The diagnostic approach to hematuria
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
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
Check for pseudohematuria
A negative urine dipstick result for blood with a concurrent negative result for bilirubin from a patient with discolored urine warrants testing a second urine sample in case RBCs are only intermittently present. If the second sample's dipstick or sediment examination results remain negative for RBCs, consider pseudohematuria—urine discoloration from pigments derived from drugs such as doxorubicin, food dyes, toxins such as mercury, or inherited conditions such as porphyria—and review the patient's historical and physical examination findings.4 If you suspect pseudohematuria, persistent discoloration of urine supernatant after centrifugation may provide further evidence of this condition. If you cannot find evidence of an abnormal exogenous or endogenous pigment resulting in discoloration, it is possible that the color is a normal manifestation of urochrome and urobilin (normal urine pigments) in a concentrated urine sample. In this instance, the urine typically has a dark-yellow to brown hue.
In rare instances, abnormal pigment in urine cannot be identified by standard evaluation methods and, if persistent, could be pursued by expanded laboratory testing. State veterinary diagnostic laboratories are good resources for investigating the possible presence of toxins, dyes, medications, and porphyrins. In addition, PennGen, the Section of Medical Genetics associated with the University of Pennsylvania School of Veterinary Medicine, may be able to provide guidance in detecting inherited disorders of heme synthesis in young patients.
STEP 2: RE-EVALUATE HISTORICAL AND PHYSICAL EXAMINATION FINDINGS
Note clinical signs, history, and variations in hematuria based on the collection method
Identifying signs of systemic disease, urination frequency, and dysuria can help establish whether the hematuria more likely originates from the upper urinary tract, lower urinary tract, or genital tract. Classic signs of lower urinary tract disease include pollakiuria and dysuria. Signs of systemic disease such as weight loss or fever support upper urinary tract or, in some cases, genital tract disease.
Although systemic hypertension is not generally considered a primary cause of hematuria, its presence is often associated with renal disease and could exacerbate bleeding and alter treatment. Historical and physical findings that may indicate systemic hypertension include polydipsia and polyuria; cardiac murmur; tachycardia; epistaxis; neurologic signs such as disorientation, ataxia, or seizures; and ocular abnormalities such as choroidopathy, retinopathy, and hemorrhage.
When blood is observed grossly in a voided sample but is not as readily apparent in urine obtained by cystocentesis or catheterization, suspect a genital or urethral source of hematuria. However, hematuria from a genital tract or urethral source can sometimes be observed in both voided and nonvoided samples since the collection technique can result in iatrogenic RBC contamination or because material from the genital tract or urethra may reflux into the bladder, especially in patients with prostatic disease.
Examine the urogenital area and observe voiding
During the physical examination, be sure to inspect the genitalia and urethral orifice as well as to abdominally palpate the bladder, kidneys, and prostate. Although sometimes involved in disorders resulting in hematuria, the uterus is generally not palpable in intact female dogs unless it is enlarged or associated with a mass. If the bladder is markedly distended with urine when palpated, palpate it again after the animal has voided to increase the chances of palpating abnormalities in the bladder wall or lumen. Extrude the penis from the prepuce for inspection in male patients, and include digital palpation of the perineal urethra as part of the urogenital examination. Vaginal examination can be enhanced by using a speculum or digital palpation. Rectal palpation allows the opportunity to palpate the prostate in male dogs, the pelvic urethra in most patients, and the posterior aspect of the bladder in some patients. Identifying pain or irregularities during palpation may indicate that organ or structure as a source of hematuria.
STEP 3: PERFORM OR RE-EVALUATE DIAGNOSTIC TESTS
After localizing the source of hematuria based on historical and physical examination findings, follow the most appropriate algorithm (Figures 3-7) to devise a diagnostic plan to identify the specific cause of hematuria.
CBC and serum chemistry profile
Urinalysis is an essential part of evaluating hematuria, regardless of the source. Always consider the method of urine collection and the urine specific gravity when evaluating urinalysis results. Urinalysis is best performed within 30 minutes of sample collection. A longer delay can result in cellular deterioration, pH changes, bacterial growth, and precipitates that can be confused with crystals.5 Refrigerate urine samples that cannot be examined within 30 minutes of collection, but gradually warm them to room temperature to obtain accurate dipstick chemical analysis.
In most cases, obtaining urine by cystocentesis is desirable to avoid genital and environmental contamination. When a coagulopathy is suspected, cystocentesis should only be cautiously attempted because of the possibility of causing excessive bleeding. When hematuria is suspected to be of genital or urethral origin, comparing urinalysis results from a voided sample with results from a cystocentesis sample may provide evidence of a genital or urethral source.
