Aims: To determine the effect of contamination of urine with 0-5% blood, varying in haematocrit and protein concentrations, on the urine protein to creatinine ratio (UPC) in dogs, and to determine whether the colour of urine can be used to aid interpretation of UPC results.
Methods: Urine samples were collected by free catch from 18 dogs, all of which had UPC <0.2. Venous blood samples were also collected from each dog, and the blood from each dog was added to its own urine to produce serial concentrations of 0.125-5% blood. The colour of each urine sample was recorded by two observers scoring them as either yellow, peach, orange, orange/red or red. Protein and creatinine concentrations were determined, and dipstick analysis and sediment examination was carried out on each sample. Based on colour and dipstick analysis, samples were categorised as either having microscopic, macroscopic or gross haematuria. A linear mixed model was used to examine the effect of blood contamination on UPC.
Results: The uncontaminated urine of all 18 dogs had a UPC <0.2. Adding blood to the urine samples resulted in an increase in UPC at all contamination concentrations compared to the non-contaminated urine (p<0.001). None of the 54 samples with microscopic haematuria had UPC >0.5. For 108 samples with macroscopic haematuria the UPC was >0.5 in 21 samples (19.4 (95% CI=13.1-27.9)%), and for 54 samples with gross haematuria 39 (72 (CI=59.1-82.4)%) had a UPC >0.5. No samples had a UPC >2.0 unless the blood contamination was 5% and only 3/18 (17%) samples at this blood contamination concentration had a UPC >2.0.
Conclusions and clinical relevance: This study showed that while blood contamination of ≥0.125% does increase the UPC, if the urine remains yellow (microscopic haematuria), then there is negligible chance that a UPC >0.5 will be solely due to the added blood. In that scenario, attributing the proteinuria present to the haematuria in the sample would be inappropriate. However blood contamination that results in discolouration of the urine sample from yellow (indicating macroscopic or gross haematuria) could increase the UPC above the abnormal range and would need to be considered as a differential for the proteinuria. Thus knowledge of urine colour, even if limited to simple colour scores (yellow, discoloured, red) could be utilised to aid interpretation of the UPC in samples with haematuria.
Keywords: Proteinuria; blood contamination; haematuria; urine colour; urine protein to creatinine ratio.