Gastroenterology

01

Malabsorption

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Celiac Disease · Clinical Picture

Diarrhea / steatorrhea · abdominal pain · bloating · weight loss · anemia · osteoporosis · enamel defects · mildly elevated transaminases · infertility · vitamin / mineral deficiencies (A · D · E · B12 · Fe · Ca) · dermatitis herpetiformis.

Risk factors: Northern European · FH · T1DM · autoimmune thyroid disease · Down syndrome · Turner syndrome · IgA deficiency.

Celiac · Diagnostic Cascade

Serology › EGD › gluten-free diet

1. Anti-tTG IgA + IgA level (rule out IgA deficiency)

2. IgA deficient › anti-DGP / anti-EMA / anti-tTG IgG

3. Confirm: EGD + duodenal biopsy › intraepithelial lymphocytosis · crypt hyperplasia · villous atrophy

4. HLA-DQ2 / HLA-DQ8 for equivocal cases

5. Treatment: lifelong gluten-free diet

Celiac · Screening Indications

Screening indications: IBS · T1DM · first-degree relative · deranged LFTs · IDA / unexplained anemia · MBD · premature osteoporosis · amenorrhea · infertility · Down syndrome · Turner syndrome · dermatitis herpetiformis.

Celiac · Monitoring & Complications

Monitoring: antibody titer at 6 months + 12 months + annually. BMD at diagnosis.

Complications: nutritional deficiencies (Fe · folate · vit D · B12) · anemia · osteopenia / osteoporosis · ↑ liver enzymes · dermatitis herpetiformis · enteropathy-associated T-cell lymphoma (refractory disease › repeat EGD) · microscopic colitis.

Dermatitis herpetiformis › gluten-free diet ± dapsone (CI: G6PD deficiency).

Bacterial overgrowth

SIBO

Abdominal discomfort · flatulence · gas · distention · bloating · diarrhea.

Causes: gastric surgery · ileocecal resection · diverticulosis · adhesions · strictures · fistula · CD · malignancy · blind loops · DM · CTD · hypothyroidism · long-term opioids · celiac · pancreatic insufficiency · AIDS · CVID · IgA deficiency · PPIs · atrophic gastritis · older age · cirrhosis.

Diagnostic: glucose breath test or lactulose breath test.

› co-amoxiclav · ciprofloxacin · doxycycline · metronidazole · neomycin · norfloxacin · rifaximin · tetracycline · cotrimoxazole.

Methanogen

IMO

IBS · constipation profile · elevated methane ≥10 ppm on carbohydrate breath test.

› neomycin + rifaximin.

Surgical

Short Bowel Syndrome

Surgical removal of small bowel › maldigestion · malabsorption · malnutrition.

Adequate nutrition is the cornerstone. Adjuncts: loperamide · PPIs · teduglutide.

Disaccharidase

Lactose / Fructose Maldigestion

Gas · bloating · diarrhea · postprandial · improves with fasting.

› Elimination diet ± lactase enzyme replacement.

02

Inflammatory Bowel Disease

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Crohn's vs Ulcerative Colitis

The shape of each disease

Feature	Crohn's Disease	Ulcerative Colitis
Symptoms	Abdominal pain · diarrhea · inflammatory masses · fever · weight loss · strictures · fistulae · perianal symptoms	Small-volume / frequent BMs · blood · tenesmus · urgency · crampy pain · fever · fatigue · weight loss
Distribution	Anywhere mouth → anus · most commonly ileum · skip lesions · no rectal involvement	Colorectal · continuous from rectum proximally · proctitis (`≤18 cm`) · left-sided (sigmoid → splenic flexure) · extensive / pancolitis (beyond splenic flexure)
Endoscopy	Linear / stellate / serpiginous ulcerations · cobblestone mucosa · granulomas · transmural	Mucosal edema · erythema · loss of vascular pattern · granularity · friability · ulceration · bleeding · crypt abscesses · lamina propria cellularity
Smoking	↑ Risk	↓ Risk
Labs	↑ CRP · anemia · ↓ albumin · ↓ Fe · ↑ fecal calprotectin · CD also › ↓ B12

