Date Published: April 6, 2017
Publisher: Public Library of Science
Author(s): Jean A. Hall, Maha Yerramilli, Edward Obare, Jun Li, Murthy Yerramilli, Dennis E. Jewell, Abelardo I Aguilera.
Serum concentrations of symmetric dimethylarginine (SDMA) correlate with renal function in cats and SDMA has been shown to be a more reliable and earlier marker for chronic kidney disease (CKD) compared with serum creatinine (Cr). Calcium oxalate uroliths tend to develop in mid-to-older aged cats and kidney stones may cause a reduction in renal function with increased SDMA, but normal serum Cr. The purpose of this retrospective study was to determine if cats with kidney stones had increased serum SDMA concentrations, and whether SDMA increased earlier than serum creatinine concentrations. Cats in the colony with kidney stones diagnosed between August 2010 and December 2015 (n = 43) were compared with healthy geriatric cats (n = 21) without kidney stones. Serum SDMA concentrations were determined by liquid chromatography-mass spectrometry and serum Cr concentrations were determined by enzymatic colorimetry. Cats with kidney stones were diagnosed antemortem by radiographic imaging (n = 12) or by postmortem necropsy (n = 31). Retrospectively, serum SDMA was found to be increased above the upper reference limit in 39 of 43 cats with kidney stones. Serum Cr was increased above the upper reference limit in 18 of 43 cats; 6 of these 18 cats had terminal azotemia only. The mean time that serum SDMA was increased before serum Cr was increased was 26.9 months (range 0 to 60 months). Kidney stones were composed of calcium oxalate in 30 of 34 cats. The lifespan for cats with kidney stones (mean, 12.5 years; range, 6.1 to 18.1 years) was shorter (P < 0.001) than for control cats (mean, 15.2 years; range, 13.0 to 17.2 years), suggesting that non-obstructive kidney stones have an effect on mortality rate or rate of CKD progression. In conclusion, if SDMA concentrations are elevated in mid-to-older aged cats, further imaging studies are warranted to check for the presence of kidney stones.
Beginning in the mid-1980s, a noticeable increase in the frequency of calcium oxalate uroliths occurred in cats concomitant with a decrease in the frequency of struvite uroliths . In 2002, approximately 55% of the 95,000 feline uroliths submitted to the Minnesota Urolith Center were composed of calcium oxalate . More recently (2007), the frequency of feline calcium oxalate uroliths declined to 41%, perhaps associated with reformulation of adult maintenance diets to minimize risk factors for calcium oxalate crystalluria . These finding are similar to the percentage of feline calcium oxalate uroliths (40%) reported from submissions to the Gerald V. Ling Urinary Stone Analysis Laboratory . The increase in calcium oxalate urolithiasis in cats has been associated with a parallel increase in the occurrence of these uroliths in the upper urinary tract (kidneys and ureters) .
The goal of this retrospective study was to determine if increased serum SDMA concentrations are detected in cats with kidney stones. We found that 92% of cats in the current study with kidney stones had increased serum SDMA concentrations at the time of diagnosis, whereas 17% were azotemic at the time of diagnosis (42% were azotemic at some time prior to death). Therefore, we conclude than using serum SDMA as a biomarker for reduced kidney function allows earlier detection of kidney stones compared with measurement of serum creatinine. Of clinical importance, if serum SDMA concentrations are increased in mid-to-older aged cats, especially with USG > 1.035, then further imaging studies are warranted to evaluate for the presence of kidney stones.