C. diff Diarrhea and Colitis: Symptoms and Clinical Course

Typical Presentation
Pseudomembranous Colitis
C. diff Diarrhea vs Colitis
Bloody Diarrhea in C. diff
Leukocytosis: The WBC Spike
Duration Without Treatment
Endoscopy Findings
Abdominal Imaging
When to Seek Emergency Care
Key Research Papers
Connections
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Typical Presentation

Clostridioides difficile infection (CDI) almost always announces itself the same way: sudden-onset watery diarrhea that arrives during or within 1–8 weeks of antibiotic use. Most patients pass loose or liquid stools 3 or more times a day; in moderate-to-severe cases, the count climbs to 10–15 trips to the bathroom in a single 24-hour period. The stool has a characteristic foul, almost sickly-sweet odor that many nurses and experienced clinicians say is immediately recognizable once they have encountered it before.

Beyond the diarrhea itself, the typical symptom cluster includes:

Lower abdominal cramping — dull to colicky pain, often worsening just before a bowel movement and easing briefly afterward.
Low-grade fever — temperatures of 38.0–38.5 °C (100.4–101.3 °F) are common; higher fevers raise the concern for more serious disease.
Nausea — present in roughly a third of patients; vomiting is less common but occurs in severe cases.
Generalized malaise and fatigue — the combination of fluid loss, electrolyte depletion, and systemic inflammation leaves most patients feeling genuinely ill.
Dehydration signs — dry mouth, decreased urine output, and lightheadedness on standing, particularly in older adults who have less physiological reserve.

The antibiotic trigger is the most important clue in the history. Clindamycin, fluoroquinolones (ciprofloxacin, levofloxacin), and broad-spectrum penicillins or cephalosporins are the highest-risk agents, but virtually any antibiotic — including those given to treat a previous CDI — can precipitate a new episode. The onset can lag the antibiotic course by weeks, so always ask about recent antibiotic exposure even if the patient stopped taking the drug a month ago.

Pseudomembranous Colitis

In a subset of CDI cases — particularly those involving hypervirulent strains such as the NAP1/BI/027 ribotype — the infection progresses beyond simple diarrhea to full-blown pseudomembranous colitis (PMC). The term refers to the yellowish-white plaques, or pseudomembranes, that coat the inner surface of the colon. These plaques are not true membranes; they are focal accumulations of fibrin, mucus, sloughed epithelial cells, and inflammatory cells that pile up over areas where the mucosal surface has been destroyed.

The driver is toxin B (TcdB), a glucosyltransferase that inactivates Rho-family GTPases inside colonocytes, causing cytoskeletal collapse, tight-junction breakdown, and ultimately cell death. Toxin A (TcdA) plays a supporting role: it disrupts epithelial barrier function and amplifies the inflammatory signal that draws massive numbers of polymorphonuclear neutrophils (PMNs) into the lamina propria and submucosa. The resulting cytokine storm — dominated by interleukin-8, tumor necrosis factor-alpha, and interleukin-1 beta — is what produces the fever, leukocytosis, and systemic toxicity that distinguish PMC from ordinary antibiotic-associated diarrhea.

Pseudomembranes are pathognomonic when seen on colonoscopy: no other condition produces those raised, adherent yellow plaques on an otherwise inflamed, bleeding mucosa. However, they are not visible on plain CT imaging in early or mild disease, and they are not always present even in severe CDI — so their absence on imaging never rules out the diagnosis.

Pseudomembranous colitis was first described in the pre-antibiotic era as a rare post-operative complication. The link to C. difficile was only firmly established in 1978, when researchers demonstrated that the organism — and specifically its cytotoxin — was responsible for most cases of antibiotic-associated PMC. Before that discovery, the condition was often fatal and its cause unknown.

C. diff-Associated Diarrhea vs Colitis

CDI exists on a spectrum, and the distinction between simple C. difficile-associated diarrhea (CDAD) and frank colitis matters for both prognosis and management decisions.

C. diff-associated diarrhea (CDAD / mild CDI): Diarrhea is present — often 3–5 loose stools per day — but the colon itself shows little or no macroscopic damage. There is no significant mucosal injury, no leukocytosis, and no systemic toxicity. This presentation is most common when the offending antibiotic is stopped early and the gut microbiome begins to recover. Many of these cases resolve without specific anti-CDI treatment, though close monitoring is essential because the disease can escalate rapidly.