Crohn's · Treatment Ladder

Induction · Maintenance

Severity	Induction	Maintenance
Mild	Sulfasalazine (colitis) · MM budesonide (ileocolonic)	Sulfasalazine (colitis)
Moderate	PO / IV glucocorticoids · MM budesonide · biologics (infliximab + AZA, adalimumab, certolizumab, vedolizumab, ustekinumab, mirikizumab, risankizumab, guselkumab) · small molecules (upadacitinib)	AZA / 6-MP · MTX · biologics · small molecules
Severe	PO / IV glucocorticoids	Biologics (infliximab + AZA) · small molecules

Upper GI CD › EGD
Small bowel › CT / MR enterography · capsule endoscopy
Strictures › constipation
Abscess (pain · fever) › CT abdomen › surgery / drainage + antibiotics › biologics
Perianal fistula › MR pelvis / EUS + EUA › infliximab or adalimumab + antibiotics ± thiopurine
Cold stricture (fibrostenotic, no active inflammation) › conservative (bowel rest · NG tube) › endoscopic balloon dilation or surgery
Hot stricture (active inflammation) › steroid bridge to maintenance therapy (biologics)
Ileitis / ileal resection › bile-salt diarrhea › cholestyramine

Surgery Indications

Surgery indications: intestinal obstruction · refractory / fulminant disease · high-grade dysplasia / cancer · severe perianal disease · fistulas · abscess · perforation.

Truelove & Witts · Severe UC

≥6 BMs · visible blood in stool · T ≥37.8°C · HR >90 · Hb ≤105 · ESR / CRP >30.

ASUC: ≥6 BMs + ≥1 feature of systemic upset.

UC · Treatment Ladder

Topical for distal · systemic for extensive

Severity	Induction	Maintenance
Mild	PO / PR 5-ASA (PO extensive · PR proctitis) · steroid suppository / enema (proctitis › suppository · left-sided › enema) · MM budesonide	PO / PR 5-ASA · MM budesonide
Moderate	PO / PR 5-ASA · MM budesonide · PO glucocorticoids · biologics (infliximab, adalimumab, golimumab, vedolizumab, ustekinumab, mirikizumab, risankizumab, guselkumab) · small molecules (tofacitinib, upadacitinib, ozanimod, etrasimod)	PO / PR 5-ASA · AZA / 6-MP · biologics · small molecules
Severe	PO / IV glucocorticoids · cyclosporine · biologics · small molecules	Biologics · small molecules

Unresponsive to 5-ASA › infliximab
Targets: clinical remission · biochemical remission · endoscopic remission

Extra-intestinal Manifestations

Peripheral arthritis · sacroiliitis · ankylosing spondylitis · erythema nodosum · pyoderma gangrenosum · aphthous ulcers · uveitis · episcleritis · PSC (UC ↑ > CD).

Pre-Biologic / Small Molecule Workup

Universal: HCV · HBV · PPD · Strongyloides serology.

Thiopurine › TPMT level. JAKi / S1PR › βhCG. JAKi › VZV titer + lipid profile. S1PR › ECG + ophthalmology assessment.

BCG vaccinated › CXR and/or IGRA. VZV vaccine as indicated.

JAKi CI: high VTE risk · pregnancy · malignancy.

Thiopurine SE: myelosuppression · infections · malignancy (hepatosplenic T-cell lymphoma).

IBD Flare · Approach

Rule out infection · Steroids · Rescue · Surgery

1. Workup: C. diff · stool C&S · stool O&P · colonoscopy + biopsy (CMV: owl-eye inclusion bodies) · AXR (toxic megacolon)

2. Resuscitate: IVF · bowel rest · VTE prophylaxis · DC immunosuppression · DC NSAIDs · DC antidiarrheals · DC mesalamine · DC opioids

3. IV steroids after ruling out infection · septic › antibiotics

4. CD › assess for abscess · fistula · perianal disease

5a. Response at 72 h › transition to PO steroids

5b. Minimal response at 72 h › infliximab or cyclosporine

5c. Minimal response by day 5–7 › colectomy

Response = <4 BMs/day × 2 d + no rectal bleeding

Surgery Indications · Flare

Surgery indications: refractory colitis · toxic megacolon · perianal / intra-abdominal abscess · complex fistulizing CD · perforation · severe refractory hemorrhage.

