November 2, 2018 Vientós-Plotts, AI, Behrend EN, et al. Effect of blood contamination on dipstick evaluation and urine protein-to-urine creatinine ratio for urine samples from dogs and cats. Am J Vet Res 2018;79:525-531
Urine is generally analyzed in three basic ways in veterinary practice: chemistry is obtained using a dry reagent strip ("dipstick"), specific gravity is evaluated using a refractometer, and sediment (cells and other solid materials) is visualized and characterized with a light microscope. A commonly used second-tier test is a urine protein-to-urine creatinine ratio (UPCR). The UPCR is used to detect clinically significant proteinuria. Blood may be present in urine either in macroscopic quantities (visually detectable color change) or microscopic quantities, and blood may enter urine in several ways: either through the presence of disease, usually in the urinary tract itself, or iatrogenically secondary to cystocentesis.
This retrospective study of urine samples from 279 dogs and 120 cats collected either through natural voiding or cystocentesis was performed at a veterinary teaching hospital. Goals of the study were to determine the influence of hematuria on dipstick analysis, urine specific gravity (USG), and UPCR. Although no information regarding patients' signalment, history, and method of urine collection was included in the study, any samples that were visibly discolored (red, orange, or brown) or containing >5 red blood cells/high-powered microscope field and/or >5 white blood cells/high-powered microscope field on sediment evaluation were excluded from the study.
To evaluate the influence of the presence of varying amounts of blood in the urine samples on laboratory assessment, the urine samples were centrifuged twice, and then the supernatants were pooled to create 30 pooled samples per species. Feline blood collected 24 hours prior and preserved in EDTA under refrigeration was then added to the undiluted pooled feline urine samples, which had been divided into aliquots that were diluted serially to 1:20, 1:40, 1:80, 1:160, 1:320, 1:1,280, and 1:5,120, such that a dilution of 1:20 represented 1 part of blood to 19 parts of urine. A color was assigned to each level of dilution by the same examiner, who also obtained a USG on each diluted sample with a refractometer. Sediment evaluations to obtain the number of red blood cells/high-powered microscope field for each dilution were also performed. In addition, an initial analysis in which EDTA only was added to three urine pools for each species demonstrated that the presence of EDTA at any of the dilution levels did not change any of the dipstick results for either cats or dogs.
Once blood was added to the feline urine samples, dipstick blood almost always read out as 3 (range 0-4) in all samples regardless of color, from light yellow through red. Feline samples diluted by blood read out significantly higher in bilirubin in dark pink and red samples, higher in ketones in red samples, and higher in protein in dark pink and red samples compared to feline urine samples undiluted by blood. In red samples, pH readings on the dipstick were significantly lower than in undiluted urine. The presence of any amount of blood in feline urine did not affect the dipstick glucose score. Specific gravity readings by refractometer were significantly higher in dark pink and red samples than in undiluted urine.
The UPCR in the undiluted feline samples was significantly lower (P < 0.001) compared to that in all blood-diluted samples and all urine colors evaluated, except for light yellow. As the color of the blood-diluted cat urine samples progressed from light yellow to red, the UPCR category obtained also progressed from non-proteinuric (< 0.2), to borderline (0.2-0.4), to proteinuric (>0.4). All (100%) of the light pink to red feline urine samples had a higher proteinuria category than the same undiluted sample, while almost half (48%) of the light to dark yellow feline urine samples also demonstrated more proteinuria than the same undiluted sample.
This study had the following important conclusions:
1) Blood contamination, even if microscopic, can increase the bilirubin dipstick reading in a urine sample. As bilirubinuria is always considered abnormal in cats, a positive bilirubin dipstick reading in cat urine with any level of blood contamination should be interpreted with caution, as it may be artifact asociated with blood contamination rather than a marker of hepatobiliary or hemolytic disease.
2) Blood contamination does not significantly alter the glucose reading on a urine dipstick.
3) Changes in urine pH secondary to blood contamination were of small magnitude. Dipstick urine pH evaluations are less accurate and less reproducible than pH meter readings, so if there are concerns about urine pH, a pH meter reading is recommended.
4) Although blood contamination has an impact on USG, this is usually not clinically significant. Urine concentrating ability can still be evaluated reliably in most animals despite the presence of hematuria. In cats with a USG slightly below 1.035, azotemia, and visible blood contamination, the hematuria could increase the USG enough for the patient to be considered to have adequate renal urine concentrating ability.
5) Dipstick protein scores in cats were not found to increase until blood-diluted urine samples were dark pink. However, even microscopic blood contamination only did influence the UPCR. Approximately 75% of the cat urine samples progressed to an increased level of proteinuria when any amount of blood was added. If there are > 250 red blood cells per high-powered micoscopic field in a urine sample, proteinuria may be falsely diagnosed. [PJS]
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urine specific gravity (USG)