Erythrocytes in Urine: Causes, What It Means, and When to See a Doctor

Quick answer: Erythrocytes (red blood cells) in urine, also called hematuria, can result from urinary tract infections, kidney stones, strenuous exercise, or more serious conditions affecting the kidneys or bladder. Visible blood in urine always warrants prompt medical evaluation. Microscopic hematuria found incidentally on urinalysis requires clinical follow-up to determine the cause, as it can be benign or may indicate kidney disease or other conditions needing assessment.

What Does it Mean to Have Erythrocytes in Urine?

Erythrocytes are red blood cells. In healthy individuals, the kidneys filter blood while retaining its cellular components, meaning red blood cells should not appear in significant numbers in urine. When they do, it indicates that the filtration barrier has been compromised somewhere along the urinary tract, from the kidneys to the urethra.

Hematuria is classified in two ways. Gross hematuria is visible to the naked eye and turns urine pink, red, or brown. Microscopic hematuria is detectable only by urinalysis and is defined as three or more red blood cells per high-power field on a properly collected urine sample. Microscopic hematuria is a relatively common incidental finding and does not always reflect serious pathology, but it reliably indicates that something is worth investigating.

Common Causes of Erythrocytes in Urine

1. Urinary tract infection (UTI)

Bacterial infection of the bladder or urethra is among the most common causes of hematuria, particularly in women. Infection causes inflammation and mucosal irritation that disrupts the integrity of the urinary lining, allowing red blood cells to enter the urine. UTI-associated hematuria is typically accompanied by urgency, frequency, burning on urination, and sometimes pelvic discomfort. Hematuria from a UTI usually resolves when the infection is successfully treated.

2. Kidney stones

Calcium oxalate, uric acid, and other mineral deposits in the urinary tract can scratch or damage the delicate mucosal lining as they pass, producing hematuria. Kidney stone-associated hematuria is often accompanied by flank or lower abdominal pain, sometimes severe. Smaller stones may cause intermittent microscopic hematuria with minimal pain, while larger stones typically produce more dramatic symptoms.

3. Strenuous exercise

Exercise-induced hematuria, sometimes called "runner's hematuria," is a recognized phenomenon in distance runners and other athletes performing sustained high-impact activity. The proposed mechanism involves repeated impact of the bladder wall against itself when the bladder is not full, combined with increased renal blood flow and potential muscle breakdown products entering the circulation. This form of hematuria is typically transient and resolves within 48-72 hours of rest. Persistent hematuria after exercise should be evaluated clinically.

4. Glomerulonephritis and kidney disease

Inflammation of the glomeruli, the tiny filtering units within the kidneys, allows red blood cells to pass into urine. Glomerulonephritis can be associated with autoimmune conditions (such as IgA nephropathy, lupus nephritis, or Goodpasture syndrome) or can follow certain infections. Hematuria from glomerular sources often produces dysmorphic red blood cells (irregularly shaped cells that have been distorted by passage through the damaged glomerular membrane), which a pathologist can identify on microscopy. This distinction matters clinically because glomerular hematuria has different implications than hematuria originating lower in the urinary tract.

5. Bladder cancer

Painless gross hematuria, particularly in adults over 40 with a history of smoking, is a recognized presentation of bladder cancer and warrants prompt evaluation. Bladder cancer can also produce microscopic hematuria without other symptoms. Urinalysis-based screening tools and cystoscopy are used in clinical evaluation when bladder malignancy is suspected. Hematuria in older adults should not be attributed to benign causes without appropriate investigation to exclude more serious pathology.

6. Enlarged prostate (BPH)

Benign prostatic hyperplasia in men can cause hematuria through increased pressure and vascular changes in the bladder and prostate. BPH-associated hematuria is most common in older men and may occur alongside other lower urinary tract symptoms such as difficulty initiating urination, incomplete emptying, and frequent nighttime urination.

7. Certain medications

Anticoagulant medications (blood thinners) such as warfarin, rivaroxaban, and aspirin do not directly cause hematuria, but can cause existing minor sources of bleeding to become clinically apparent. Cyclophosphamide (a chemotherapy agent) causes a form of hemorrhagic cystitis. Non-steroidal anti-inflammatory drugs (NSAIDs) taken long-term can affect renal function in ways that may produce hematuria. If hematuria begins after starting a new medication, this should be discussed with a prescribing provider.

8. Menstrual contamination

Urinalysis collected during or shortly after menstruation may show erythrocytes as a result of contamination rather than true urinary hematuria. A clean-catch midstream specimen collected outside of the menstrual period should be used to confirm or exclude hematuria in this context.

What Does the RBC Urine Test Measure?

Urinalysis can detect erythrocytes in two ways: the dipstick test detects hemoglobin (and is positive for both intact red blood cells and free hemoglobin from lysed cells) and microscopic examination counts actual cells per high-power field. A positive dipstick result should be confirmed with microscopy when possible, as myoglobinuria (from muscle breakdown) can produce false positives on dipstick. Superpower's RBC urine test provides a direct count of urinary red blood cells.

When to See a Doctor

Gross hematuria (visible blood in urine) should always prompt same-day or urgent clinical evaluation, regardless of whether it is painful or painless. Painless gross hematuria in particular should not be assumed benign without assessment.

Microscopic hematuria found incidentally on a routine urinalysis should be followed up with your provider to determine whether repeat testing, additional blood work, or urological evaluation is indicated. Current clinical guidelines generally recommend evaluation of asymptomatic microscopic hematuria in adults, particularly in those over 35 or with risk factors for urological malignancy.

Which Blood Tests Are Relevant Alongside Urinary Hematuria?

When hematuria is found, additional blood tests help evaluate kidney function and rule out systemic causes.

• Creatinine + eGFR — Kidney filtration function; reduced in kidney disease. Included in the Baseline Panel
• BUN (blood urea nitrogen) — Kidney waste clearance; assessed alongside creatinine. Included in the Baseline Panel
• hs-CRP — Systemic inflammation; elevated in inflammatory kidney disease
• Hemoglobin + RBC — Anemia secondary to chronic blood loss or kidney disease

Superpower's Baseline Blood Panel includes creatinine, eGFR, BUN, hemoglobin, and RBC count, providing a baseline picture of kidney function and blood health alongside a comprehensive metabolic assessment.