CRC Surveillance

Colonoscopy: 8 y after diagnosis or at diagnosis of PSC › every 1–5 y.

Toxic Megacolon · Criteria

TC diameter >6 cm + ≥3 of (T >38°C · HR >120 · N# >10.5 · anemia) + ≥1 of (dehydration · altered sensation · electrolyte disturbance · hypotension).

Microscopic Colitis

Relapsing / remitting watery diarrhea · mild abdominal pain · weight loss · women aged 45–60y.

Diagnosis: colonoscopy + biopsy › lamina propria inflammation (lymphocytic vs collagenous).

Risk factors: smoking · medications (NSAIDs · SSRIs · PPIs) · autoimmune (celiac · thyroid · RA).

Treatment: DC causative meds + smoking cessation + loperamide. Moderate / severe › budesonide › PO 5-ASA · bismuth salicylate.

03

Gastrointestinal Bleeding

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Transfusion & Reversal Targets

Hb

>70 · >80 if symptomatic / IHD / unstable

Platelets

>50 if actively bleeding / unstable

NSAIDs / ASA 1° px

Stop

Stable

Continue ASA 2° prevention · no anticoagulation reversal

Life-threatening

Hypovolemic shock · ↓ Hb >50 · transfusion ≥5 U PRBC › PCC (warfarin · anti-Xa) · idarucizumab (dabigatran). Restart anticoagulation after hemostasis.

Recent ACS / PCI

Restart P2Y12i within 5 d

UGIB · Clinical Reading

Hematemesis · melena · major / rapid › hematochezia (clues: melena · liver disease · hemodynamic instability · ↑ BUN:Cr).

Causes: PUD · varices · Mallory-Weiss tear · NSAIDs · H. pylori · aortoenteric fistula.

Glasgow-Blatchford Risk Stratification

Hb · BUN · SBP · HR · melena / syncope / CHF / liver disease › very low risk 0–1 › out-patient.

High risk: tachycardia · hypotension · age >60y · major comorbidities.

High re-bleed EGD features: active bleeding · non-bleeding visible vessels · adherent clots.

Variceal UGIB · Management

Pharmacology + endoscopy + prophylaxis

1. IV PPI + IV vasoactive (octreotide · somatostatin · terlipressin · vasopressin) — continue 2–5 d

2. Cirrhotic › IV antibiotics (ceftriaxone 1 g OD × 5 d)

3. EGD + EVL within ≤12 h

4. Secondary prevention: NSBBs (carvedilol · propranolol · nadolol)

5. Refractory › TIPS

Non-Variceal UGIB · Management

PPI › endoscopy › risk-stratify

1. IV PPI (bolus + infusion or BD)

2. EGD ± endoscopic therapy within 12–24 h · high-risk ulcers: Forrest IA · IB · IIA

3. Refractory › repeat EGD ± IR embolization / surgery

4. Tumor bleeds › radiation

Post-Endoscopy PPI Strategy

High-risk ulcers › IV PPI BD / infusion × 72 h › PO PPI BD ≥2 weeks.

Low-risk ulcers › PO PPI BD × 2–4 weeks.

Test & treat H. pylori.

Long-term PPI Indications

Long-term PPI: APT · NSAIDs · anticoagulants · glucocorticoids.

Hematochezia › colonoscopy
Unstable / refractory › CTA + embolization

Diverticular

Diverticular Bleed

Painless · self-limited · massive hematochezia.

Neoplastic

Tumor / Polyp / Angiodysplasia

Chronic blood loss · acute painless hematochezia · older adult.

Vascular

Colonic Ischemia

Vascular disease · older adult · LLQ abdominal pain / tenderness · sudden cramping pain › hematochezia.

Sources: cardio-embolic · athero-embolic · shock · vasculitis.

Heyde

Heyde Syndrome

Angiodysplasia + aortic stenosis.

Aortoenteric

Aortoenteric Fistula

Post aortic-aneurysm repair.

Congenital

Meckel Diverticulum

Painless hematochezia · young patient · normal EGD / colonoscopy.

