Estimation of Kidney Function and SDMA as a New Biomarker

Daniela Vrabelova Ackley, DVM DACVIM angell.org/internalmedicine
internalmedicine@angell.org
781-902-8400
MSPCA-Angell West, Waltham

Methodologies of GFR measurement and their limitations

Chronic kidney disease (CKD) is an insidious, progressive and prevalent illness in dogs and cats that is frequently recognized only after most of the kidney function is lost. Recent efforts of the International Renal Interest Society (IRIS) resulted in development of guidelines for CKD staging focused on early detection and intervention. Early indicators of kidney damage include a reduction in glomerular filtration rate (GFR) and impaired urine concentrating ability.

The most accurate estimate of GFR is measurement of plasma clearance of an injected marker over a given interval of time. If a marker is eliminated intact by the kidneys via glomerular filtration and is not reabsorbed nor secreted by the tubules, then renal clearance is equal to plasma clearance, and is the most exact estimate of GFR. Renal clearance of inulin is the gold standard for measuring GFR, however, iohexol clearance appears to be the simplest method for GFR determination in clinical practice. A comprehensive review of currently used methodologies for GFR estimation including endogenous and exogenous creatinine, radiolabeled markers, iohexol, contrast-enhanced computed tomography and cystatin C, was published in The Veterinary Journal in 2011.¹

Unfortunately, these techniques suffer from considerable limitations. The major disadvantage for general practitioners is the need for serial, precisely timed blood draws. The most accurate clearance calculation requires as many as 8 post-injection blood samples over 6 hours, although reasonable estimates can be obtained from 2 to 3 samples. Another limitation of plasma clearance is the large variability in what is considered to be “normal”. Different studies offer markedly wide 95% confidence intervals, which makes it difficult to define a normal GFR value for any given patient without a baseline. Determination of plasma clearance can be time and cost-prohibitive, especially when monitoring renal function over a period of time. In addition, numerous technique-specific, biological, and analytical factors must be considered, which makes the use of these techniques limited to specialty centers.

Serum creatinine

Serum creatinine (sCr) is the most commonly used endogenous marker for estimating GFR in clinical practice. However, sCr is an insensitive marker of kidney function because it remains in the reference interval until GFR is reduced by 75%.²  For this reason, IRIS established new reference ranges for sCr (dog < 1.4 mg/dl and cat < 1.6 mg/dl) independently of the reference range provided by the laboratory. The first stage of CKD includes dogs and cats with normal sCr having some other renal abnormality (e.g., decreased urinary concentrating ability without identifiable nonrenal cause, abnormal renal palpation or renal imaging, proteinuria of renal origin). Another limitation of sCr is its dependence on muscle mass and breed variations in dogs. Therefore, sCr may overestimate GFR in cachectic, geriatric, and very young patients. If muscle loss is not present, serial determinations of sCr are likely to reflect worsening renal function due to little individual variation in otherwise healthy adult animals, even over several years. In addition to biological variability, analytical variability is another important issue in the assessment of sCr especially in moderately to markedly azotemic samples. For these reasons, it is important that the same instrument is used for repeated measurements of sCr, ideally one that is subjected to a strict quality assurance program.

Symmetric Dimethylarginine (SDMA)

Methylated arginines are byproducts of protein methylation, released into blood during protein degradation.  There are three main species of methylated arginine: monomethylarginine (MMA), assymetric dimethylarginine (ADMA), and SDMA. SDMA has the advantage to be excreted primarily (>90%) by renal clearance in humans ⁴, which is extrapolated to dogs. Because SDMA is excreted by the kidneys, plasma concentrations are affected by changes in GFR. Dietary content of L-arginine does not influence serum SDMA concentration because serum SDMA is derived from methylated nuclear proteins.⁵ Many studies support SDMA as a marker of renal disease, including a meta-analysis compiled of 18 human studies, where SDMA concentrations correlated highly with inulin clearance and serum creatinine.⁶

SDMA in dogs

A recent prospective study validated SDMA measurement using liquid chromatography with excellent performance.⁷ SDMA was highly stable in serum and plasma up to 7 days at room temperature and 14 days at 4°C. This study also showed that SDMA strongly correlated with GFR in dogs with progressive hereditary nephropathy. Furthermore, SDMA identified decreased renal function earlier than sCr and GFR. Even though serial sCr measurements detected <50% loss of renal function, SDMA identified <20% decrease in GFR using either a general reference limit or serial monitoring. In 4 unaffected littermates, SDMA remained unchanged.

Another recent prospective study concluded that SDMA concentration was not affected by lean body mass in 41 aging Beagles.⁸ Both, sCr and BUN were correlated with muscle mass, which was significantly lower in older dogs and females. Serum creatinine decreased with increasing age overestimating GFR in older animals. BUN is also influenced by extra-renal factors including protein metabolism, hepatic function and nutrition, making it a less reliable marker of renal function. In this study, dogs were fed a reduced protein and phosphorus diet enhanced with fish oil, medium-chain triglycerides, and L-carnitine. SDMA concentrations, but not sCr, decreased significantly from baseline the longer dogs remained on this diet. Thus, SDMA could have clinical advantages over sCr in monitoring response to nutritional interventions.

Studies on non-renal influences in dogs showed no influence of asymptomatic mitral regurgitation, age, breed, sex, or exercise on serum SDMA.^9,10

SDMA in cats

One of the first feline articles measuring ADMA and SDMA documented an increase in both of them in cats with CKD and their concentrations correlated with sCr.³

A recent study established a relationship between declining GFR and SDMA concentration in 10 client-owned cats, both azotemic and nonazotemic.¹¹ GFR was measured by plasma iohexol clearance and a linear relationship was observed between declining GFR and increasing SDMA concentration in these older cats (11-17 years old).

Another study measured SDMA and sCr concentrations retrospectively from banked serum samples in 21 cats developing CKD and 21 healthy geriatric controls.¹² Both SDMA and sCr were significantly correlated to GFR. SDMA increased above the reference interval of 14 ug/dl on average 17 months earlier than sCr (upper reference limit was 2.1 mg/dl). SDMA had higher sensitivity (100%) compared with sCr (17%), but lower specificity (91% vs 100%).

The goal of another recent study was to evaluate whether feeding cats a reduced protein and phosphorus diet with added fish oil, L-carnitine, and medium-chain triglycerides for 6 months altered serum biomarkers of renal function.¹³ This study enrolled 32 healthy cats with a mean age of 14 years (8-19 years old). Although the enriched diet altered plasma fatty acids, carnitine and lysophospholipid concentrations, there were no changes in renal function biomarkers including sCr, SDMA and GFR. There was, however, a benefit in using SDMA versus sCr to assess kidney function in older cats with muscle wasting. Serum SDMA concentration was more accurately correlated with GFR in these cats, and, unlike sCr, SDMA increased as GFR declined with age.

Conclusions

The ability to detect declining GFR earlier than sCr supports SDMA as a more suitable marker of declining renal function compared to sCr in both dogs and cats. In addition, a single measurement of SDMA might be sufficient to detect a mild decrease in GFR when trending (multiple measurements over time) is not possible. SDMA can prove especially useful in the initial diagnosis of CKD in older patients or patients with poor muscle mass, for which sCr might not provide a reliable estimate of GFR.  Additional studies are needed to evaluate extra-renal influences in both dogs and cats.

For more information on SDMA or Angell-West’s Internal Medicine Service, please contact Dr. Ackley at dvrabelova@angell.org or 781-902-8400

References:

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