Urine chemical and sediment examination. Urine chemical and sediment findings such as protein, infectious organisms, white blood cells, casts, and crystals can help determine definitive or contributing causes of hematuria. Always assess the clinical relevance of these findings in relationship to the urine collection technique and the total clinical evaluation of the patient. For example, crystals can be a normal finding in urine samples and are not considered a cause of hematuria, although their presence may be associated with potential sources of hematuria such as uroliths.
If you suspect a lower urinary tract infection (UTI), it is reasonable to try to eradicate the infection in an attempt to resolve the hematuria. Ideally, treat bacterial infections by selecting antibiotics based on bacterial culture and antimicrobial sensitivity testing results. Treat patients with multiple episodes of UTIs, systemic signs of illness, systemic fungal infections, genital tract infections, or upper UTIs with longer courses of antimicrobial drugs. These patients should undergo further diagnostic procedures to identify complicating factors and other organ system involvement. If parasites are identified in urine, such as Capillaria or Dioctophyma species ova or Dirofilaria species microfilariae, administer an appropriate parasiticide. In patients with Dioctophyma or Dirofilaria species infections, perform additional diagnostic procedures before treatment to assess kidney function and structure as well as other organs that might be affected by these parasites.
Blood pressure measurement
Routine screening for systemic hypertension by performing blood pressure measurements is advisable if you suspect upper urinary tract disorders or in cases in which localizing the source of hematuria cannot be reasonably accomplished through historical and physical examination findings.
Diagnostic imaging plays an important role in tracking down the site and cause of hematuria. Abdominal radiography and ultrasonography are usually indicated in cases of hematuria. Imaging is indicated early in the diagnostic process in patients with a recent history of trauma or physical examination findings such as abdominal masses or abnormalities associated with a kidney, the bladder, the uterus, or the prostate. Use imaging to thoroughly examine the urogenital tract for irregularities, including masses, abnormalities in the shape and size of structures, and uroliths or bladder sediment.
Although blind aspirates and biopsies of suspicious areas of the urogenital tract can be performed, ultrasonography provides a potentially safer, more accurate method of obtaining samples for cytologic or histologic examination or culture. In patients with lower urinary tract signs, a retrograde contrast study may be indicated to identify urethral abnormalities, further evaluate bladder masses, and recognize radiolucent calculi. Patients with evidence of upper urinary tract disease or hematuria that has yet to be localized may be candidates for an excretory urogram to identify functional or subtler structural abnormalities. To avoid introducing bacteria or other contaminants, collect samples from the urogenital tract for bacterial culture or other analysis before performing contrast procedures. Advanced imaging such as computed tomography or magnetic resonance imaging may be indicated when the results of traditional imaging methods are inconclusive.
If less-invasive diagnostic tests have not revealed the source of the hematuria, abdominal exploratory surgery may be indicated. An exploratory not only allows gross examination and biopsy of the urogenital system, the ureters can be catheterized individually to determine whether blood is present in the urine produced by one or both kidneys.
Before performing biopsies or exploratory surgery in patients with hematuria, perform a coagulation profile to establish that excessive bleeding is unlikely to occur. An activated clotting time could be used as a screening test, but it is not as sensitive of an indicator of bleeding potential as are the prothrombin time, partial thromboplastin time, and platelet count included in a standard coagulation profile.
STEP 4: PINPOINT THE CAUSE BASED ON DIAGNOSTIC TEST RESULTS
Exercise-induced hematuria may represent a rare subcategory of trauma that is diagnosed by excluding other causes of hematuria and noting a history of vigorous exercise that occurred within minutes to a few days before blood became apparent in the urine.8,9 Exercise-induced hematuria, as described in people, is transient and is thought to be primarily related to bladder mucosal damage that occurs as a result of repetitive or vigorous movement of the bladder during exercise. In some cases, the source of exercise-induced hematuria may be nontraumatic and occur as a result of increased glomerular excretion of RBCs into the urine.10,11
Although uncommon, leakage of RBCs through the glomerulus can occur because of chronic congestive mechanisms resulting from disorders such as heart failure8,12 or as a consequence of ischemic and congestive mechanisms associated with exercise-induced hematuria.11
Renal hematuria can fall in an idiopathic category. Primary (idiopathic) renal hematuria is thought to result from benign vascular abnormalities that are not readily recognizable through typical diagnostic procedures.13-15
Handling patients with hematuria begins with confirming RBCs in the urine since a variety of pigmented substances can discolor the urine. Hematuria can be confused with hemoglobinuria, myoglobinuria, bilirubinuria, and pseudohematuria. Once RBCs have been confirmed, historical and physical examination findings can often localize the source of hematuria to the upper urinary tract, lower urinary tract, or genital tract. Coagulopathies must be differentiated from disease of the urogenital tract as the source of urine RBCs. After localizing the source of hematuria, you can follow a series of algorithmic steps delineating a logical diagnostic plan to identify categories of disorders that can lead to a specific cause of hematuria.
Mary Bowles, DVM, DACVIM
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