Bloody Diarrhea: Causes, Warning Signs, Diagnosis, and Treatment

Bloody diarrhea is loose or mushy stool in which blood is visible. It is important to distinguish it from other types of intestinal bleeding: bloody diarrhea does not include isolated streaks of blood on the surface of formed stool, as occurs with an anal fissure, nor does it include black, tarry stool, which is more often associated with digested blood from the upper gastrointestinal tract. [1]

This symptom is important because it often indicates not just accelerated bowel movements, but inflammation, damage to the mucosa, or even an infectious lesion of the colon. Among the most common acute causes, doctors primarily consider bacterial intestinal infections, while among non-infectious ones, they consider inflammatory bowel disease and ischemic colitis. [2]

Bloody diarrhea is not a separate diagnosis. It is a clinical sign that can indicate a wide range of conditions, from shigellosis and infection with Shiga toxin-producing Escherichia coli to ulcerative colitis, a severe allergic proctocolitis-like reaction in infants, or intestinal ischemia in the elderly. This is why the same complaint in a child, a young adult, and an elderly patient carries different diagnostic weight. [3]

Another cause for concern is the risk of complications. The US Centers for Disease Control and Prevention emphasizes that severe foodborne infections can be accompanied by dehydration, kidney damage, and hemolytic uremic syndrome, while Shiga toxin-producing E. coli can lead to acute renal failure, especially in children. [4]

Therefore, the practical approach to bloody diarrhea is always more rigorous than to regular watery diarrhea. The National Institute of Diabetes and Digestive and Kidney Diseases (US) and the US Centers for Disease Control and Prevention (CDC) clearly state that blood in the stool is a sign that requires a medical evaluation, and self-treatment with antidiarrheal medications may be unsafe. [5]

Sign	How to understand it correctly
Loose stools with visible blood	Real bloody diarrhea
A streak of blood on the surface of a hard stool	More often than not, it is not bloody diarrhea, but local anal bleeding.
Black tarry stool	Most often a sign of bleeding from the upper gastrointestinal tract
Bloody diarrhea with pain and fever	Often requires urgent search for infection or severe inflammation
Bloody diarrhea with decreased urine and severe weakness	Severe dehydration and hemolytic uremic syndrome must be excluded.

Sources for the table: [6]

The main causes of bloody diarrhea

In acute situations, infectious causes are the most significant. Guidelines from the Infectious Diseases Society of the United States indicate that in adults and children, bloody diarrhea is often associated with inflammatory or invasive enteric pathogens, among which the most frequently identified are Salmonella, Campylobacter, Shigella, Yersinia, and Shiga toxin-producing E. coli. However, blood in the stool is not considered an expected manifestation of Clostridioides difficile infection. [7]

Shiga toxin-producing Escherichia coli occupies a special place. A modern review from 2025 describes a typical picture: stool may initially be non-bloody, then become bloody within 48-72 hours; especially dangerous is the possibility of developing hemolytic uremic syndrome with microangiopathic hemolytic anemia, thrombocytopenia, and kidney damage within 1-2 weeks. The greatest risk of severe progression is noted in children under 10 years of age and in the elderly. [8]

Among chronic and recurrent causes, ulcerative colitis is a significant one. The National Institute of Diabetes and Digestive and Kidney Diseases (USA) notes that the symptoms of ulcerative colitis vary in severity but include bloody diarrhea, and in severe and fulminant cases, the number of bloody stools can exceed 10 per day. Therefore, recurrent bloody diarrhea without obvious infection requires not only a stool culture but also a gastrointestinal evaluation for inflammatory bowel disease. [9]

Another important non-infectious cause is ischemic colitis. The Mayo Clinic notes that reduced blood flow to the colon can cause cramping abdominal pain, urgency, and bright red or dark purple blood in the stool, sometimes even blood without a significant amount of stool. This is especially important in older people and in patients with vascular risk factors. [10]

In children, the differential diagnosis is broader than in adults. Children's Health Ireland guidelines indicate that in infants, non-immunoglobulin-E-mediated food allergy, intestinal ischemia, intussusception, volvulus, and inflammatory bowel disease should be considered as causes, while in children over 1 year of age, infection and inflammatory bowel disease remain the leading causes. However, in acute presentations with fever and abdominal pain, infectious enterocolitis is still the primary suspect. [11]