Hereditary

HHT

Mucocutaneous telangiectasias.

Other

Etcetera

Cancer · ulcers · hemorrhoids · anal fissure · radiation proctitis · infection · IBD.

Overt or occult · anemia › EGD ± push enteroscopy + colonoscopy
Most common etiology: angiodysplasia
Stable › capsule endoscopy · CT enterography (if RF strictures)
Unstable › CTA
Further: double-balloon enteroscopy · surgery · IR embolization

04

Esophageal Disorders

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Features: heartburn · regurgitation · PPI-responsive
Lifestyle: weight loss · avoid trigger foods · smoking cessation · elevate HOB
› PO PPI OD × 8 weeks › try to wean
Nocturnal symptoms › anti-H2

EGD Indications

EGD indications: atypical symptoms · red flags (dysphagia · weight loss · GI bleeding · anemia · vomiting · odynophagia) · refractory to PPI.

Normal EGD › 24 h pH monitoring.

Extra-Esophageal Symptoms

Chest pain · cough · asthma · laryngitis · dental erosions › PFTs · referral (respirology / ENT / dentist) › 24 h pH monitor › PPI.

Surgery Indications

Surgery indications (Nissen fundoplication · magnetic sphincter · Roux-en-Y if obese · endoscopic fundoplication): confirmed GERD (EGD or pH) + normal manometry · refractory symptoms · large hiatal hernia · severe esophagitis.

Barrett's Esophagus

Metaplasia: squamous epithelium → columnar epithelium ≥1 cm proximal to GEJ. EGD + biopsy › columnar metaplasia, goblet cells. Increased risk of esophageal adenocarcinoma.

Risk factors: chronic GERD · age >50y · male · smoking · obesity · Caucasian · FH.

Treatment: smoking cessation + weight loss if overweight + PO PPI OD.

Screening indications: GERD >5y + ≥3 RFs.

Barrett's Surveillance

Frequency by dysplasia grade

Grade	Surveillance
No dysplasia	EGD `3–5 yearly`
LGD	Dysplasia › EET (EMR + RFA / cryotherapy) · EGD at 1 y › 3 y › 3–5 yearly
HGD	EET · EGD at `3 months` › `6 months` › `12 months` › yearly + biopsies

Dysphagia · Decision Tree

Localize the symptom · choose the test

Oropharyngeal (CVA · Parkinson's · muscular) › difficulty initiating swallowing · coughing · choking · nasal regurgitation · drooling · symptoms above sternal notch › video fluoroscopic swallowing study › dietary adjustment + speech therapy

Esophageal (mechanical obstruction · motility disorder) › food "stuck" below sternal notch · regurgitation › EGD ± biopsy (eosinophilic esophagitis)

Structural (stricture · ring · tumor) › solids · progressive (stricture · carcinoma) · intermittent (web · Schatzki ring)

Motility (achalasia · spasm) › liquids ± solids › manometry

Red flags: age >50y · weight loss · odynophagia · vomiting · GI bleeding › EGD

Zenker: pharyngoesophageal diverticulum › dysphagia + late regurgitation of undigested food › cricopharyngeal myotomy

Achalasia

Progressive dysphagia to solids + liquids · regurgitation · chest pain · weight loss · PPI-resistant · age 30–60y. Megaesophagus · sigmoid esophagus · ↑ risk esophageal SCC.

Causes: idiopathic · viral · autoimmune · neurodegenerative · Chagas.

Diagnostics: EGD › manometry (LES impaired relaxation · abnormal peristalsis). Barium swallow › bird's beak.

Good surgical candidate › laparoscopic Heller myotomy · peroral endoscopic myotomy (POEM) · pneumatic dilatation
Non-surgical › endoscopic botox injection › CCB · nitrates
Megaesophagus / sigmoid esophagus / surgery failure › esophagectomy

Esophagitis Subtypes

Match the cause to the treatment

Type	Clues	Treatment
Candida	Oral candidiasis	PO fluconazole or itraconazole
HSV	Ulcerations	Acyclovir · famciclovir · valacyclovir
CMV	Ulcerations	Ganciclovir · valganciclovir
Pill-induced	Severe substernal chest pain with swallowing. doxycycline · NSAIDs · KCl · iron · alendronate	Stop offending agent
Eosinophilic (EoE)	Male · age `30–44y` · atopy · food allergies · autoimmune disease › dysphagia · food impaction · reflux. EGD › edema · trachealization · exudates · furrows · strictures. Biopsy › eosinophilic infiltration.	Diet elimination · PO PPI BD · topical steroids › dupilumab

Strictures › endoscopic dilatation

Surgical emergency

Boerhaave Syndrome

Spontaneous esophageal rupture.

