Blood in urine has a wide range of causes, from urinary tract infections and kidney stones to vigorous exercise and, less commonly, cancer. Most cases turn out to be benign, but visible blood in your urine always warrants a medical evaluation because the cause isn’t something you can determine on your own. Even blood that’s only detectable under a microscope can signal a condition that needs attention.
Visible vs. Invisible Blood
Blood in urine falls into two categories. Gross hematuria is blood you can see: your urine looks pink, red, or cola-colored. It only takes a tiny amount of blood to change the color noticeably. Microscopic hematuria, on the other hand, is invisible to the naked eye and only shows up during a lab test. The clinical threshold is three or more red blood cells per high-power field on a urine sample examined under a microscope. Many people discover microscopic hematuria during a routine physical and are surprised, since they had no symptoms at all.
The distinction matters because visible blood carries a higher statistical risk of a serious underlying cause. In a large pooled analysis, about 17% of people with visible blood in their urine were eventually diagnosed with bladder cancer, compared to roughly 3.3% of those with microscopic blood only. That still means the vast majority of people in both groups had a non-cancerous explanation, but it’s the reason doctors take visible hematuria especially seriously.
The Most Common Causes
Urinary Tract Infections
UTIs are one of the most frequent reasons blood shows up in urine, particularly in women and children. Bacteria enter through the urethra and inflame the bladder lining, which can bleed. You’ll typically also notice burning during urination, a strong urge to go frequently, and cloudy or strong-smelling urine. Blood caused by a UTI usually clears once the infection is treated. In roughly 4 to 22% of UTI cases, hematuria is present.
Kidney and Bladder Stones
Minerals in your urine can crystallize into hard deposits in the kidneys or bladder. These stones scrape the lining of the urinary tract as they move, causing bleeding that can range from microscopic to clearly visible. The hallmark symptom is intense, wave-like pain in your side or lower back, often accompanied by nausea. Small stones sometimes pass on their own over days to weeks; larger ones may need medical intervention.
Enlarged Prostate
In middle-aged and older men, the prostate gland (which sits just below the bladder and wraps around the upper urethra) naturally enlarges with age. This growth can compress the urethra and irritate nearby tissue, leading to blood in the urine along with a weak stream, difficulty starting urination, or feeling like the bladder never fully empties.
Kidney Disease
A group of conditions called glomerulonephritis inflame the tiny filters inside the kidneys. When these filters are damaged, blood and protein leak into the urine. Clues that the blood is coming from the kidneys rather than the bladder or urethra include protein in the urine, foamy or frothy urine, swelling in the legs or face, and high blood pressure. Under a microscope, red blood cells that have passed through damaged kidney filters look misshapen, which helps doctors pinpoint the source.
Cancer
Bladder, kidney, and prostate cancers can all cause blood in urine, and the risk increases after age 50. Visible blood in urine that’s painless is often the first and only warning sign of bladder cancer. The overall risk is relatively low for any individual person, but it rises with smoking history, older age, male sex, and exposure to certain industrial chemicals. A pooled analysis found kidney cancer in about 2% of people with visible hematuria and 0.6% of those with microscopic blood.
Less Common and Surprising Causes
Intense exercise can trigger blood in the urine, sometimes called “runner’s hematuria.” During prolonged running with an empty bladder, the bladder’s back wall repeatedly slaps against the bladder neck with each stride, creating small bruise-like injuries on the lining. The resulting blood typically clears within a few days of rest without any treatment. It can also happen after contact sports like football.
Certain medications are linked to hematuria. Blood thinners can unmask bleeding from an otherwise minor source in the urinary tract. Some antibiotics, including penicillin, and certain chemotherapy drugs are also associated with bloody urine.
Inherited conditions play a role in some cases. Sickle cell disease can cause blood in the urine because abnormally shaped red blood cells damage small blood vessels in the kidneys. Alport syndrome, a genetic condition that weakens the kidney’s filtering membranes, is another inherited cause.
A blow to the kidney from an accident, fall, or contact sport can also produce bleeding that shows up in urine, sometimes dramatically.
When It’s Not Actually Blood
Before assuming the worst, consider whether something you ate or took could be coloring your urine. Beets, blackberries, and rhubarb can all turn urine pink or red. So can certain medications: rifampin (a tuberculosis drug) turns urine reddish-orange, phenazopyridine (a common over-the-counter bladder pain reliever) turns it bright orange-red, and laxatives containing senna can cause a similar color shift. None of these involve actual blood. A simple urine test can confirm whether red blood cells are present or the color has another explanation.
How Doctors Figure Out the Cause
The evaluation depends on your age, symptoms, and risk factors. A urine sample examined under a microscope is always the starting point. If infection is suspected, a urine culture identifies the specific bacteria involved.
For people at intermediate risk of a serious cause, current guidelines recommend an ultrasound of the kidneys combined with cystoscopy, a procedure where a thin camera is passed through the urethra to visually inspect the bladder lining. It sounds uncomfortable, and there is some brief discomfort, but it typically takes only a few minutes and is done in an office setting.
For higher-risk patients (older age, smoking history, significant amount of blood), a CT scan with contrast dye provides detailed images of the kidneys, ureters, and bladder. If contrast dye or CT isn’t an option due to allergies or kidney problems, MRI or other imaging alternatives are used instead.
When doctors suspect the blood originates from the kidneys themselves rather than the bladder or ureters, the microscopic appearance of the red blood cells offers a strong clue. Misshapen cells and protein in the urine point toward kidney disease, while normal-looking red blood cells without protein suggest the bleeding is coming from somewhere lower in the urinary tract. This distinction often determines whether you’re referred to a urologist or a kidney specialist.
Symptoms That Need Prompt Attention
Any visible blood in your urine warrants a call to your doctor, even if it happens once and then stops. Certain combinations of symptoms raise the urgency:
- Blood clots in urine with difficulty urinating, which can signal a blockage
- Painless visible blood in anyone over 50, because painless hematuria is the classic presentation of bladder cancer
- Fever and flank pain alongside bloody urine, suggesting a kidney infection that can worsen quickly
- Unexplained weight loss combined with blood in urine, a pattern associated with malignancy
- Blood in urine after an injury to the back or abdomen, which may indicate kidney damage
Microscopic hematuria found incidentally on a routine test is less urgent but still worth following up on, especially if it persists on repeat testing. In many cases, no serious cause is found, but the evaluation itself is what provides that reassurance.