ABSTRACT. Chronic kidney disease is one of the most common disorders in dogs and cats. The plasma urea nitrogen (P-UN) and creatinine (P-Cre) concentrations are not sufficiently sensitive for early diagnosis of renal dysfunction. Although urine and plasma clearance methods allow earlier detection of reductions in the GFR, it is difficult to estimate a mildly reduced GFR from the values obtained by these methods, as they are also affected by physiological factors, such as body weight (BW) and age. The present study is a retrospective survey designed to assess the factors that affect markers of kidney function and to revaluate the clinical utility of the markers, including P-UN, P-Cre and GFR determined by plasma iohexol clearance (PCio) in dogs and cats. The P-UN, P-Cre and PCio values in dogs and the P-Cre and PCio values in cats were significantly correlated with BW (P<0.001). PCio in smaller dogs (15.0 kg) was significantly and inversely correlated with age. In smaller dogs, increase of P-UN alone might warrant a suspicion of a decreased GFR, but in contrast, P-Cre may be inefficient for detecting renal dysfunction or determining the severity of CKD compared with that in larger dogs ( 15.1 kg). P-Cre in larger dogs correlated better with PCio than in smaller dogs, suggesting that P-Cre in larger dogs was a more sensitive marker of reduced GFR.KEY WORDS: body weight, plasma creatinine, plasma iohexol clearance, plasma urea nitrogen.J. Vet. Med. Sci. 72(9): 1129-1136, 2010 Chronic kidney disease (CKD) is one of the most common disorders and is a common cause of death in dogs and cats. The prevalence of renal diseases has been reported to range from 0.5 to 7% in dogs and from 1.6 to 20% in cats [29,43]. Although CKD occurs in dogs and cats of all ages, the mortality of CKD commonly increases with age. In human and veterinary medicine, CKD is diagnosed by detecting chronic decreases in the glomerular filtration rate (GFR) or the presence of chronic renal damage [4,36]. GFR can be evaluated indirectly from the concentrations of plasma and urine markers, which are dependent on the amount eliminated by the kidneys. The plasma urea nitrogen (P-UN) and creatinine (P-Cre) concentrations are widely used as endogenous markers to evaluate renal function in dogs and cats because they can be easily, rapidly and cheaply measured. GFR is measured directly by urine or plasma clearance methods using inulin, creatinine or iohexol [7,14,15,20,21,33,34]. Although the urinary clearances of inulin and creatinine are considered to be the gold standards for measuring GFR in humans and animals, these methods are impractical in clinical settings because they require insertion of a bladder catheter, which is associated with a risk of developing a urinary tract infection or urethral injury; complete collection of a timed urine sample; and continuous infusion of the marker. The single injection technique can be used to determine plasma clearance from the elimination curve of a marker in plasma after a bolus injection. Plasma clearance ca...