Glomerular filtration rate in dogs as estimated via plasma clearance of inulin and iohexol and use of limited-sample methods.

OBJECTIVE: To compare plasma clearance of inulin and iohexol determined by use of 9 plasma samples for evaluation of glomerular filtration rate in dogs and to evaluate limited-sample approaches for evaluation of plasma clearance of these markers. ANIMALS: 43 dogs of various breeds that weighed between 5.5 and 63 kg and that had various degrees of renal function. PROCEDURES: 9 plasma samples were obtained from each dog at 5 minutes to 6 hours after IV bolus injection of iohexol and inulin. Clearance was calculated by use of results for all 9 samples (ie, reference method). Results for 3 limited-sample strategies for determination of plasma clearance of iohexol and inulin were compared with results for the reference method. RESULTS: Mean clearance of inulin and iohexol for the reference method was 2.72 and 2.48 mL/min/kg, respectively. The mean difference between clearance of these 2 markers for the reference method was 0.24 mL/min/kg. In general, use of the limited-sample strategies yielded clearance values similar to those for the reference method. More accurate estimates of clearance were obtained for iohexol than for inulin by use of the limited-sample methods. CONCLUSIONS AND CLINICAL RELEVANCE: Use of iohexol and inulin yielded similar but not identical results for plasma clearance. Accuracy for limited-sample methods would be acceptable for many clinical and research situations.

Comparison of serum-free thyroxine concentrations determined by standard equilibrium dialysis, modified equilibrium dialysis, and 5 radioimmunoassays in dogs.

Measurement of serum-free thyroxine (fT4) concentration provides a more accurate assessment of thyroid gland function than serum thyroxine (T4) or 3,5,3'-triiodothyronine (T3). Techniques for measuring serum fT4 concentration include standard equilibrium dialysis (SED), radioimmunoassay (RIA), and a combination of both (modified equilibrium dialysis [MED]). This study compared results of serum fT4 measurements by means of SED, MED, and 5 RIAs in 30 healthy dogs, 10 dogs with hypothyroidism, and 31 euthyroid dogs with concurrent illness for which hypothyroidism was a diagnostic consideration. Serum fT4 concentrations were comparable when determined by the SED and MED techniques, and mean serum fT4 concentrations were significantly (P < .01) lower in dogs with hypothyroidism than in healthy dogs and euthyroid dogs with concurrent illness. Significant (P < .05) differences in fT4 concentrations were identified among the 5 RIAs and among the RIAs and MED and SED. Serum fT4 concentrations were consistently lower when fT4 was determined by the RIAs, compared with either equilibrium dialysis technique. Serum fT4 concentrations were significantly lower (P < .01) in dogs with hypothyroidism than in healthy dogs for all RIAs; were significantly lower (P < .05) in dogs with hypothyroidism than in euthyroid dogs with concurrent illness for 4 RIAs; and were significantly lower (P < .01) in euthyroid dogs with concurrent illness than in healthy dogs for 4 RIAs. RIAs had the highest number of low serum fT4 concentrations in euthyroid dogs with concurrent illness. This study documented differences in test results among fT4 assays, emphasizing the importance of maintaining consistency in the assay used to measure serum fT4 concentrations in the clinical or research setting.