CXR › pneumomediastinum · subcutaneous emphysema.

CTD

Systemic Sclerosis

Esophageal dysmotility › lifestyle changes · anti-H2 · PPI.

05

Stomach & Duodenum Disorders

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H. pylori · Testing

Tests: urea breath test · stool antigen · biopsy.

Risk regions: Africa · South America · Asia. Chronic H. pylori › atrophic gastritis › gastric intestinal metaplasia › gastric adenocarcinoma.

Testing indications: PUD · MALT lymphoma · dyspepsia · long-term NSAIDs / ASA · unexplained IDA · ITP · household members of +ive HP · 1° / 2° gastric adenocarcinoma prevention.

Repeat testing: ≥4 weeks after eradication + ≥2 weeks after PPI.

False -ive: PPI · antibiotics · atrophic gastritis · intestinal metaplasia · MALToma.

H. pylori · Eradication Regimens

All ×14 days

Regimen	Components
PBMT	PPI + bismuth + metronidazole + tetracycline
PAR	PPI + amoxicillin + rifabutin
PAL	PPI + amoxicillin / metronidazole + levofloxacin

Gastric Adenocarcinoma

RF: FH · atrophic gastritis · intestinal metaplasia / dysplasia · autoimmune gastritis · gastric polyps · East Asian · Latino · Black · American Indian · Alaska Native.

Surveillance: EGD 3-yearly.

Peptic Ulcer Disease

Features: epigastric pain · worse during fasting · improves with food / antacid / PPI › EGD + H. pylori testing › PO PPI OD ± H. pylori eradication.

Causes: H. pylori · NSAIDs · cocaine · methamphetamine · bisphosphonates · SSRIs · smoking · alcohol · stress.

Complications:

Perforation: severe sudden pain · shock · peritonitis › CT abdomen
Gastric outlet obstruction: nausea · vomiting · early satiety · distention · succussion splash
Bleeding: hematemesis · melena · hematochezia · IDA. H. pylori biopsy -ive › urea breath test or stool antigen

DC NSAIDs · continue ASA 2° prevention. NSAIDs needed › COX-2 + PPI.

Repeat EGD indications: persistent symptoms · ulcer size >2 cm · ulcer of unknown cause.

Non-Ulcer Dyspepsia

No GERD or PUD · bothersome postprandial fullness · early satiety · epigastric burning.

Workup: H. pylori testing · celiac screen · abdominal imaging · gastric emptying studies.

Treatment: PPI × 4–6 weeks › buspirone. DC causative drugs (NSAIDs · antibiotics · bisphosphonates · K supplements).

EGD indications: age ≥60y · red flags · FH gastric cancer · Asian · Russian · South American.

Motility

Gastroparesis

Nausea · vomiting · abdominal pain · early satiety · bloating · weight loss.

Causes: systemic sclerosis · DM · hypothyroidism · anticholinergics · opioids · post-viral.

Workup: EGD (rule out gastric outlet obstruction) › gastric scintigraphy or gastric emptying carbon breath testing.

Diabetic › tight glycemic control. Tx: small / frequent / low-fat meals ± metoclopramide (SE: dystonia · dyskinesia).

Autoimmune

Autoimmune Gastritis

› Pernicious anemia · iron deficiency · SIBO · gastric neuroendocrine tumors.

› B12 supplementation + iron replacement + EGD 3-yearly.

Neuroendocrine

GI NETs

RF: Zollinger-Ellison (gastrinoma) · atrophic gastritis.

Allergic

Eosinophilic Gastritis

› Elimination diet.

