The presence of blood in the urine is medically termed hematuria. When blood is present in a large enough quantity to change the color of the urine to pink, red, or cola-colored, it is called gross hematuria. If blood cells from the urinary tract are present in amounts too small to be seen with the naked eye, it is known as occult blood in urine, or microscopic hematuria. This condition is often discovered incidentally during routine health screenings. While any finding of blood warrants investigation, microscopic hematuria frequently stems from non-serious, temporary causes.
Understanding Occult Blood in Urine
Occult blood means the red blood cells (RBCs) are only detectable using laboratory methods, not by visual inspection of the urine. The standard initial screen is a chemical dipstick test performed on a urine sample. This test detects the peroxidase activity of hemoglobin, the oxygen-carrying protein found within red blood cells.
A positive dipstick result is a preliminary indicator, but it does not confirm the presence of actual RBCs. The test can sometimes yield a false positive result, reacting to substances like myoglobin, which is released into the bloodstream after severe muscle injury, or certain cleaning chemicals.
For a definitive diagnosis, the sample must undergo a microscopic examination. Microscopic hematuria is confirmed when the count reveals three or more red blood cells per high-powered field (RBC/HPF) in the centrifuged urine sediment. This established threshold differentiates a clinically significant finding requiring further evaluation from incidental red blood cells that may naturally pass into the urine.
The Most Common Causes of Microscopic Hematuria
The causes of microscopic hematuria range widely from temporary events to more complex conditions involving the urinary system.
Temporary Causes
Temporary causes often do not require extensive follow-up once the factor is identified and resolved. Common temporary findings include strenuous exercise, particularly long-distance running, which can cause minor trauma to the bladder lining. Dehydration can also concentrate the urine, leading to irritation and a positive test result. Additionally, in women, menstrual contamination of the urine sample is a frequent cause.
Urinary Tract Conditions
Conditions within the urinary tract require more focused investigation. Urinary tract infections (UTIs) are the most common cause, as inflammation of the bladder or urethra lining can lead to bleeding. Kidney stones cause bleeding when their rough edges scrape the delicate tissues of the urinary tract as they pass or move. In men over 50, benign prostatic hyperplasia (BPH), or an enlarged prostate, is a frequent source of bleeding because the vascular tissue of the enlarged gland is prone to rupture. Diseases affecting the kidney’s filtering units, known as glomerular diseases (e.g., glomerulonephritis), can also cause microscopic hematuria. These conditions are sometimes indicated by the presence of red blood cell casts or dysmorphic cells in the urine. Though less common, persistent occult blood necessitates ruling out malignancies in the urinary tract, including bladder, kidney, and prostate cancer. Bleeding from these sources is often silent and can be an early warning sign, which is why a structured diagnostic evaluation is necessary for confirmed cases.
Diagnostic Steps Following a Positive Result
Following an initial positive dipstick test, the first step is to confirm the finding with a microscopic urinalysis. If the microscopic analysis confirms three or more RBCs/HPF, the investigation attempts to rule out the most common temporary causes. This often involves repeating the test after a period of abstinence from intense physical activity or ensuring the sample was not contaminated by menstruation. If a urinary tract infection is suspected, a urine culture is performed to identify the bacteria and guide antibiotic treatment. After successful treatment, the urinalysis is repeated to confirm the hematuria has resolved.
Persistent microscopic hematuria in the absence of a benign cause triggers a structured diagnostic pathway to evaluate the entire urinary system. This evaluation involves laboratory blood tests, such as serum creatinine, to assess overall kidney function. Imaging studies are commonly ordered to visualize the kidneys and ureters, with specialized tests like CT urography often preferred for a detailed view of the upper urinary tract.
For high-risk patients or those with persistent hematuria, a referral to a urologist is common. The urologist may perform a cystoscopy, a procedure where a thin, lighted camera is inserted into the urethra and bladder. This allows for a direct visual inspection of the bladder lining to identify subtle sources of bleeding.