangeable use of assays should be done with caution because analytical variation could be interpreted as clinically relevant change.