Hematuria is the medical term for blood in the urine, a finding often discovered during routine health screening. When the amount of blood is too small to be seen with the naked eye, it is called microscopic hematuria. Asymptomatic microscopic hematuria (AMH) specifically refers to the presence of red blood cells without corresponding pain, discomfort, or other urinary symptoms. This common finding often leads to an evaluation to determine the underlying source of the bleeding.
Understanding the Condition
AMH is technically defined by the presence of three or more red blood cells per high-power microscopic field (RBCs/hpf) in a properly collected urine sample. This quantification is confirmed via microscopic examination of the urine sediment, usually after an initial positive dipstick test. Because the blood loss is minute, the urine appears normal in color, differentiating it from gross hematuria, where the urine is visibly pink or red.
The “asymptomatic” designation means the patient is not experiencing symptoms like pain, difficulty urinating, or fever that would suggest an infection or stone. Since AMH is often discovered incidentally during a check-up, medical guidelines recommend a repeat test. This ensures the finding is persistent and not a temporary anomaly.
Why Blood Appears in the Urine
Blood cells can enter the urinary tract anywhere from the kidneys, which filter the blood, down through the ureters, bladder, and urethra. Identifying the source is a primary goal of the medical evaluation process.
Many cases of AMH are transient. Temporary causes include intense physical activity, dehydration, minor trauma, sexual activity, or a recent viral illness. In women, menstrual contamination of the urine sample is a frequent, non-disease-related cause of a positive test.
Common, treatable causes include urinary tract infections (UTIs) and stones in the kidney or bladder (urolithiasis). A UTI causes inflammation and irritation that leads to bleeding, which clears up after antibiotic treatment. In men, an enlarged prostate (benign prostatic hyperplasia or BPH) can cause bleeding due to increased pressure and irritation within the bladder neck.
AMH can also signal a more serious underlying issue, such as a urinary tract malignancy, including cancer of the bladder, kidney, or ureter. Up to 5% of patients with AMH are ultimately diagnosed with a malignancy, necessitating a thorough workup. Other serious causes include medical kidney diseases, such as glomerulonephritis, where damaged filtering units allow red blood cells to leak into the urine.
Steps for Determining the Source
Once AMH is confirmed, the evaluation focuses on systematically ruling out potential sources of bleeding. Initial steps involve a comprehensive physical examination, a detailed patient history, and specialized laboratory work. Blood tests are used to check kidney function by measuring serum creatinine and estimating the glomerular filtration rate.
The urine sample is analyzed further for specific markers that help localize the bleeding. The presence of protein or cellular casts alongside red blood cells may suggest the bleeding originates from the kidney’s filtering units, indicating possible medical kidney disease. Additionally, dysmorphic (abnormally shaped) cells often point toward a kidney source, while normal-looking cells suggest bleeding from the lower tract.
Imaging studies are necessary to visualize the structure of the urinary system, including the kidneys and ureters. Computed tomography (CT) urography is often the preferred method, providing detailed images to identify masses, stones, or structural abnormalities. Ultrasound is used as an alternative when radiation exposure is a concern.
Direct visualization of the lower urinary tract is performed through cystoscopy, where a thin, lighted tube examines the bladder lining. This procedure is typically recommended for patients at higher risk for malignancy, such as those over 35 or those with a history of smoking. Cystoscopy allows the physician to directly inspect the bladder for any growths or lesions that might be the source of the bleeding.
Following Up After Diagnosis
If a specific cause is identified during the workup, treatment is directed toward that condition. This may involve antibiotics for an infection, a procedure to remove kidney stones, or specialized care for a diagnosed malignancy or kidney disease.
In a significant number of cases, the evaluation finds no definitive cause for the bleeding, a situation referred to as idiopathic AMH. For these patients, immediate treatment is unnecessary, but careful, long-term monitoring is instituted. This involves periodic check-ups with repeat urinalysis, blood pressure measurements, and laboratory tests to ensure the condition does not progress.
Monitoring is generally continued for a period, often three years, to watch for the development of gross hematuria, new symptoms, or signs of kidney function decline. If repeat tests remain negative and the patient remains without symptoms, monitoring can usually be discontinued. This surveillance approach detects any delayed or slow-growing condition while avoiding unnecessary interventions.