Microhematuria is the medical finding of an abnormally elevated number of red blood cells in the urine, a condition not visible without magnification. It is distinct from gross hematuria, where urine appears visibly pink, red, or cola-colored due to a larger quantity of blood. Microhematuria is a sign of an underlying issue, not a disease itself. It is frequently discovered incidentally during routine health screenings and requires investigation because it signals a breach within the urinary tract system.
The Mechanism of Detection
The initial detection of blood often occurs through a simple dipstick test, a rapid chemical screen for hemoglobin. While highly sensitive, this test can yield false-positive results from things like certain medications or dehydration. A positive dipstick result must be confirmed through a microscopic examination of a urine sample before a diagnosis of microhematuria is made.
A healthcare provider spins the urine sample in a centrifuge to create a sediment pellet. This sediment is examined under a high-power field (HPF) of a microscope. Microhematuria is officially defined by the presence of three or more red blood cells per HPF in the urine sediment. Microscopic confirmation is necessary because the dipstick test reacts to hemoglobin, which can be present even if red blood cells have lysed or if other substances are present.
Common Causes
The finding of microscopic blood is often linked to temporary or easily treatable conditions originating anywhere from the kidneys to the urethra. One of the most common causes is a urinary tract infection (UTI), where bacterial irritation and inflammation of the bladder or urethra lining cause minor bleeding. Kidney or bladder stones can also cause bleeding as the hard, crystalline deposits scrape against the inner lining of the urinary passages.
Strenuous physical activity, particularly distance running, can induce temporary microhematuria, possibly due to minor trauma to the bladder wall. Certain medications, including common blood thinners or nonsteroidal anti-inflammatory drugs (NSAIDs), can cause the bleeding. Other benign causes include minor trauma to the lower back or pelvis, a recent viral illness, or an enlarged prostate gland in older men, which causes irritation around the urethra.
Diagnostic Steps
Once microhematuria is confirmed, a doctor first rules out and treats common, reversible causes like a UTI, and then repeats the urinalysis. If the bleeding persists, a comprehensive investigation begins to identify the source of the red blood cells. The workup focuses on the entire urinary system, including the kidneys, ureters, bladder, and urethra.
Advanced imaging of the upper urinary tract (kidneys and ureters) is a primary investigation area. A computed tomography (CT) scan with contrast (CT urography) or an ultrasound is often performed to check for stones, structural abnormalities, or suspicious masses. Blood tests, such as measuring serum creatinine levels, are also used to assess kidney function.
The lower urinary tract (bladder and urethra) is often examined directly using a cystoscopy. During this outpatient procedure, a thin, flexible tube with a camera is inserted through the urethra to visually inspect the lining of the bladder for inflammation or growths. This diagnostic protocol aims to pinpoint the specific location and nature of the issue causing the microscopic bleeding.
Underlying Conditions
While many causes of microhematuria are benign, the thorough diagnostic process is necessary to exclude more serious underlying conditions. Certain kidney diseases, which affect the organ’s filtering units (the glomeruli), can present with microscopic blood. Glomerulonephritis is a condition where the tiny blood vessels that filter waste become inflamed, allowing red blood cells to leak into the urine.
The evaluation assesses the link between microhematuria and urinary tract malignancy. Cancers of the bladder, kidney, or prostate can cause microscopic bleeding, often without other noticeable symptoms in early stages. Although the overall risk of cancer is low for the general population, a complete workup is important for early detection in high-risk individuals. Identifying these conditions early, while the blood is only microscopic, improves the chances for successful treatment.