Red blood cells (RBCs) are typically absent from urine because the kidneys filter waste products while retaining larger components, such as blood cells, in the bloodstream. A urinalysis is a common test that checks the composition of urine, providing insight into the health of the kidneys and urinary tract. The presence of RBCs suggests a breach in this filtration barrier or bleeding somewhere along the urinary pathway, from the kidneys to the urethra. This test is important for the early detection of various conditions affecting the urinary system.
How Red Blood Cells Are Measured in Urine
The initial screening for RBCs in a urine sample is often performed using a chemical dipstick test. This method relies on a reagent strip that changes color when it detects the peroxidase activity of hemoglobin. A dipstick provides a quick, qualitative result, indicating the potential presence of blood, but it cannot count the actual cells or distinguish between intact cells and free hemoglobin.
If the dipstick test is positive, a microscopic examination of the urine sediment is performed to confirm the finding and provide a quantitative count. For this definitive measurement, a sample of urine is centrifuged, and the concentrated sediment is placed under a high-power field (HPF) of a microscope. The microscopic count is the standard because it directly visualizes and enumerates the red blood cells, helping to rule out false-positive results caused by other substances.
Defining the Normal Threshold
A normal finding in a microscopic urinalysis is defined as having a very low or absent number of red blood cells. The widely accepted standard for a normal range is zero to two, or zero to three, RBCs per high-power field (HPF) in the urine sediment. Trace amounts within this range are considered an acceptable finding and not a cause for concern.
Any finding consistently above the three RBC/HPF threshold is termed hematuria, meaning blood in the urine. Hematuria is categorized into two types: gross hematuria, where the blood is visible to the naked eye, and microscopic hematuria, where the blood is only detectable under a microscope. Because laboratory methods and equipment can vary, it is important to understand the specific normal range provided by the testing facility.
Conditions That Cause Elevated RBC Counts
When the red blood cell count exceeds the normal limit, it indicates an issue somewhere along the urinary tract, which encompasses the kidneys, ureters, bladder, and urethra. One of the most frequent causes is a urinary tract infection (UTI), including bladder or kidney infections. The inflammation and irritation of the urinary tract lining caused by the bacterial infection allow blood cells to leak into the urine.
Kidney stones and bladder stones are another common cause, as these hard mineral deposits can scrape against the delicate inner lining of the urinary structures, leading to irritation and bleeding. Temporary causes can include vigorous physical activity, such as long-distance running, or recent sexual intercourse, both of which may cause minor irritation to the urinary tissues. Menstruation can also contaminate a urine sample, leading to a falsely elevated RBC count.
More serious conditions can lead to hematuria and require investigation. Kidney diseases, such as glomerulonephritis, damage the filtering units of the kidney, allowing RBCs to pass into the urine. In men, an enlarged prostate gland (benign prostatic hyperplasia or BPH) can compress the urethra and cause bleeding. Although less common, the presence of red blood cells can also be a sign of cancer of the kidney, bladder, or prostate.
What Happens After an Abnormal Finding
If a urinalysis reveals an elevated red blood cell count, the first step is often to determine if the finding is persistent or temporary. A healthcare provider will typically order a repeat urinalysis, perhaps weeks later, to confirm that the hematuria is not transient, which can happen after strenuous exercise or mild irritation. If the hematuria is confirmed, a comprehensive diagnostic workup is initiated to pinpoint the underlying cause.
The workup may include additional urine tests, such as a urine culture to check for infection or cytology to look for abnormal cells. Imaging tests are commonly ordered to visualize the internal structures of the urinary tract and kidneys, including ultrasound, CT scans, or MRI to detect tumors or kidney stones. For a more direct view of the bladder and urethra, a procedure called a cystoscopy may be performed using a flexible tube with a camera. Consulting a healthcare provider for proper diagnosis is necessary, as the treatment plan depends entirely on the specific cause of the elevated RBC count.