Asymptomatic microscopic hematuria (AMH) is a common finding discovered incidentally during routine health screening. This condition is defined by the presence of red blood cells in the urine that are not visible to the naked eye. While often benign, AMH can sometimes indicate a more serious underlying condition within the urinary tract. Therefore, an evaluation is necessary to determine the source of the bleeding and rule out potential malignancies or kidney diseases.
Defining Asymptomatic Microscopic Hematuria
The diagnosis of microscopic hematuria relies on a precise measurement of red blood cells (RBCs) found in a properly collected urine sample. The standard medical definition requires finding three or more RBCs per high-powered field (\(\ge\) 3 RBCs/HPF) upon microscopic examination of the urine sediment. A simple urine dipstick test may indicate blood, but this is not sufficient for an AMH diagnosis because dipsticks can produce false-positive results from other substances, like myoglobin. Therefore, a positive dipstick must be confirmed by a full urinalysis using a microscope.
The patient is not experiencing pain, burning, or visible blood in the urine, which might suggest an immediate cause like a urinary tract infection or kidney stone. While older guidelines sometimes required two or three positive samples, current evidence suggests that a single finding of \(\ge\) 3 RBCs/HPF is sufficient to initiate a workup. This is due to the intermittent nature of hematuria, even when caused by a serious condition. The correct collection of a clean-catch midstream sample is also required to prevent contamination.
Potential Sources and Etiology
The source of the blood can generally be divided into two categories: glomerular (originating from the kidney’s filtering unit) or non-glomerular (originating from the collecting system, ureters, bladder, or urethra). Glomerular causes often point toward medical conditions of the kidney, such as IgA nephropathy or thin basement membrane disease. These conditions are often suggested by additional findings, such as misshapen (dysmorphic) red blood cells, cellular casts, or protein in the urine.
Non-glomerular causes are far more common and represent a diverse range of conditions. Transient and benign causes include strenuous exercise, recent viral illnesses, mild dehydration, and recent urological procedures. For women, menstruation must also be excluded as a source of contamination. Once these temporary causes are ruled out, the persistence of AMH warrants further investigation for underlying structural issues.
More persistent non-glomerular causes involve structural abnormalities or chronic conditions within the urinary tract. These include kidney stones (urolithiasis), benign prostatic hyperplasia (BPH) in men, and urinary tract infections. Malignancy is a primary concern, as up to five percent of patients with AMH are found to have urinary tract cancer (e.g., bladder or kidney cancer). Risk factors that increase the likelihood of malignancy include age over 35 years, a history of smoking, exposure to certain occupational chemicals, and prior pelvic radiation.
The Standard Diagnostic Protocol
Once AMH is confirmed and transient causes are excluded, a structured diagnostic protocol is followed to identify the source of the bleeding. The initial steps involve ruling out infection and assessing kidney function. A urine culture checks for a urinary tract infection (UTI), and if positive, the hematuria is re-tested after antibiotic treatment. Blood tests measure serum creatinine and estimated glomerular filtration rate (eGFR) to evaluate overall kidney health.
Imaging studies are a core component of the workup, primarily to examine the upper urinary tract (kidneys and ureters). The preferred imaging method is often a multiphasic computed tomography (CT) urogram, which provides detailed pictures of the urinary system and can identify masses, strictures, or stones. In some cases, a renal ultrasound may be used as an alternative to avoid radiation exposure.
The lower urinary tract (bladder and urethra) is evaluated using direct visualization through cystoscopy. During this procedure, a thin, lighted tube is inserted into the bladder, allowing the physician to inspect the lining for tumors, lesions, or other abnormalities like an enlarged prostate. Cystoscopy is generally recommended for patients at higher risk for malignancy, such as those over 35 or those with a history of smoking. Urine cytology, which screens for cancerous cells, is no longer a standard part of the routine workup unless the patient has specific risk factors.
Managing Findings and Long-Term Surveillance
The management plan for AMH depends on the outcome of the initial diagnostic workup. If the workup identifies a specific cause, such as a kidney stone or a urinary tract infection, treatment is directed at resolving that underlying issue. For example, a stone may be removed or managed, or a bladder mass may be biopsied and treated. Afterward, a repeat urinalysis is performed to ensure the microscopic hematuria has resolved.
When the complete workup, including imaging and cystoscopy, yields no identifiable cause for the bleeding, the patient is considered to have isolated AMH. The risk of a missed malignancy in this scenario is low. For these individuals, the focus shifts to a long-term surveillance plan rather than immediate further testing. Annual monitoring is recommended, which includes a repeat urinalysis, blood pressure checks, and assessment of kidney function.
This extended follow-up is designed to catch any disease that was too small to detect during the initial evaluation. If the hematuria resolves after a few years of monitoring, the patient may be released from further specialized care. Patients are advised to seek immediate re-evaluation if they develop new symptoms, such as flank pain, visible blood in the urine (gross hematuria), or significant changes in voiding habits. If the AMH persists after several years, a full repeat evaluation may be considered, though the yield of new malignancies is minimal.