Hematuria can be undetectable on gross examination of urine samples, or the red blood cells (RBCs) can impart a variety of colors to urine and must be distinguished from other components that discolor urine. Hematuria is a common finding in dogs and cats, but identifying the source can be difficult since RBCs can be introduced anywhere along the urinary tract and may arise from the genital tract as well. Underlying causes of hematuria range from iatrogenic sources such as sample collection technique to disorders that produce marked morbidity. This article and the accompanying algorithms are intended to guide practitioners through the series of steps necessary to confirm the presence of hematuria, localize the source of the RBCs, and identify the specific cause.
STEP 1: CONFIRM THE HEMATURIA
Depending on the degree of hematuria, urine can range from a relatively normal yellow-to-amber color to abnormal red, orange, or brown shades. Thus, RBCs may be detected incidentally when you evaluate urine as a part of a routine health examination or discovered when you examine urine as part of a work-up in patients with a variety of clinical signs. Distinguishing substances other than RBCs as a source of urine discoloration is an important first step in correctly identifying and pursuing patient problems (Figure 1).
Dipstick colorimetric testing is a readily available, economic way to identify blood in urine, but keep in mind that a positive result is not specific for intact RBCs but is also compatible with hemoglobinuria or myoglobinuria. And regardless of pigment type, grossly discolored urine can make accurate interpretation of dipstick test pads difficult.
Estimate the number of RBCs
If the dipstick test result is positive for blood, examine the urine sediment to confirm RBCs and to approximate their number. A small number of RBCs is normal depending on the case and the urine collection method. Voided urine samples may normally have 0 to 8 RBCs/HPF, catheterized samples may normally have 0 to 5 RBCs/HPF, and cystocentesis samples may normally have 0 to 3 RBCs/HPF.1,2 While these numbers may seem counterintuitive, keep in mind that some cystocentesis and catheterized samples may be more difficult to obtain than others, and the degree of potential trauma or contamination during collection must be taken into account when excessive RBCs are present in urine samples obtained by any of these methods. Conversely, if RBCs are absent or relatively lacking in a urine sample obtained by cystocentesis or catheterization when compared with a voided sample, suspect a genital or urethral source of hematuria.
Rule out hemoglobinuria and myoglobinuria
In the absence of a marked number of RBCs in a urine sample that had a positive dipstick test result for blood, rule out hemoglobinuria and myoglobinuria by inspecting the patient's plasma. Pink plasma supports hemoglobinuria, while clear plasma is compatible with myoglobinuria or RBC lysis in the urine before sediment examination. Myoglobinuria may be confirmed with an ammonium sulfate test at a commercial laboratory.
Rule out bilirubinuria
If the result for blood on a dipstick is negative and the result for bilirubin is positive, bilirubinuria most likely accounts for the urine discoloration. The greater the quantity of bilirubin present in the urine, the more likely that discoloration will result. The clinical relevance of bilirubinuria is a separate issue and depends on the patient's urine specific gravity, species, and sex. Any amount of bilirubin in a cat's urine is considered abnormal. Urine with < 1+ bilirubin and a specific gravity ≥ 1.020 is generally considered normal for male and female dogs. Male dogs may normally have up to a 1+ bilirubin reading in concentrated urine. Urine with a bilirubin ≥ 1+ and a specific gravity ≥ 1.020 is a potentially clinically relevant finding, especially in female dogs. Canine urine with a bilirubin > 1+ and a specific gravity < 1.020 is clinically relevant regardless of sex.3