Today's Veterinary Practice

SEP-OCT 2018

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PEER REVIEWED 10 SEPTEMBER/OCTOBER 2018 todaysveterinarypractice.com Blood testing. An annual blood screen that includes a complete blood count and chemistry profile with electrolytes is recommended. The complete blood count can help detect anemia, which is commonly mild and nonregenerative with CKD. After overt signs of CKD are present, some degree of azotemia (increased blood urea nitrogen [BUN] and creatinine) will be detected. Varying degrees of hyperphosphatemia will also be present and often worsen as CKD progresses. Like serum creatinine concentrations and glomerular filtration rate, symmetric dimethylarginine (SDMA) 5,6 is a means of measuring renal function in cats. Serum SDMA concentrations are inversely proportional to glomerular filtration rate and may move out of the reference range before serum creatinine concentrations are increased. Thus, measurement of SDMA may allow earlier detection of renal disease. Although SDMA testing can be a complementary tool for CKD diagnosis, a single abnormal result does not confirm CKD, and you should not substitute SDMA findings for urinalysis and other diagnostic procedures. SDMA levels can fluctuate from day to day and be affected by factors such as hydration, diet, toxin ingestion, or medication use. A recent study showed significant variation in SDMA concentrations in healthy dogs sampled 9 times at various intervals. 7 Therefore, you should always view SDMA findings within the context of the entire clinical picture. If the SDMA concentration is the only finding suggestive of CKD, you should re-test within 1 to 3 months to confirm repeatability. Urinalysis. The importance of this screening test cannot be overemphasized, given that concentrating ability is often lost before BUN or creatinine increase. The urinalysis (including a microscopic sediment examination) measures urine specific gravity and determines presence or absence of proteinuria, red and white blood cells, bacteria, crystals, and casts. The International Renal Interest Society (IRIS; iris-kidney. com) guidelines on CKD in cats suggest that those with specific gravity <1.030 be further evaluated, but other data 7 suggest that older, apparently healthy cats with specific gravity <1.035 undergo additional diagnostics, including blood pressure and SDMA measurements. Multiple factors can play a role in decreasing a cat's urine concentrating ability (e.g., age, sex, dietary moisture content, drinking avidity, and fasting status). 8 If proteinuria is detected on a dipstick, or if there is other evidence to support a diagnosis of CKD, the urine protein:creatinine ratio should be determined. If proteinuria is confirmed and the ratio is >0.4, medical and/or dietary intervention is warranted because proteinuria is associated with decreased survival times in cats. 9 THYROID TESTING As cats reach their senior years, adding a total thyroxine test to screen for hyperthyroidism is also advised. Hyperthyroidism may mask CKD because increased renal blood flow may lower BUN and creatinine. The influence of hyperthyroidism on SDMA metabolism is unknown, although a laboratory survey suggested that SDMA concentrations are more accurate than creatinine concentrations for identifying CKD in cats with hyperthyroidism. In cats that receive iodine-131 therapy, SDMA testing has high specificity but relatively low sensitivity for predicting development of azotemia. 10 CKD CONFIRMATORY TESTS When screening test results alert practitioners to the possibility of CKD, confirming these findings completes the picture. Illness and dehydration, as well as acute toxin ingestion, can lead to abnormal screening findings, which may later be reversed. You should not make a CKD diagnosis on the basis of 1 elevated creatinine or SDMA value without determining if the results are repeatable. For a confirmed diagnosis of CKD, abnormal findings should persist for at least 3 months. You should not make a CKD diagnosis on the basis of 1 elevated creatinine or SDMA value without determining if the results are repeatable. For a confirmed diagnosis of CKD, abnormal findings should persist for at least 3 months.

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