Microscopic hematuria refers to the presence of blood in the urine that is not visible to the naked eye, meaning the urine appears normal in color. It is typically discovered during a routine urinalysis and is defined by an elevated number of red blood cells (RBCs) found when the urine is examined under a microscope. While this finding can frequently be temporary and harmless, it warrants medical attention because it can also be the sole indication of a more serious, underlying health condition. Therefore, microscopic hematuria always requires professional follow-up to determine its origin.
How Microscopic Hematuria is Detected
The initial detection of blood in the urine often occurs through a quick screening method called a dipstick test, which detects the presence of hemoglobin, the protein in red blood cells. However, a positive dipstick result does not definitively confirm hematuria, as it can be caused by other substances like myoglobin or certain medications. This preliminary finding must be confirmed by a microscopic examination of the urine sample.
A medical professional centrifuges the urine sample and examines the sediment under a high-power field (HPF) microscope. Microscopic hematuria is officially defined by the American Urological Association (AUA) as the presence of three or more red blood cells per HPF. This threshold is used to distinguish a true finding from the trace amounts of red blood cells that can naturally pass through the urinary system.
Temporary and Non-Serious Causes
The appearance of red blood cells in the urine is often transient and not indicative of a chronic disease. Strenuous physical activity, particularly distance running, is a well-known cause of temporary hematuria. The exact mechanism is not fully understood but may involve mild trauma to the bladder or changes in kidney blood flow during intense exercise.
Recent infections, even a recovering viral illness or a mild urinary tract infection (UTI), can also cause temporary inflammation and bleeding. Furthermore, in women, menstruation or a recent pelvic examination can inadvertently contaminate the urine sample, leading to a false-positive finding. If a transient cause is suspected, the initial management involves repeating the urinalysis several weeks later to see if the finding has resolved.
Potential Serious Medical Conditions
When microscopic hematuria is persistent or accompanied by other symptoms, it can signal a condition within the urinary tract or kidneys that requires specific treatment. Urinary tract infections (UTIs) are the most common cause, where bacteria irritate the lining of the bladder or urethra, causing inflammation and minor bleeding. Similarly, kidney stones (urolithiasis) or bladder stones can cause bleeding as they move and scrape against the delicate lining of the urinary system.
Conditions affecting the filtering units of the kidney, known as glomeruli, can also be the source, a process termed glomerular hematuria. Examples include glomerulonephritis, where inflammation damages the kidney’s filters, allowing blood cells to leak into the urine. This type of bleeding is often suggested by the presence of protein in the urine or dysmorphic (misshapen) red blood cells.
Microscopic hematuria is a potential early warning sign for cancers of the urinary tract, including the bladder, kidney, or prostate. The risk for a malignancy is higher in older individuals, especially those with a history of smoking. Although cancer is found in a small percentage of patients with microscopic hematuria, its possibility is the primary reason for a thorough diagnostic evaluation.
What Happens After a Positive Test
Following a confirmed finding of persistent microscopic hematuria, the next step involves a risk assessment based on factors like age, smoking history, and the degree of hematuria. Patients considered low-risk may only require a repeat urinalysis in six months to monitor the situation. For those with intermediate or high-risk factors, a comprehensive workup is typically recommended to locate the source of the bleeding.
This evaluation often begins with diagnostic imaging to visualize the upper urinary tract, which includes the kidneys and ureters. A computed tomography (CT) urogram or a renal ultrasound may be used to look for masses, stones, or structural abnormalities. Evaluation of the lower urinary tract, including the bladder, is often completed by a specialist using a procedure called a cystoscopy. During a cystoscopy, a thin, lighted tube is inserted into the urethra to directly examine the lining of the bladder for any growths or lesions.