Dumping Syndrome · Early vs Late

Post gastric surgery

Type	Timing	Features	Treatment
Early	`≤30 min`	Palpitations · flushing / pallor · diaphoresis · lightheadedness · hypotension · fatigue · diarrhea · nausea · bloating · cramping · borborygmus	Small / frequent / high-protein / high-fiber / low-carbohydrate meals. Late dumping › acarbose. Confirm with oral glucose challenge.
Late	`1–3 h`	Hypoglycemia · ↓ concentration · faintness · altered consciousness · protein-wasting malnutrition

06

Pancreatic Disorders

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Acute Pancreatitis · 2 of 3 Criteria

Characteristic abdominal pain (acute severe epigastric / LUQ · radiates to back · improves leaning forward) · lipase / amylase ≥3× ULN · characteristic imaging findings.

Initial workup: US abdomen.

Resuscitation: IVF RL 1.5 mL/kg/h × 12–24 h + enteral nutrition (low-fat) within 24–48 h.

Causes: gallstones · alcohol · hypertriglyceridemia (>11) · hypercalcemia · GLP1RA · 5-ASA · thiazides · 6-MP / AZA · steroids · sulfonamides · estrogens · didanosine · valproate · pentamidine · furosemide · autoimmune · ERCP · trauma · malignancy · hereditary · smoking · scorpion venom · vasculitis · idiopathic.

Poor prognosis: altered mental status · pulmonary infiltrates · pleural effusion · SIRS · ↑ BUN / Hct · age >55y · obesity · comorbidities.

No improvement at 48–72 h or uncertain › CT abdomen.

Acute Pancreatitis · Etiology-Specific Care

Gallstones + cholangitis / jaundice › ERCP ≤24 h + rectal NSAID + pancreatic duct stent › cholecystectomy before discharge

Hypertriglyceridemia › insulin infusion + NPO ± plasmapheresis › long-term fibrates + low-fat diet

Acute Pancreatitis · Complications Timeline

Window	Complications
Early `<4 wk`	Interstitial edematous pancreatitis (IEP) · acute peripancreatic fluid collection · pancreatic necrosis · acute necrotic collection · infected pancreatic necrosis
Late `≥4 wk`	Pseudocysts · abscess · walled-off pancreatic necrosis

Pseudocyst Drainage Indications

Drainage indications: severe symptoms · infection.

Autoimmune Pancreatitis · Two Faces

Type	Demographics	Associations	Lab clue
Type I	Male · age `>60y`	Sjögren · PSC · bile duct strictures · autoimmune thyroiditis · retroperitoneal fibrosis · sclerosing sialadenitis · interstitial nephritis	↑ IgG4
Type II	Age `>40y`	Chronic pancreatitis · IBD	—

Autoimmune pancreatitis: abdominal pain · obstructive jaundice › prednisone 40 mg OD × 4–6 weeks then taper
Imaging: CT / MRCP › narrowed main pancreatic duct · parenchymal swelling (sausage pancreas)
Histology: EUS + biopsy › lymphoplasmacytic infiltrate · fibrosis
Relapse › retrial steroids or AZA or rituximab

Chronic Pancreatitis

Severe abdominal pain · fat-soluble vitamin deficiency (ADEK) · steatorrhea · maldigestion · osteoporosis · weight loss · DM.

Imaging: CT pancreas › pancreatic calcifications. CT -ive › MRI / MRCP / EUS.

Causes: chronic alcohol · hypertriglyceridemia · smoking · hypercalcemia · medications · toxins · idiopathic · CF · genetic (PRSS1 · CTRC) · autoimmune · recurrent AP · pancreatic divisum · stricture · stone · tumor.

Young adult › sweat chloride test (CF). Exocrine status › fecal elastase.

Management: alcohol / smoking cessation. Exocrine insufficiency › pancreatic enzymes. Pain › stepwise › celiac plexus block › surgery. Obstructive › ERCP · EUS. End-stage › total pancreatectomy + islet cell autotransplant.

Pancreatic Cancer

Abdominal pain · back pain · weight loss · jaundice · new-onset DM › CT pancreas › EUS + biopsy.