C. diff colitis (moderate-to-severe CDI): The colon wall is inflamed. Key distinguishing features include:

Elevated white blood cell count (leukocytosis), often 15,000 cells/mm³ or higher.
Rising serum creatinine, reflecting third-spacing of fluid and early hemodynamic compromise.
Fever above 38.5 °C (101.3 °F).
Abdominal tenderness on examination — often diffuse, sometimes peritoneal in fulminant cases.
Colonic wall thickening visible on CT scan.

Recognizing where a patient sits on this spectrum determines which antibiotic regimen they receive (metronidazole for mild-moderate vs. oral vancomycin or fidaxomicin for severe CDI per IDSA/SHEA 2021 guidelines) and whether they need hospitalization.

Bloody Diarrhea in C. diff

Bloody diarrhea is not a defining feature of CDI the way it is in Salmonella typhi, Campylobacter jejuni, or enterohemorrhagic E. coli infections. When clinicians think of bloody diarrhea from a bacterial pathogen, C. difficile is not usually the first organism that comes to mind — and that intuition is broadly correct.

However, gross blood in the stool does occur in 5–10% of CDI cases, and its presence is clinically significant. It signals that the mucosal destruction has extended deep enough to reach submucosal blood vessels — a level of tissue injury associated with more severe disease and a higher risk of complications. Possible explanations include:

Hemorrhagic colitis — in severe or fulminant CDI, diffuse mucosal necrosis can cause frank hemorrhage.
Pseudomembrane sloughing — as pseudomembranes detach, they expose raw bleeding surfaces underneath.
Superimposed pathology — underlying inflammatory bowel disease (IBD) or diverticular disease may bleed independently in the context of CDI.

It is also worth distinguishing gross (visible) blood from a heme-positive stool on fecal occult blood testing. In CDI, occult-positive stools are more common than frank bleeding; this low-level oozing from inflamed mucosa may not be visible to the naked eye but is detectable with guaiac or immunochemical stool tests. When a patient with suspected CDI also has gross rectal bleeding, consider whether an alternative or co-existing diagnosis — ischemic colitis, IBD flare, colorectal cancer — is contributing.

Leukocytosis: The WBC Spike

Among all the laboratory abnormalities that CDI can produce, leukocytosis — an elevated white blood cell count — is the one that clinicians have learned to treat as a red flag. A WBC above 15,000 cells/mm³ is a criterion for severe CDI in virtually every major guideline. Counts of 30,000–50,000 cells/mm³ or higher are not rare in fulminant disease, and extreme leukocytosis of this magnitude in a patient with diarrhea should immediately trigger concern for CDI even before stool test results return.

Why does CDI cause such dramatic leukocytosis? The answer lies in the toxin-driven inflammatory cascade. When toxin B destroys colonocytes, it triggers:

Release of interleukin-8 (IL-8), a powerful neutrophil chemoattractant that floods the colon wall with PMNs.
Systemic release of tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β), which signal the bone marrow to rapidly release mature and immature neutrophils into the bloodstream — producing the "left shift" (bandemia) often seen on the differential count.
A stress response mediated by cortisol that further demarginalizes neutrophils from vessel walls into the circulating pool.

In practice, the WBC is incorporated into clinical severity scoring. The IDSA/SHEA 2021 guidelines define severe CDI as WBC ≥ 15,000 cells/mm³ or serum creatinine ≥ 1.5 mg/dL; fulminant CDI is defined by hypotension, shock, ileus, or megacolon — any of which mandates ICU-level care. A rising WBC on serial measurements is especially ominous because it suggests the infection is not responding to treatment and may be escalating toward fulminant disease.

Duration Without Treatment

What happens if CDI is not treated — either because the diagnosis is missed or because the patient declines antibiotics? The answer depends heavily on whether the triggering antibiotic has been stopped and on the baseline health of the patient.

If the offending antibiotic is discontinued, a minority of patients — estimates vary from 15–25% in older studies — will experience spontaneous resolution. The reasoning is mechanistically sound: removing the selective pressure that killed off the protective gut microbiome gives commensal bacteria a chance to repopulate and restore colonization resistance against C. difficile. This outcome is more likely in younger, healthier patients with mild disease.

