N-acetyl-beta-D-glucosaminidase index as an early biomarker for chronic kidney disease in cats with hyperthyroidism.

Hyperthyroid cats are at risk of developing azotemic chronic kidney disease (CKD) and diagnostic tools currently used to screen for CKD in hyperthyroid cats are either unreliable or impractical. Urine N-acetyl-beta-D-glucosaminidase index (NAG(i)) is a good biomarker for azotemic CKD in hyperthyroid cats. Twenty-four newly diagnosed nonazotemic hyperthyroid cats and 10 healthy cats. All cats were evaluated for hyperthyroidism at baseline. Hyperthyroid cats were treated with methimazole and reevaluated once euthyroid. At the end of the study, cats were divided into 3 groups: healthy cats, nonazotemic, and azotemic euthyroid cats. Baseline group characteristics were compared to predict azotemic CKD. The influence of treatment on NAG(i) was evaluated. Baseline NAG(i) was significantly different among groups (P= .004). Azotemic cats had a higher median value (13.12 U/g) when compared with healthy cats (1.38 U/g). With NAG(i) >2.76 U/g, negative and positive predictive values for development of azotemia were 77.7 and 50%, whereas the combination of a urine specific gravity (USG) <or=1.035 and T(4) >7.80 microg/dL enhanced predictive values to 88.9 and 83.3%, respectively. NAG(i) values decreased significantly over time in treated nonazotemic cats. Baseline NAG(i) did not differentiate azotemic from nonazotemic euthyroid cats. NAG(i) could be used to assess renal function during medical therapy allowing the clinician to adjust methimazole dosage accordingly. The combination of USG and T(4) could optimize identification of appropriate candidates for permanent treatment of hyperthyroidism.