Cause	What is typical for her?
Salmonella, Shigella, Campylobacter, Yersinia	Acute abdominal pain, inflammatory infectious diarrhea, sometimes fever
Shiga toxin-producing Escherichia coli	Initially, there may be watery stools, then blood; risk of hemolytic uremic syndrome
Ulcerative colitis	Recurring or chronic diarrhea with blood, pain, and urgency
Ischemic colitis	Sudden abdominal pain, bloody stools, more common in the elderly
Food allergic proctocolitis-like reaction in infants	Bloody and mucous liquid stools in an apparently healthy infant
Intussusception, volvulus, severe intestinal distress in children	Bloody diarrhea along with acute abdomen

Sources for the table: [12]

When urgent help is needed

The mere presence of blood in loose stools is a serious reason to seek medical attention immediately. The Centers for Disease Control and Prevention (CDC) classifies severe symptoms as bloody diarrhea, diarrhea lasting longer than 3 days, high fever, repeated vomiting, and signs of dehydration such as infrequent urination, dry mouth, and dizziness upon standing. [13]

Particularly dangerous is the combination of blood in the stool with severe abdominal pain, fever, and a general severe condition. The Infectious Diseases Society of America guidelines emphasize that blood in the stool, along with fever, severe abdominal pain, or signs of sepsis, requires testing for intestinal bacterial pathogens, the results of which can impact treatment and the urgency of the approach. [14]

Infection with Shiga toxin-producing E. coli poses a distinct risk, with hemolytic uremic syndrome becoming a distinct threat. The U.S. Centers for Disease Control and Prevention advises seeking immediate medical attention if bloody diarrhea is followed by decreased urine output, blood in the urine, marked pallor, unusual drowsiness, weakness, bruising, or a fine-point rash. This is no longer simply an intestinal symptom, but a possible sign of a systemic disorder. [15]

In children, the threshold for concern is even lower. The Irish Paediatric Guidelines recommend immediate reassessment if bloody diarrhea is accompanied by severe abdominal pain, no urine output for 12 hours or more, irritability, swelling of the face or ankles, bleeding from the nose or mouth, blood in the urine, bruising, or a rash. This is done specifically to detect dehydration, hemolytic uremic syndrome, and severe intestinal disease. [16]

There are also situations where bloody diarrhea may indicate not an infection, but a vascular accident. The Mayo Clinic recommends going to the emergency room if you experience abdominal pain so severe that you cannot sit still, as this can occur with severe ischemic bowel disease and other conditions requiring immediate intervention. [17]

An alarming sign	Why is it dangerous?
Bloody diarrhea lasting more than 3 days	Higher risk of severe infection, dehydration and non-infectious pathology
High fever and severe abdominal pain	Invasive infection or severe inflammation is possible.
Frequent vomiting and inability to drink	Dehydration is rapidly increasing
Infrequent urination, dryness, dizziness	Signs of fluid deficiency
Paleness, bruising, little urine after bloody diarrhea	It is urgent to rule out hemolytic uremic syndrome.
Very severe pain in the abdomen	Ischemic colitis or other acute surgical process is possible.

Sources for the table: [18]

How is a patient with bloody diarrhea examined?

The examination begins not with a colonoscopy, but with an assessment of the severity of the condition. The doctor determines the duration of symptoms, bowel frequency, fever, recent travel, suspicious food intake, contact with sick people, antibiotic use, chronic intestinal diseases, immunodeficiency, and signs of dehydration. This stage alone helps determine whether the condition is more likely to be infectious colitis, inflammatory bowel disease, or ischemic process. [19]

In acute bloody diarrhea, stool testing is key. The Infectious Diseases Society of America guidelines recommend stool testing for Salmonella, Shigella, Campylobacter, Yersinia, Clostridioides difficile, and Shiga toxin-producing E. coli in people with diarrhea accompanied by fever, bloody or mucous stools, severe abdominal pain, or signs of sepsis. For Shiga toxin-producing E. coli, either an O157 culture, a Shiga toxin test, or genetic methods are important. [20]

In addition to stool, blood tests are often necessary. The Mayo Clinic and the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) indicate that a complete blood count, electrolytes, and kidney function tests help assess the severity of diarrhea, anemia, inflammation, and the degree of dehydration. This is especially important if blood in the stool is accompanied by weakness, tachycardia, dryness, low urine output, or suspected hemolytic uremic syndrome. [21]