However, the majority of patients do not improve spontaneously, and the risk of rapid clinical deterioration is real and unpredictable:

Fluid and electrolyte losses from persistent diarrhea cause progressive dehydration, hypokalemia, and hypoalbuminemia — all of which worsen the prognosis.
Colonic inflammation can extend transmurally over days, raising the risk of perforation or toxic megacolon.
In immunocompromised patients (transplant recipients, those on chemotherapy, patients on high-dose steroids), progression to fulminant disease can occur within 24–48 hours without treatment.
Untreated CDI in hospitalized patients is associated with significantly longer length of stay and higher mortality.

The practical implication: in a patient with confirmed or strongly suspected CDI, waiting to see if the diarrhea resolves on its own — beyond the initial 24–48 hours after stopping the triggering antibiotic — is rarely the right call. Start treatment early.

Endoscopy Findings

Colonoscopy or flexible sigmoidoscopy is not a routine part of CDI diagnosis — stool testing (glutamate dehydrogenase antigen, toxin immunoassay, and/or nucleic acid amplification testing) is faster, cheaper, and avoids the procedural risk of perforating an already-inflamed colon. But when the clinical picture is unclear, when stool tests are negative despite high clinical suspicion, or when a patient is critically ill and an immediate diagnosis is needed, endoscopy can be decisive.

What the endoscopist sees in CDI:

Pseudomembranes — the signature finding. Raised, yellow-white or tan plaques, 2–10 mm in diameter, studding the colonic mucosa. They have a stuck-on appearance and bleed when touched with the biopsy forceps. When pseudomembranes are present, no other diagnosis is needed — they are pathognomonic for CDI. Histology shows a "summit lesion": a mushroom-shaped eruption of fibrin, mucin, and inflammatory cells erupting through a focal mucosal defect.
Edematous mucosa — the mucosa appears swollen, pale, and loss of its normal vascular pattern.
Friability and erosions — the mucosa bleeds easily on contact; superficial erosions are visible between pseudomembranes in moderate-to-severe disease.
Diffuse erythema — in cases without obvious pseudomembranes, the endoscopist may see only nonspecific redness and edema, which is less specific but still consistent with colitis.

The distribution is important. CDI colitis characteristically involves the left colon (descending colon, sigmoid, rectum), which is why sigmoidoscopy alone captures most cases. However, in roughly 10–20% of patients — more common with the hypervirulent NAP1 strain — disease may be predominantly right-sided or pancolonic, in which case sigmoidoscopy can be falsely negative.

In fulminant or near-fulminant CDI, colonoscopy is generally avoided because insufflating an inflamed, thin-walled colon risks perforation. Sigmoidoscopy without air insufflation (or with very careful, minimal insufflation) is sometimes performed in the ICU setting to confirm the diagnosis and guide surgical decision-making.

Abdominal Imaging

CT scanning of the abdomen and pelvis with intravenous contrast has become the most important imaging modality in CDI. It is not used to make the initial diagnosis — again, that role belongs to stool tests — but it is invaluable for assessing disease severity, detecting complications, and guiding triage decisions.

Key CT findings in CDI:

Colonic wall thickening — the most consistent finding. The normal colon wall measures 3–4 mm; in CDI it often exceeds 10–15 mm, reflecting edema and inflammatory infiltration. Thickening can be segmental (most often left-sided) or pancolonic in severe disease.
The "accordion sign" — a striking pattern seen when oral contrast or stool material becomes trapped between thickened, edematous haustral folds, producing an alternating high-density and low-density stripe pattern that resembles the bellows of an accordion. It is highly associated with pseudomembranous colitis.
Pericolic fat stranding — haziness in the fat surrounding the thickened colon, indicating extension of inflammation beyond the bowel wall. When fat stranding is marked, the risk of perforation increases.
Ascites — free fluid in the peritoneal cavity signals significant protein loss and capillary leak from the inflamed gut; it correlates with more severe disease and worse outcomes.
Thumbprinting — nodular impressions on the colon wall from submucosal edema; can also be seen on plain abdominal radiograph.
Pneumatosis coli or free air — gas within the bowel wall or free in the peritoneal cavity indicates impending or actual perforation; this is a surgical emergency.
Megacolon — colonic diameter exceeding 6 cm (by convention) on plain film or CT, a sign of toxic megacolon — one of the most feared complications of CDI.

