Abstract
Objectives
The aim of the present study was to determine the frequency and clinical significance of discordant serum symmetric dimethylarginine (SDMA) and creatinine concentrations relative to glomerular filtration rate (GFR) in mature and senior hyperthyroid cats treated with radioactive iodine (RAI). Secondary aims of the study were to compare the sensitivity and specificity of two SDMA upper reference intervals (RIs; >14 and ⩾18 µg/dl) and creatinine (⩾175 µmol/l) for detecting decreased GFR and to evaluate the intra-individual variability of both biomarkers.
Methods
A prospective longitudinal study of 27 client-owned hyperthyroid cats receiving low-dose RAI therapy was conducted. At baseline, 1, 6 and 12 months after RAI, cats were evaluated for thyroid status, GFR, SDMA and creatinine concentrations. Discordance was defined as one renal biomarker exceeding its RI with the other within/below the RI, using laboratory and age-specific SDMA RIs. In euthyroid cats, sensitivity, specificity and predictive values of SDMA/creatinine for decreased GFR and intra-individual variability were determined.
Results
Discordance between SDMA and creatinine was in the range of 10–75% and was highest with the SDMA limit >14 µg/dl. SDMA-GFR discordance exceeded creatinine-GFR discordance at all time points and was highest at baseline, when all cats were hyperthyroid. In cats with euthyroid status at 12 months, SDMA >14 µg/dl was more sensitive (83%) but less specific (50%) for decreased GFR than SDMA ≥18 µg/dl (sensitivity 50%, specificity 71%) and creatinine (sensitivity 50%, specificity 85%). Intra-individual variability was similar for both biomarkers (~15%).
Conclusions and relevance
Discordant SDMA and creatinine concentrations are frequent in older hyperthyroid cats and are influenced by thyroid status and SDMA RIs, complicating interpretation. The SDMA limit ≥18 µg/dl improves specificity for decreased GFR and is more appropriate for senior cats. SDMA concentrations in the range of 15–18 µg/dl are less likely to reflect true GFR reduction when creatinine is normal; therefore, non-renal influences such as hyperthyroidism should be considered and ongoing monitoring of renal parameters is recommended.