Microscopic hematuria, or blood in the urine, is a common finding often discovered during routine testing. Hematuria means the presence of red blood cells (RBCs) in the urine, but the microscopic type is invisible to the naked eye, meaning the urine appears normal in color. While often harmless and temporary, it is never considered definitively “normal” until a medical professional has ruled out potentially serious underlying conditions. The presence of blood signals a need for investigation to identify the source of the bleeding.
What Microscopic Hematuria Means
Microscopic hematuria is defined by the presence of a specific number of red blood cells when a urine sample is analyzed under a microscope. The American Urological Association defines the threshold as three or more red blood cells per high-power field (RBC/HPF) in a properly collected urine specimen. This finding is typically discovered accidentally during a routine urinalysis or a dipstick test. A positive dipstick result needs confirmation with a microscopic examination, since the dipstick can sometimes react to other substances like myoglobin or hemoglobin that are not intact red blood cells.
The crucial distinction is between microscopic hematuria and gross hematuria, where the urine is visibly pink, red, or tea-colored. Gross hematuria is associated with a significantly higher risk of malignancy (10% to 20%), compared to the 2.6% to 4% risk associated with microscopic hematuria. Therefore, a finding of microscopic hematuria initiates a diagnostic process to determine the source of the bleeding within the urinary tract, including the kidneys, ureters, bladder, and urethra.
Temporary and Benign Explanations
Many causes of microscopic hematuria are temporary and resolve without treatment. One common benign cause is recent strenuous physical activity, often seen in long-distance runners. This exercise-induced hematuria is thought to be related to trauma to the bladder or increased blood flow to the kidneys during intense exertion.
Infections of the urinary tract, including UTIs or viral illnesses, are frequent causes of microscopic hematuria. The inflammation irritates the lining of the urinary tract, leading to a small amount of bleeding. The hematuria is expected to clear up after the infection is successfully treated with antibiotics.
Other temporary factors include contamination from menstruation, which can lead to a false-positive result. Certain medications, such as blood thinners (anticoagulants) or NSAIDs, can also be associated with hematuria, sometimes by unmasking an underlying condition. For men, an enlarged prostate (BPH) is a common non-cancerous cause, particularly over the age of 50.
Underlying Conditions Requiring Investigation
Microscopic hematuria must be investigated because it can be an early sign of more serious, chronic conditions. One significant category is urolithiasis, which involves kidney, ureteral, or bladder stones. These crystallized particles can scrape the delicate lining of the urinary tract as they pass, causing bleeding that may only be visible microscopically.
Conditions affecting the glomeruli, the filtering units of the kidney, represent another area of concern. Glomerulonephritis, often caused by diseases like IgA nephropathy or lupus nephritis, results in blood leaking into the urine due to damage to the kidney’s filtration barrier. The presence of abnormally shaped red blood cells or red blood cell casts, along with protein in the urine, often suggests a glomerular source of bleeding.
Most concerning is the possibility of urinary tract cancers, including those of the bladder, kidney, or prostate. Although this occurs in a small percentage of cases (typically less than 5%), a comprehensive workup is mandatory. The risk for malignancy is increased in patients over 40, those with a history of smoking, and individuals with occupational exposure to certain chemicals.
The Diagnostic Path Forward
Once microscopic hematuria is confirmed, a medical workup is initiated to determine the source of the bleeding. The first step involves a repeat urinalysis to confirm the persistence of the hematuria after ruling out temporary causes like infection or exercise. If blood cells are still present, the investigation proceeds using imaging and direct visual inspection.
Imaging the Upper Tract
Imaging tests evaluate the upper urinary tract, which includes the kidneys and ureters. A renal ultrasound is often the initial, non-invasive method used to check for kidney stones, masses, or structural abnormalities. For a more detailed view, especially in patients with risk factors for malignancy, a multiphasic computed tomography (CT) urogram may be used to visualize the entire urinary collecting system.
Evaluating the Lower Tract
The lower urinary tract, particularly the bladder and urethra, is evaluated using a procedure called cystoscopy. During this procedure, a urologist inserts a thin tube with a camera through the urethra into the bladder to inspect the lining for tumors, inflammation, or other lesions.
Patients whose test results suggest a kidney-related issue, such as those with significant protein in their urine or impaired kidney function, are often referred to a nephrologist for specialized care. Those whose symptoms point to the bladder or ureters are managed by a urologist.