This article is sponsored content brought to you by IDEXX.
Today there are a number of methods available for veterinarians to evaluate renal function in dogs and cats. The aim of many of these methods is to estimate the Glomerular Filtration Rate (GFR). GFR is the rate the fluid moves from the plasma to the glomerular filtrate. The measurement of GFR can be achieved by evaluating the clearance of a specific marker administered intravenously, such as inulin or iohexol, but this is not practical as a routine test in practice, however.
On this basis, it is common in veterinary practice to use other biomarkers as a substitute for the direct measurement of GFR. Examples include urine concentrating ability, urea and creatinine. There are some challenges with these tests however, as they are later stage markers of disease (approximately 66% loss of renal mass to lose concentrating ability, and 75% loss for azotaemia), and they can be affected by other factors (for example other disease processes such as some endocrinopathies may affect urine concentrating ability, and lean body mass can affect creatinine).
Symmetric dimethylarginine (SDMA) by comparison has some distinct advantages. It increases earlier with a lower degree of reduction of renal mass (reliably at 40% loss and as early as 25% loss), and it is unaffected by lean body mass. An increased SDMA could be pre-renal, renal or post-renal in nature.
A persistently increased SDMA is most commonly a result of chronic kidney disease (CKD), but could reflect acute kidney injury, pyelonephritis etc.
The International Renal Interest Society, a group of independent international renal specialists, produce guidelines on the diagnosis, staging and management of CKD (and AKI). Patients with IRIS Stage 1 disease, or even some with early Stage 2 disease, will typically be asymptomatic. In the past, ways to diagnose IRIS Stage 1 disease included;
- Creatinine increasing within the reference interval
- Persistent renal proteinuria
- Abnormal kidney imaging
There was the challenge however that unless any of the above were incidentally detected through other screening processes, they may well be missed. An elevated SDMA is now another tool to be able to easily detect stage 1 disease.
Many clinics undertake Pre GA screening—at a minimum a history and physical examination, and in many cases combinations of haematology, a limited or comprehensive biochemical profile, a TT4 in cats over 7 years of age, and sometimes urine testing. Chemistry profiles will typically at a minimum include liver enzymes, total protein and urea and creatinine.
Including IDEXX SDMA allows the opportunity to potentially detect a reduced GFR, which if persistent, could reflect Stage 1 kidney disease. IDEXX provides a guide on how to approach a patient with mildly increased SDMA, but other parameters are within reference intervals. At a minimum if there is a mildly increased SDMA a urinalysis should be evaluated.
The question needs to be raised however—does a mildly elevated SDMA mean that a procedure should be delayed? If the patient is clinically well, and other parameters are normal, the answer is likely no. An increased SDMA could be pre-renal, and fluid therapy will be the solution. If potentially the result of renal disease, suggestions would include intra-procedure fluids, monitoring of blood pressure to avoid hypotension, and consideration of alternatives to NSAIDs for analgesia. As in any case with an increased SDMA, renal parameters and a urinalysis should be re-evaluated in 4 weeks to determine if the increase in SDMA is persistent or not.