A kidney infection does not directly cause blood in your stool. These are two separate symptoms involving different organ systems, and a standard kidney infection (pyelonephritis) does not produce gastrointestinal bleeding. However, there are a few indirect connections and overlapping conditions that could explain why someone might experience both at the same time.
What Kidney Infections Actually Cause
Kidney infections cause symptoms centered on the urinary tract: fever, chills, flank pain, frequent urination, burning during urination, and often blood in the urine. That last symptom, bloody or pink-tinged urine, is the one most commonly confused with blood in the stool. If you’re noticing red in the toilet bowl, it’s worth considering whether the blood is coming from urine rather than from a bowel movement. This distinction matters because blood in urine is a well-known kidney infection symptom, while blood in stool is not.
If you’re unsure of the source, paying attention during urination versus during a bowel movement can help clarify. Blood that appears only when you urinate, or that turns toilet water pink even without a bowel movement, almost certainly originates from the urinary tract.
Why Both Symptoms Might Appear Together
Several scenarios could produce kidney-area pain alongside blood in the stool, even though one doesn’t cause the other.
Shared bacterial origins. The bacterium most often responsible for kidney infections is E. coli, which normally lives in the gut. Certain strains of E. coli, particularly the O157:H7 strain, produce a toxin that damages the lining of the small intestine and can cause bloody diarrhea. While the strains that cause urinary infections are typically different from those that cause intestinal bleeding, a person dealing with a gut infection from a harmful E. coli strain could develop both gastrointestinal bleeding and a urinary tract infection around the same time. The two problems would be related by the same bacterial family but not by one directly causing the other.
Antibiotic side effects. Antibiotics prescribed for a kidney infection can disrupt the normal balance of bacteria in the gut. This sometimes leads to diarrhea, and in more serious cases, an overgrowth of harmful bacteria in the colon that can cause inflammation and bloody stool. If blood in your stool appeared after starting antibiotics, that timing is important information for your doctor.
Sepsis complications. A kidney infection that goes untreated can allow bacteria to enter the bloodstream, a condition sometimes called blood poisoning. In severe cases progressing to septic shock, decreased blood flow to the digestive tract can erode the tissue lining of the stomach or intestines, potentially causing gastrointestinal bleeding. This is a medical emergency with obvious signs of severe illness: high fever, confusion, rapid heart rate, and a dramatic drop in blood pressure. Someone experiencing GI bleeding from sepsis would be critically ill, not just uncomfortable.
Other Conditions That Cause Both Symptoms
When flank pain and blood in stool show up together, the explanation is sometimes a single condition that affects both areas rather than a kidney infection branching into the gut. An abdominal aortic aneurysm, for example, can cause back or flank pain alongside gastrointestinal bleeding. Inflammatory bowel disease can produce bloody stool and, in some cases, kidney complications. Certain cancers in the abdominal or pelvic region can also press on or involve both the urinary and gastrointestinal tracts.
These are less common than a simple coincidence of two unrelated problems, but they underscore why the combination of flank pain and rectal bleeding deserves medical evaluation rather than assumption.
Blood in Urine vs. Blood in Stool
Because the toilet bowl collects everything, it can be genuinely difficult to tell whether blood is urinary or fecal in origin. A few practical clues help:
- Blood on toilet paper after wiping typically points to a rectal source, such as hemorrhoids or an anal fissure.
- Pink or red toilet water without a bowel movement suggests blood in the urine.
- Dark, tarry stool indicates bleeding higher in the digestive tract, like the stomach or small intestine.
- Bright red blood mixed into stool usually comes from the lower colon or rectum.
If you have a confirmed kidney infection and are noticing what looks like blood in your stool, identifying the actual source is the most important first step. Blood in urine during a kidney infection is expected and typically resolves with treatment. Blood in stool alongside a kidney infection likely has a separate cause that needs its own attention.
When the Combination Is Serious
Bloody urine with nausea and vomiting during a kidney infection warrants immediate medical care, as these can signal the infection is worsening. If left untreated, kidney infections can lead to kidney scarring, chronic kidney disease, and bacteria entering the bloodstream.
If you’re passing blood in your stool while running a high fever from a kidney infection, the concern shifts toward whether the infection has become systemic. Signs that things are escalating include confusion, rapid breathing, a heart rate that won’t slow down, and feeling significantly worse rather than gradually better on antibiotics. Rectal bleeding that is heavy, persistent, or accompanied by dizziness and weakness also needs prompt evaluation regardless of whether a kidney infection is present.