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

Longitudinal Study of Renal Health Screening in Apparently Healthy Aging Dogs

BACKGROUND

Combined measurement of functional, glomerular, and tubular markers in aging dogs is essential to detect early renal disease.

OBJECTIVES

Prospective longitudinal study to describe renal function and assess which biomarkers are associated with the development of early renal disease or death.

ANIMALS

One hundred and twenty-two apparently healthy senior and geriatric dogs.

METHODS

Prospective longitudinal study. Renal function was evaluated at baseline (T0) and every 6-12 months over 2 years, using systolic blood pressure measurements (SBP) and validated serum (creatinine, symmetric dimethylarginine, cystatin C [sCysC]), and urinary (specific gravity [USG], protein:creatinine [UPC], albumin:creatinine, retinol-binding protein:creatinine [uRBPcr]) biomarkers. Glomerular filtration rate (GFR) was measured in a subgroup. Survival models were used to assess the predictive value of measured biomarkers at baseline for the onset of azotemic chronic kidney disease (CKD) or death, respectively.

RESULTS

A total of 122 dogs were included; follow-up was available in 106 (T12) and 92 (T24); and GFR was estimated in 18 (T0), 11 (T12), and 10 (T24) dogs. Throughout the study, 15/122 (12%) dogs showed evidence of non-azotemic CKD, and in 11/106 (10%) dogs, azotemic CKD developed. Proteinuria was not associated with azotemic CKD, in contrast to muscle condition score, functional markers, and uRBPcr. Death was weakly associated with USG, UPC, and sCysC.

CONCLUSIONS AND CLINICAL IMPORTANCE

Over a 2-year period, 20% (26/122) of older dogs developed CKD, mostly persistent renal proteinuria (15/122). Muscle wasting and functional markers combined with uRBPcr had the best predictive value for the onset of azotemic CKD in these older, previously apparently healthy dogs.

Impact of interlaboratory differences on the measurement of serum creatinine and symmetric dimethylarginine values: does this affect clinical decision-making?

ObjectivesThis study aimed to assess the impact of interlaboratory differences (in analysis technique and reference intervals [RIs]) on the measurement of serum creatinine (sCr) and serum symmetric dimethylarginine (sSDMA) concentrations in cats and on classification by stage of chronic kidney disease (CKD).MethodsFor the interlaboratory discrepancy study, samples from 30 client-owned cats (healthy or diagnosed with CKD) with sCr in the range of 130-200 µmol/l and urine specific gravity <1.035 were retrospectively selected. A single batch of samples was analysed for sCr and sSDMA with one in-house laboratory machine and in three commercial laboratories. In addition, sCr values from 63 healthy cats aged ⩾10 years were used to calculate an age-specific RI for the in-house analyser.ResultsDespite a relatively good correlation (interclass correlation coefficient [ICC] = 0.875), the in-house analyser consistently reported lower sCr values, while for sSDMA, systematically higher values were obtained in one commercial laboratory (ICC = 0.707). A total of 22/30 (73%) cases had at least one laboratory reporting inconsistent sSDMA values in relation to the RI, and 23/30 (77%) for sCr. For subsequent staging using sSDMA, at least 1/4 laboratories indicated a different stage in 22/30 (73%) cases. For sCr, all laboratories classified the cats as stage 2; however, the in-house analyser classified 8/30 (27%) cats as stage 1. The RI (71-212 µmol/l) provided by the company for sCr measurement on the in-house machine was subsequently recalculated, resulting in a considerably lower upper limit (163 µmol/l) and a significantly (adjusted P value = 0.016) different proportion of cats with measurements outside the RI.Conclusions and relevanceClinicians must be careful when interpreting sCr and sSDMA values, as they may lead to discrepant outcomes due to interlaboratory variability. Moreover, age-specific RIs should be applied when available, facilitating the early detection of CKD in older cats.

Influence of Carcinoma and Sarcoma on Neutrophil Gelatinase-Associated Lipocalin and Symmetric Dimethylarginine Concentrations in Dogs

BACKGROUND

It is unknown if tumors or concomitant renal disease influence neutrophil gelatinase-associated lipocalin (NGAL) and symmetric dimethylarginine (SDMA) concentrations in tumor-bearing dogs.

OBJECTIVES

Determine the effect of tumor presence, tumor type, and metastasis on concentrations of serum NGAL (sNGAL), SDMA, urinary NGAL (uNGAL), and uNGAL-to-creatinine ratio (uNGAL/Cr) in dogs with carcinoma or sarcoma without clinically relevant renal disease.

ANIMALS

Twenty-one dogs with carcinoma, 18 with sarcoma, and 20 healthy age-controlled dogs.

METHODS

Concentrations of sNGAL, SDMA, and uNGAL, and uNGAL/Cr ratio were measured from banked samples collected during a previous prospective study. Patient clinicopathological and histopathology records were reviewed, and those with renal azotemia or moderate to severe histopathological renal abnormalities were classified as having clinically relevant renal disease. Biomarker concentrations were compared between tumor-bearing dogs without clinically relevant renal disease and healthy age-controlled dogs. Additionally, comparisons were made between dogs with carcinoma and sarcoma, as well as between dogs with and without metastasis. Correlations between uNGAL and sNGAL concentrations, along with acute phase protein (APP) concentrations, were also analyzed.

RESULTS

Tumor-bearing dogs without clinically relevant renal disease had increased uNGAL/Cr (p < 0.001), but not sNGAL, compared with healthy controls. Although median SDMA concentrations did not significantly differ between groups, increased concentrations were found in 32% of dogs with carcinoma and 20% of dogs with sarcoma. No differences were found between dogs with carcinoma and those with sarcoma, or between dogs with metastasis and those without. Urinary and serum NGAL concentrations were moderately correlated, while weak to no correlations were observed with APPs.

CONCLUSION

Carcinomas and sarcomas, but not metastasis, influence uNGAL/Cr and SDMA concentrations in dogs.

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

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°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 (r2  = .99) and precision (coefficient of variation <10%), with a laboratory RI between 8.0 and 14.0 μg/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) μg/dL using ELISA. Both methods significantly differed with a mean difference of 2.2 μg/dL. The RI for older dogs for LC-MS/MS was 4.4-15.0 μg/dL, and for ELISA was 6.4-17.4 μg/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.