If infection is not confirmed or bloody diarrhea becomes recurrent, endoscopy comes into play. For ulcerative colitis, the US National Institute of Diabetes and Digestive and Kidney Diseases recommends colon endoscopy with biopsies, while for Crohn's disease, the most accurate diagnostic method remains endoscopic examination with biopsy, supplemented by imaging as indicated. This is a step in the search for a chronic inflammatory cause, not just an acute infection. [22]

If ischemic colitis is suspected, the examination is often supplemented by CT scanning, colonoscopy, and stool analysis to rule out infection and confirm a vascular cause. The Mayo Clinic emphasizes that colonoscopy helps diagnose ischemic colitis and rule out cancer and other diseases, while CT scanning is useful for initially distinguishing between ischemia and inflammatory bowel disease. [23]

Study	When is it especially useful?
Stool analysis for bacterial pathogens	For acute bloody diarrhea with fever and pain
Shiga toxin test and O157 culture	If Shiga toxin-producing E. coli is suspected
Complete blood count	To assess anemia, inflammation and complications
Electrolytes and renal function tests	To assess dehydration and risk of kidney damage
Colonoscopy with biopsy	If ulcerative colitis, Crohn's disease, or ischemic colitis is suspected
Computed tomography	When to distinguish between ischemia, complications, and other causes of pain

Sources for the table: [24]

How is bloody diarrhea treated and why is it dangerous to follow a "standard" treatment plan?

Treatment depends on the cause, but the general first step is almost always the same: fluid and salt replacement. The World Health Organization and the US National Institute of Diabetes and Digestive and Kidney Diseases emphasize the importance of oral rehydration solutions, and in cases of severe dehydration or vomiting, they switch to intravenous solutions. For children with Shiga toxin-producing E. coli, early intravenous rehydration can reduce the risk of kidney failure. [25]

The main rule of thumb is not to automatically use antidiarrheal medications for bloody diarrhea. The National Institute of Diabetes and Digestive and Kidney Diseases and the Centers for Disease Control and Prevention recommend avoiding over-the-counter antidiarrheals in children and in people with bloody stools or fever. For Shiga toxin-producing E. coli, such medications are especially undesirable because they can increase the risk of complications, including toxic megacolon and hemolytic uremic syndrome. [26]

The situation with antibiotics is also not as straightforward as it seems. Guidelines from the Infectious Diseases Society of the United States indicate that empirical antibacterial therapy for bloody diarrhea in immunocompetent children and adults is generally not recommended until results are available. Exceptions are possible for infants under 3 months with a suspected bacterial cause, for seriously ill patients with symptoms of shigellosis dysentery, and for immunocompromised individuals with severe diarrhea. It is specifically emphasized that if Shiga toxin-producing E. coli is clinically suspected, antibiotics may increase the risk of hemolytic uremic syndrome. [27]

If ulcerative colitis is the cause, treatment follows a completely different logic. The US National Institute of Diabetes and Digestive and Kidney Diseases indicates that for ulcerative colitis, aminosalicylates, glucocorticosteroids, immunosuppressants, biologics, and small molecules are used depending on the severity, while severe and fulminant colitis often requires hospitalization. For ischemic colitis, treatment depends on the severity: in mild cases, fluids, bowel rest, and treatment of underlying causes are often helpful, while in cases of necrosis, perforation, or severe bowel damage, surgery may be necessary. [28]

In children, the approach to bloody diarrhea is especially cautious. Irish pediatric guidelines recommend sending a stool sample for culture in all cases, reserving antibiotics for severe infection, immunodeficiency, and certain confirmed causes such as Shigella or Entamoeba histolytica. Persistent or recurrent bloody diarrhea with negative cultures should prompt consideration of inflammatory bowel disease, and if signs of an acute abdomen are present, urgent surgical intervention is warranted. [29]

Tactics	When it is appropriate
Oral rehydration solutions	With the ability to drink and moderate severity
Intravenous fluids	In case of severe dehydration, vomiting, severe condition
Over-the-counter antidiarrheals	Not usually suitable for bloody diarrhea
Empirical antibiotics	Not recommended routinely in immunocompetent adults and children
Targeted antibacterial treatment	After assessing the clinical picture, tests and the probable pathogen
Anti-inflammatory treatment for ulcerative colitis	If inflammatory bowel disease is confirmed
Surgical treatment	In complicated ischemic colitis or other acute surgical pathology

