Diare Akut

01Pathophysiology

The gastrointestinal tract normally processes approximately 9 liters of fluid daily, of which about 2 liters come from oral intake and the remaining 7 liters from endogenous secretions (salivary, gastric, biliary, pancreatic, and intestinal). The small intestine absorbs the bulk of this volume, the colon absorbs another 1 to 2 liters, and only 100 to 200 mL is excreted in normal stool. Diarrhea results when this absorptive-secretory equilibrium is disrupted, and the mechanism of disruption determines how the patient presents clinically.
Secretory diarrhea occurs when enterotoxins activate intracellular second messengers such as cyclic AMP (cholera toxin, ETEC heat-labile toxin) or cyclic GMP (ETEC heat-stable toxin). These messengers open chloride channels on the luminal surface of enterocytes, driving active chloride and water secretion into the bowel lumen. Because this process is driven by active secretion rather than an ingested solute, the diarrhea is characteristically large-volume, watery, and persists even when the patient is fasting. The stool osmotic gap is low (below 50 mOsm/kg) because the stool osmolality is accounted for by electrolytes.
Osmotic diarrhea results when a poorly absorbed or non-absorbable solute draws water into the intestinal lumen by osmotic force. Classic examples include lactose in a lactase-deficient patient, sorbitol-containing foods, and magnesium-based laxatives or antacids. The hallmark is that diarrhea resolves when the offending agent is removed (i.e., with fasting), and the stool osmotic gap is elevated (above 50 mOsm/kg) because unmeasured solutes are responsible for retaining water.
The formula to calculate the stool osmotic gap is:
$\text{Stool Osmotic Gap}=290-2\times ({\text{Stool Na}}^{+}+{\text{Stool K}}^{+})\backslash text\{Stool\; Osmotic\; Gap\}\; =\; 290\; -\; 2\; \backslash times\; (\backslash text\{Stool\; Na\}^+\; +\; \backslash text\{Stool\; K\}^+)$
A gap greater than 50 mOsm/kg points to an osmotic cause, while a gap less than 50 mOsm/kg suggests a secretory mechanism.
Inflammatory (invasive) diarrhea occurs when organisms directly invade the intestinal mucosa or elaborate cytotoxins that cause mucosal destruction. Shigella, Salmonella, Campylobacter, and Entamoeba histolytica are classic invasive pathogens. C. difficile toxins A and B cause mucosal necrosis with pseudomembrane formation. EHEC produces Shiga toxin, which damages vascular endothelium in the colon and kidneys. Inflammatory diarrhea typically presents as frequent, small-volume, bloody stools (dysentery) with fever, tenesmus, and positive fecal leukocytes or lactoferrin.
Altered motility is a less commonly tested mechanism in acute settings but contributes when increased peristalsis reduces mucosal contact time, impairing fluid absorption. This mechanism is more relevant in chronic functional disorders.

02Classification and Clinical Manifestation

Classification by Mechanism

Common Pathogens and Their Distinguishing Clinical Features

03Diagnostic Workup

The majority of acute diarrheal episodes are self-limited and do not require any laboratory workup. The best initial approach for most patients presenting with acute, non-bloody, watery diarrhea without alarm features is clinical assessment and supportive care alone. Alarm features that should trigger a diagnostic workup include: bloody stools, fever exceeding 38.5 C, signs of severe dehydration, duration beyond 7 days, recent hospitalization or antibiotic exposure, and an immunocompromised state.
When workup is indicated, the best initial test is a combination of fecal leukocytes (or fecal lactoferrin) and clinical context. Fecal leukocytes serve as a rapid screening test to determine whether the diarrhea is inflammatory in nature. A positive result narrows the differential toward invasive bacterial pathogens or inflammatory bowel disease and justifies obtaining a stool culture, which is the gold standard for identifying bacterial enteropathogens. It is important to note that when EHEC (O157:H7) is suspected (bloody diarrhea with minimal or no fever, ground beef exposure), the laboratory must be notified to plate on sorbitol-MacConkey agar, as standard culture media will not identify this organism.
For patients with recent antibiotic use or nosocomial diarrhea, C. difficile testing should be ordered. The recommended approach per current guidelines is either a stool PCR (nucleic acid amplification test), which is highly sensitive, or a two-step algorithm using glutamate dehydrogenase (GDH) screening followed by toxin enzyme immunoassay (EIA) confirmation. The rationale behind the two-step approach is that GDH is sensitive but not toxin-confirmatory, while toxin EIA is less sensitive on its own. Discordant results (GDH-positive, toxin-negative) can be resolved with PCR.
Stool examination for ova and parasites is reserved for patients with a relevant travel history, camping or hiking exposure (think Giardia), immunocompromised status, or symptoms lasting beyond 7 to 14 days. At least three specimens collected on separate days are recommended to improve sensitivity. If Cryptosporidium is suspected, a modified acid-fast stain should be requested separately.

