Introduction

Glomerular filtration rate (GFR) is acknowledged to give a significantly more accurate assessment of renal function than standard biochemistry and urinalysis. Inulin clearance is well established as a 'gold standard' measure of GFR in humans. Further, inulin is commercially available, stable in plasma, requires a small sample volume, is inexpensive to assay and a canine reference range has been established.

Aims and Objectives

Retrospective study of the clinical utility of using plasma clearance of inulin to measure GFR in dogs looking at ease of use, quality of the clearance curve produced, potential side effects and clinical benefit.

Method

Inulin clearance was performed on 45 dogs with 4 dogs having more than one test performed over time. A standard method was used giving 3000 mg inulin/m² body surface area (Inutest 25% solution Fresenius Kabi) as an intravenous bolus via a catheter. Six timed blood samples were obtained over a 6 hour period (approximately 5–10, 20, 40, 100, 180 and 360 minutes). The exact time that the blood sample was acquired was recorded and used for data analysis.

High pressure liquid chromatography was used to measure the inulin level in each sample by the same external laboratory (Alomed).

GFR was calculated using commercial software (Inusoft-Laevosan).

Clearance curve from a 23 month old Vizsla with abnormal renal ultrasound. GFR = 140 ml/min/m2

Results

Timed blood samples were obtained in all dogs (weight range 4–55 kg, 17 breeds, 10–156 months of age).

No side effects were observed following inulin injection, during the test period or over subsequent hospitalisation.

The results obtained produced a good curve-fit in the majority of cases. Occasional aberrant results occurred that were unexplained and were discarded by the software.

Calculated inulin clearances were from 12 to 140 ml/min/m² [ref: 85–144].

Conclusions

The use of inulin to measure GFR was straightforward, reproducible and without observed side effects. It was felt to be of value to diagnose or rule-out early renal disease (IRIS stage I and II) as a cause of polyuria/polydipsia, to assess the level of long term renal damage following acute renal failure in patients whose urea and creatinine had returned to normal and to monitor progression of cases to help decide whether further intervention (e.g., biopsy) was appropriate. The value in dogs with established IRIS stage III-IV renal disease was unclear.