Microscopic slide preparation in patients without significant urologic symptoms, microscopic hematuria is occasionally detected on routine urinalysis. At present, routine screening of all adults for microscopic hematuria with dipstick testing is not recommended because of the intermittent occurrence of this finding and the low incidence of significant associated urologic disease. However, once asymptomatic microscopic hematuria is discovered, its cause should be investigated with a thorough medical history including a review of current medications and a focused physical examination. Laboratory and imaging studies, such as intravenous pyelography, renal ultrasonography or retrograde pyelography, may be required to determine the degree and location of the associated disease process. Cystourethroscopy is performed to complete the evaluation of the lower urinary tract. Microscopic hematuria associated with anticoagulation therapy is frequently precipitated by significant urologic pathology and therefore requires prompt evaluation. Microscopic hematuria is defined as the excretion of more than three red blood cells per high-power field in a centrifuged urine specimen. Because the degree of hematuria bears no relation to the seriousness of the underlying cause, hematuria should be considered a symptom of serious disease until proved otherwise. The widespread use of dipstick urinalysis in clinical practice and health screening has resulted in increased recognition of microscopic hematuria and has raised concerns about the appropriate diagnostic investigation.
The prevalence of asymptomatic microscopic hematuria in adult men and postmenopausal women has been reported to range from ten percent to as high as twenty percent. Routine screening of all adults for microscopic hematuria with dipstick testing is not currently recommended because of the intermittent occurrence of this finding and the low incidence of significant associated urologic disease. Several factors can influence the microscopic detection of erythrocytes in urine. Procurement of a urine sample using a catheter may cause urethral trauma that result in variable degrees of hematuria. A clean catch midstream urine specimen should be obtained using aseptic technique to avoid contamination from the external genitalia. The first urine in the morning is typically the best specimen because erythrocytes are heat preserved in acidic and concentrated urine. A prolonged delay from specimen collection to analysis can result in a false test interpretation. When a urine specimen cannot be examined within one hour of collection, it should be refrigerated to prevent overgrowth of bacteria, changes in urinary pH and disintegration of red and white cell casts. These conditions may occur if the specimen remains at room temperature for a long period. Standardization of the analysis procedure is also essential to achieve an accurate result. Centrifugation is typically performed on a fixed volume of urine for five minutes at 3,000 rotations per minute, after which the supernatant is poured off and the remaining sediment is resuspended in the centrifuge tube by gently tapping the bottom of the tube. A pipette is used to sample the residual fluid and transfer it to a glass slide; a coverslip is applied to the slide for the microscopic evaluation. The specimen is examined under high magnification to determine cell type and distinct morphologic features. Results are recorded as the number of red blood cells per high power field. On phase contrast microscopy, erythrocytes may display morphologic features that are helpful in differentiating glomerular and nonglomerular causes of microscopic hematuria.. Dysmorphic erythrocytes are characterized by an irregular outer cell membrane and suggest hematuria of glomerular origin. Red blood cell casts are also associated with a glomerular cause of hematuria. Acanthocytes, which are ring formed erythrocytes with one or more membrane protrusions of variable size and shape, may represent an early form of dysmorphic erythrocytes and are a marker for hematuria of glomerular origin. Erythrocytes of uniform character are classified as isomorphic and suggest hematuria of lower urinary tract origin. Microscopic clots of clumped erythrocytes in urine are also suggestive of lower urinary tract bleeding. The presence of both dysmorphic and isomorphic erythrocytes in urine represents a mixed morphologic pattern of nonspecific origin The initial laboratory studies are determined by pertinent information obtained from the medical history and physical examination. Formal urinalysis is performed to document the degree of hematuria, determine the morphologic features of erythrocytes and evaluate urinary crystals and casts. If pyuria or bacteriuria is present, a urine culture with sensitivity testing should be obtained to rule out infectious urinary tract pathogens. Screening laboratory tests typically consist of coagulation studies, a complete blood count, serum chemistries and serologic studies for glomerular causes of hematuria as directed by the medical history. Further urologic evaluation is warranted if more than three red blood cells per high power field are found on at least two of three properly collected urine specimens or if high grade microscopic hematuria with more than 100 red blood cells per high power field is found on a single urinalysis. The only exceptions are children with persistent microscopic hematuria without proteinuria, in whom the most likely diagnoses include thin glomerular basement membrane nephropathy, idiopathic hypercalciuria, IgA nephropathy and Alport syndrome.