Sources for the table: [30]

FAQ

Is bloody diarrhea a simple case of food poisoning that will resolve on its own?
Sometimes blood does appear with a bacterial food infection, but the presence of blood makes the situation more serious. The Centers for Disease Control and Prevention (CDC) lists bloody diarrhea as a sign that requires medical attention rather than simply observing it at home. [31]

Why shouldn't loperamide be taken immediately if bloody diarrhea occurs?
Because drugs that inhibit intestinal motility can be unsafe in the presence of bloody stool and fever, and in the presence of Shiga toxin-producing E. coli, they can increase the risk of complications. This is clearly noted by both the National Institute of Diabetes and Digestive and Kidney Diseases in the US and the Centers for Disease Control and Prevention in the US. [32]

Are antibiotics always necessary if there is blood in the stool?
No. In immunocompetent adults and children, empirical antibiotics for bloody diarrhea are generally not recommended until testing results are available, except in certain severe cases. It is especially important not to prescribe them randomly if Shiga toxin-producing E. coli is suspected. [33]

What other conditions besides infection can cause bloody diarrhea?
The most important non-infectious causes are ulcerative colitis, some forms of Crohn's disease, and ischemic colitis. In infants, food allergy proctocolitis-like reactions should also be considered, and in children with severe abdominal pain, intussusception and other surgical conditions should be considered. [34]

When should kidney damage be especially feared after bloody diarrhea?
When an infection caused by Shiga toxin-producing E. coli is suspected, especially in children. If, during or after diarrhea, urination decreases, pallor, severe weakness, bruising, or blood in the urine appear, hemolytic uremic syndrome must be urgently ruled out. [35]

When is a colonoscopy needed, not just a stool test?
When blood in the stool recurs, cultures don't explain the symptom, there is weight loss, signs of chronic inflammation, anemia, or suspected ulcerative colitis, Crohn's disease, or ischemic colitis. For ulcerative colitis and Crohn's disease, endoscopy with biopsy remains a key step in confirming the diagnosis. [36]

Key points from experts

Andrea L. Shane, MD, MPH, is a professor of pediatric infectious diseases and chief of the Division of Pediatric Infectious Diseases at Emory University School of Medicine. Her work focuses on enteric infections and diarrheal diseases in children. A practical lesson from modern infectious gastroenterology is this: when dealing with bloody diarrhea, it's more important not to rush to a generic antibiotic but to quickly identify whether shigellosis, Campylobacter, Salmonella, or Shiga toxin-producing E. coli is the cause, as a mistake in early management can alter the risk of complications. [37]

Stephen B. Freedman, MD, Professor of Pediatrics and Emergency Medicine at the University of Calgary, is a researcher specializing in pediatric gastroenteritis and Shiga toxin-producing E. coli. His research is particularly relevant to this topic because it relates to the treatment of childhood intestinal infections and the prevention of hemolytic uremic syndrome. The practical implication of these studies is that bloody diarrhea in children requires more than just "observation"; it requires active hydration assessment, early recognition of renal risk, and caution with medications that may worsen the infection. [38]

David T. Rubin, MD, professor of medicine and pathology, chief of the division of gastroenterology, hepatology, and nutrition, and director of the Inflammatory Bowel Disease Center at the University of Chicago. He is a co-author of the 2025 American College of Gastroenterology guideline for ulcerative colitis. His expertise emphasizes a key clinical principle: if bloody diarrhea is not consistent with an acute infection or becomes recurrent, promptly pursue a search for inflammatory bowel disease rather than endlessly changing symptomatic medications.[39]

William D. Chey, MD, professor of gastroenterology and nutrition and chief of the Division of Gastroenterology and Hepatology at the University of Michigan, is a leading expert in the field of diagnosing and treating intestinal diseases. His work reflects a common approach in modern gastroenterology: the decision to treat bloody diarrhea should be based on risk stratification and appropriate testing, not a one-size-fits-all approach. Therefore, stool tests, renal evaluation, endoscopy, and imaging are used selectively but promptly. [40]