Measurement of symmetric dimethylarginine (SDMA) is used for diagnosis and monitoring of kidney disease in veterinary medicine. Recently, IDEXX introduced a novel immunoassay to measure SDMA on the Catalyst® chemistry analyzers. The assay performance for feline samples has not been published elsewhere.
This study evaluates the in-clinic Catalyst SDMA Test (CatalystSDMA) for SDMA measurement in feline samples by comparing results with those obtained by liquid chromatography–mass spectrometry (LC-MS; technique previously validated for feline samples).
Serum samples from 113 cats were obtained from IDEXX Reference Laboratories (RefLab). The samples were originally submitted for clinical purposes and, after all requested testing was completed and the standard retention period (7 days; 4 ˚C), the RefLab typically disposes of remaining material. Instead, under the RefLab Terms and Conditions, the residual samples were frozen until use in this study.
Samples were thawed in batches and analyzed once using CatalystSDMA, on a Catalyst One® Chemistry Analyzer, and once using LC-MS (RefMethod). The study was completed over several days, however, for each sample, all testing was completed within four hours. All testing was completed at IDEXX.
Results are reported with 95 % confidence limits in parentheses.
RefMethod median SDMA was 21.5 µg/dL; interquartile range (IQR) of 11.7 to 32.5 µg/dL; range: 5.1 to 85.3 µg/dL. CatalystSDMA median SDMA was 21.7 µg/dL; IQR: 11.3 to 32.9 µg/dL; range: 1.9 to 79.5 µg/dL. No statistical difference existed between SDMA concentrations obtained by the two methods (P = 0.88; Mann‑Whitney U test).
Passing-Bablok regression analysis: intercept 1.0 µg/dL (0.0 to 2.0); slope 1.0 (0.9 to 1.1); Tau 0.84. The Pearson’s correlation coefficient (r) was 0.94. The mean difference (Bland-Altman plot) was 0.0 µg/dL; standard deviation: 5.7 µg/dL (4.9 to 6.4). Limits-of-agreement (LOA) plot showed no fixed or proportional bias, with increased variation at the higher end of the dynamic range where unlikely to impact clinical decisions. LOA upper limit 11.1 µg/dL (9.4 to 13.0). LOA lower limit -11.1 µg/dL (-13.0 to -9.3).
For each assay, results to the nearest whole number were assigned to one of three categories: ≤ 14 µg/dL; 15 to 19 µg/dL; ≥ 20 µg/dL. The percentage agreement (concordance) between the two assays was calculated. There was good agreement (85 %) on results classification.
The minimal bias, good concordance and excellent correlation to the reference method provide confidence that CatalystSDMA can be used for in-clinic measurement of feline SDMA.
Medical Affairs Manager
IDEXX LABORATORIES INC
Dr. Graham E.A. Bilbrough MA VetMB CertVA MRCVS
After graduation from the University of Cambridge, England, Graham Bilbrough spent a year in mixed practice in Oxfordshire, before returning to Cambridge to become the BSAVA Petsavers Resident in Anaesthesia and Critical Patient Care. Later he became the Senior Anaesthetist at the Queen’s Veterinary School Hospital, responsible for the clinical anaesthesia service and intensive care unit at the university hospital. Graham joined IDEXX Laboratories UK in October 2006. After working for IDEXX across Europe, he moved to their headquarters in Maine, USA where he is a Medical Affairs Manager.
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