Plain abdominal radiographs are less sensitive than CT but remain useful for rapidly identifying megacolon, free air, or small-bowel obstruction when CT is not immediately available. In patients too unstable to leave the ICU for CT, a portable abdominal X-ray can provide critical information in seconds.

When to Seek Emergency Care

Most people with CDI are sick, but not in immediate danger. They can be managed with oral antibiotics as outpatients if they are otherwise well, adequately hydrated, and reliable about follow-up. But certain warning signs demand same-day or immediate emergency evaluation — do not wait these out.

Call 911 or go to the emergency room immediately if you have:

Severe abdominal pain or a rigid, board-like abdomen — may indicate perforation or peritonitis.
Signs of shock — rapid heart rate (over 100 bpm at rest), very low blood pressure, extreme lightheadedness, or feeling faint; pallor or mottled skin.
High fever above 39.5 °C (103 °F), especially with shaking chills or confusion.
Inability to keep any fluids down combined with profuse diarrhea — severe dehydration can develop within hours, especially in older adults.
Mental status changes — confusion, unusual drowsiness, or disorientation in someone with active diarrhea is a medical emergency.
Markedly distended abdomen — a sudden increase in abdominal girth may indicate toxic megacolon developing.
No urine output for 8 or more hours — a sign of severe dehydration or kidney injury from sepsis.

Seek urgent (same-day) medical evaluation if:

You are passing more than 10 watery stools in a 24-hour period.
You notice significant blood in your stool.
Your symptoms are not improving after 48–72 hours on prescribed antibiotic treatment for CDI.
You are immunocompromised (organ transplant, chemotherapy, HIV, high-dose steroids) — CDI can progress far faster in this population.
You are over 65 and have severe diarrhea — older adults decompensate quickly and are at higher risk for fulminant disease.

If you have been diagnosed with CDI and you are on treatment, contact your doctor the same day if your symptoms are getting worse rather than better after 2–3 days of antibiotics, or if you develop any of the warning signs above. CDI can escalate from manageable to life-threatening within 24–48 hours.

Key Research Papers

The following peer-reviewed studies and guidelines form the evidence base for understanding C. diff diarrhea and colitis.

Bartlett JG. Narrative review: the new epidemic of Clostridium difficile–associated enteric disease. Ann Intern Med 2006;145:758–64. PMID 17116919
Surawicz CM, Brandt LJ, Binion DG, et al. Guidelines for diagnosis, treatment, and prevention of Clostridium difficile infections. Am J Gastroenterol 2013;108:478–98. PMID 23439232
Kelly CP. A 76-year-old man with recurrent Clostridium difficile-associated diarrhea. JAMA 2009;301:954–62. PMID 19244193
Khanna S, Pardi DS. The growing incidence and severity of Clostridium difficile infection in inpatient and outpatient settings. Expert Rev Gastroenterol Hepatol 2010;4:409–16. PMID 20678013
Hookman P, Barkin JS. Clostridium difficile associated infection, diarrhea and colitis. World J Gastroenterol 2009;15:1554–80. PMID 19340896
Price AB, Davies DR. Pseudomembranous colitis. J Clin Pathol 1977;30:1–12. PMID 320768
Fekety R. Guidelines for the diagnosis and management of Clostridium difficile–associated diarrhea and colitis. Am J Gastroenterol 1997;92:739–50. PMID 9149181
Dallal RM, Harbrecht BG, Boujoukas AJ, et al. Fulminant Clostridium difficile: an underappreciated and increasing cause of death and complications. Ann Surg 2002;235:363–72. PMID 11882758
Leffler DA, Lamont JT. Clostridium difficile infection. N Engl J Med 2015;372:1539–48. PMID 25875259
Johnson S, Lavergne V, Skinner AM, et al. Clinical practice guideline by the IDSA and SHEA: 2021 focused update on management of Clostridioides difficile infection in adults. Clin Infect Dis 2021;73:e1029–e1044. PMID 34164374

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