Blood in your urine can indicate anything from a urinary tract infection to kidney stones to, in a smaller percentage of cases, cancer of the bladder or kidney. The cause depends heavily on whether the blood is visible to the naked eye, whether you have pain, your age, and your overall health history. Most causes are treatable, but blood in the urine always warrants investigation because it can be the first sign of something serious.
Visible Blood vs. Hidden Blood
There are two forms. Visible blood (called gross hematuria) turns your urine pink, red, or brownish, sometimes with clots. Hidden blood (microscopic hematuria) looks completely normal to you but shows up on a lab test, defined as three or more red blood cells per high-power field under a microscope. Both forms share many of the same causes, but visible blood is more likely to prompt a thorough workup because it’s associated with a higher chance of a serious underlying condition.
Common Causes
Urinary tract infections are one of the most frequent explanations, especially in women. The infection irritates the bladder lining, and you’ll typically notice burning during urination, urgency, and sometimes cloudy or strong-smelling urine alongside the blood. Kidney infections can do the same, often with flank pain and fever.
Kidney stones are another major cause. As a stone moves through the urinary tract, it scrapes the lining and produces blood that can be visible or microscopic. The pain from a kidney stone is usually intense and comes in waves, radiating from the back or side down toward the groin. If you have blood in your urine with that kind of pain, a stone is high on the list.
In men over 50, an enlarged prostate is a common culprit. As the prostate grows, the blood vessels within it also enlarge and become fragile. These vessels can rupture and release blood into the urine, sometimes without any pain at all. A study of 166 men evaluated in a hematuria clinic confirmed that vascular enlargement of the prostate is a well-established source of bleeding.
When Blood in Urine Points to Cancer
This is the concern most people searching this topic are really asking about. Painless visible blood in the urine is the hallmark warning sign of bladder cancer and, less commonly, kidney cancer. Among patients who show up with visible blood in their urine, the prevalence of a urological malignancy typically ranges between 3% and 6%, though some studies have reported rates as high as 19% to 24% depending on the population studied. The risk increases with age, a history of smoking, and exposure to certain industrial chemicals.
The key word here is “painless.” If you see blood in your urine and have no burning, no pain, and no obvious explanation like a recent intense workout, that’s exactly the scenario that doctors take most seriously. It doesn’t mean cancer is likely in absolute terms, but it means the possibility needs to be ruled out with imaging and sometimes a direct look inside the bladder using a small camera.
Kidney Filter Problems
Sometimes the blood originates not from the bladder or urinary tract but from the kidneys’ filtering units themselves. Conditions that inflame these filters push red blood cells into the urine, often alongside protein. Doctors can distinguish this source because the red blood cells look misshapen under the microscope, or they appear clumped together in tiny casts. These findings point toward conditions like IgA nephropathy (a common kidney inflammation), Alport syndrome (a genetic condition), or thin basement membrane disease, which is usually mild and may only cause small amounts of blood without significant protein loss.
If your doctor finds both blood and protein in your urine, especially with high blood pressure or swelling in your legs, the investigation shifts toward kidney-specific causes rather than bladder or prostate issues.
Exercise, Foods, and Medications
Intense physical activity can cause temporary blood in the urine. Long-distance running is the classic example, sometimes called “runner’s bladder,” where repeated impact causes minor trauma to the bladder wall. Research shows exercise-induced blood in the urine occurs in 20% to 100% of people depending on the activity’s intensity, and it typically resolves within 24 to 48 hours. If the blood clears on its own after a hard workout and doesn’t return, it’s generally not concerning.
Before assuming you have blood in your urine, consider what you’ve eaten. Beets are the most well-known offender. The pigments in beetroot can turn urine pink or red while containing zero actual blood cells. Spinach, rhubarb, and other foods high in oxalic acid can increase the absorption of these pigments and make the color change more dramatic. A simple urine test will confirm whether red blood cells are actually present.
Several medications can also cause or worsen hematuria. Blood thinners like warfarin, apixaban, and dabigatran don’t cause bleeding on their own in a healthy urinary tract, but they can make an existing small bleed much more visible. Other drugs known to cause hematuria as a side effect include certain antibiotics (penicillin-type and aminoglycoside antibiotics), some antidepressants, diuretics, pain relievers, oral contraceptives, and seizure medications. If you started a new medication and noticed a color change, mention it to your doctor, but don’t assume the medication is the only explanation. Blood thinners in particular can unmask an underlying problem that needs its own evaluation.
What Happens During Evaluation
Your doctor will start with a urine test to confirm that blood is actually present and check for signs of infection or protein. From there, the workup depends on your risk level. Younger patients with a clear trigger (a UTI, vigorous exercise, a recent kidney stone) may need nothing more than a repeat urine test after the trigger resolves.
For older adults, smokers, or anyone with painless visible blood, the evaluation is more involved. A CT scan of the urinary tract can reveal stones, tumors, or structural abnormalities in the kidneys. A cystoscopy, where a thin camera is passed through the urethra into the bladder, lets the doctor inspect the bladder lining directly. Current guidelines from the American Urological Association use a risk stratification system that weighs your age, smoking history, the amount of blood detected, and other factors to determine how aggressively to investigate.
Symptoms That Need Emergency Attention
Most blood in the urine can be evaluated by your regular doctor within a reasonable timeframe. But certain combinations of symptoms suggest something more urgent: passing multiple blood clots, urine so dark with blood that you can’t see through it, an inability to urinate at all, or blood accompanied by fever, chills, nausea, or vomiting. These can signal a urinary tract obstruction or a severe infection, both of which need emergency treatment to prevent kidney damage or sepsis.