RF: age >50y · smoking · obesity · chronic pancreatitis · mucinous cystic lesions · FH · genetic (BRCA1/2 · ATM · PALB2 · Lynch) · T2DM · high red meat consumption · excess alcohol · FH ≥2 1° degree relatives.

Screening in high-risk age ≥50y: annual EUS or MRCP.

07

Diarrhea, Constipation & IBS

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IBS · Rome IV Criteria

Recurrent abdominal pain ≥1/week ≤3 months + 2 of: defecation-related pain · change in stool frequency · change in stool consistency.

Workup: tTG-IgA + IgA (celiac) · Giardia · fecal calprotectin (IBD).

Colonoscopy indications: age >50y · red flags · refractory.

First line for IBS: low-FODMAP diet, avoid gluten / dairy, and add soluble fiber (psyllium). Abdominal pain › dicyclomine · peppermint oil · TCAs.

Constipation-predominant

IBS-C

First: polyethylene glycol (PEG).

Then: linaclotide · plecainatide · tenapanor · lubiprostone.

Diarrhea-predominant

IBS-D

First: loperamide.

Then: rifaximin · eluxadoline (CI: hepatobiliary dysfunction · excess alcohol) · alosetron (SE: ischemic colitis).

Chronic Diarrhea · Approach

≥4 weeks › colonoscopy + biopsies ± EGD (CMV · TB · Whipple's · microscopic colitis · colon cancer · VIPoma · gastrinoma · carcinoid).

Osmotic: associated with eating · improves with fasting · not nocturnal › lactase deficiency typical.

Secretory: large-volume · watery · nocturnal BMs · unchanged by fasting.

Fecal osmotic gap = 290 − (2 × [stool Na + stool K]) · >100 osmotic · <50 secretory.

Chronic Diarrhea · Targeted Workup

Picking the right lab for the right pattern

Suspect	Test
IBD	Fecal calprotectin
Laxative abuse	Stool / urine laxative screen
Chronic pancreatitis	Fecal elastase
Celiac	tTG-IgA + IgA
Celiac / SIBO / SBS / pancreatic insufficiency	72 h fecal fat
Hyperthyroidism	TSH
Infection	Stool cultures · C. diff · O&P (Giardia)
Lactose / carb intolerance / SIBO	Hydrogen breath test
Carcinoid	24 h urinary 5-HIAA
DM autonomic neuropathy	HbA1c

Medication-induced: antibiotics · metformin · colchicine · NSAIDs · PPIs · chemotherapy · Mg supplements
Bile acid diarrhea › cholestyramine
Immune checkpoint inhibitor diarrhea › DC medication · glucocorticoids / biologics

Acute Diarrhea · <2 weeks

Pathogens: C. difficile · norovirus · E. coli · cholera · Cryptosporidia (AIDS) · Giardia.

Dysentery: Campylobacter · EHEC · Entamoeba histolytica · Shigella · Salmonella · Yersinia enterocolitica.

Diagnostic testing indications: mucoid / bloody diarrhea · fever · suspected sepsis · immunocompromised · require hospitalization · diarrhea >1 week · severe disease · hemodynamic instability.

Acute Diarrhea · Treatment

Scenario	Treatment
General	Oral hydration · loperamide (CI: dysentery · fever)
Dysentery + T `>38`	Azithromycin
Mild traveller's	Bismuth
Moderate / severe traveller's	Azithromycin
Giardia / Entamoeba histolytica	Metronidazole · tinidazole · nitazoxanide
EHEC	Self-limiting (antibiotics may worsen HUS)

Chronic Constipation · ≥3 months

Stop causative medications. Increase physical activity + dietary fiber.

Colonoscopy indications: age ≥45y · red flags.

Agents: PEG · bisacodyl · sodium picosulfate · linaclotide · plecanatide · prucalopride · psyllium · magnesium salts · senna · lubiprostone · fruit-based laxatives · yogurt with galactooligosaccharide · prunes · linseed oil.

Refractory › combination laxatives (different mechanisms) › prosecretory agents (lubiprostone · linaclotide) or serotonergic (prucalopride).

Pelvic floor dyssynergia › neuromuscular biofeedback.

Refractory to laxatives › colon transit testing · anorectal manometry · balloon expulsion testing · defecography.