04Management and Treatment

Acute Stabilization
The first and most critical step in managing any patient with acute diarrhea, regardless of etiology, is assessment and correction of dehydration. Oral rehydration therapy (ORT) using a balanced oral rehydration solution is the treatment of choice for mild-to-moderate dehydration and is superior to plain water because it exploits the sodium-glucose cotransporter (SGLT1), which remains intact even during secretory diarrhea. This is a frequently tested principle: the glucose in ORS is not there for calories but to drive sodium and water absorption. For severe dehydration (altered mental status, hemodynamic instability, inability to tolerate oral fluids), IV fluid resuscitation with isotonic crystalloid is indicated before transitioning to oral rehydration.
Empiric Antibiotic Therapy
Most acute diarrhea does not require antibiotics. Empiric therapy is warranted in two clinical scenarios: (1) moderate-to-severe traveler's diarrhea and (2) febrile dysentery with bloody stools and systemic toxicity. Azithromycin has become the preferred empiric agent due to rising fluoroquinolone resistance, particularly in Southeast Asia and South Asia. Fluoroquinolones (ciprofloxacin) remain an alternative in regions with lower resistance rates.
Critical Contraindication: EHEC. When the vignette describes a patient with bloody diarrhea and no fever, particularly after consuming undercooked ground beef, the answer is supportive care only. Antibiotics in EHEC infection increase the risk of hemolytic uremic syndrome (HUS) by promoting Shiga toxin release from lysed bacteria. This is one of the most heavily tested management traps in this topic.
C. difficile Treatment
Current guidelines now favor fidaxomicin or oral vancomycin as first-line treatment over metronidazole for initial C. difficile infection. Metronidazole is now reserved as an alternative when fidaxomicin and vancomycin are not available. A key testable point: vancomycin for C. difficile must be given orally, because IV vancomycin does not achieve therapeutic concentrations in the colonic lumen. In fulminant disease (hypotension, ileus, megacolon), the regimen is high-dose oral vancomycin (500 mg four times daily) combined with IV metronidazole (500 mg every 8 hours), and rectal vancomycin enemas may be added if ileus prevents oral delivery.
Antidiarrheal Agents
Loperamide (an opioid receptor agonist that slows intestinal motility) can be used for symptomatic relief in non-inflammatory, watery diarrhea only. It is contraindicated in dysentery (bloody diarrhea), suspected C. difficile, and suspected EHEC, because slowing transit time in invasive infections increases toxin exposure and risk of toxic megacolon or HUS. Bismuth subsalicylate is a safer symptomatic option that can reduce stool frequency; avoid it in patients with aspirin allergy or those taking anticoagulants.
Long-Term Considerations
After viral gastroenteritis, patients (particularly children) may develop transient lactose intolerance due to brush border damage. If watery diarrhea persists after the acute illness resolves, a temporary lactose-free diet should be considered before pursuing additional workup.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most commonly tested trap in acute diarrhea is the management of EHEC (E. coli O157:H7). The vignette will describe a patient, often a child, who ate undercooked hamburger and now presents with bloody diarrhea and notably no fever or only low-grade fever. The instinct to prescribe antibiotics for bloody diarrhea is exactly what the question is testing. Antibiotics in this setting increase the release of Shiga toxin and elevate the risk of hemolytic uremic syndrome (microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure). The correct answer is supportive care alone.
The second major trap involves C. difficile treatment. Students frequently select IV vancomycin, which is the standard route for treating MRSA or endocarditis. However, for C. difficile colitis, vancomycin must be given orally because the IV formulation does not reach the colonic lumen. Additionally, metronidazole is no longer the preferred first-line agent for C. difficile per updated guidelines; fidaxomicin or oral vancomycin should be chosen first.
A third common error is giving loperamide to a patient with inflammatory or bloody diarrhea. Loperamide slows gut motility, which in the setting of invasive infection or toxin-mediated disease prolongs mucosal exposure and can precipitate toxic megacolon. It is only appropriate for non-inflammatory, watery diarrhea without systemic signs.
Students also confuse non-typhoidal Salmonella gastroenteritis management. In an immunocompetent adult, uncomplicated Salmonella gastroenteritis is self-limited and antibiotics are not indicated. Treating with antibiotics can paradoxically prolong the carrier state. Exceptions requiring treatment include bacteremia, extremes of age (infants or elderly), immunocompromised hosts, and patients with prosthetic vascular grafts or implants.
Finally, watch for the post-infectious transient lactose intolerance vignette: a child who recovered from viral gastroenteritis but continues to have watery diarrhea after reintroducing milk. The mechanism is temporary brush border lactase deficiency, not persistent infection. The osmotic gap will be elevated, and the correct answer is temporary dairy avoidance rather than additional antibiotics or further infectious workup.
The core competency tested across all acute diarrhea questions is the ability to match the clinical scenario to the correct pathogen, determine when workup versus empiric treatment is appropriate, and recognize the situations where standard treatment (antibiotics, antidiarrheals) causes more harm than the disease itself.