Updated: 5/27/2019

Irritable Bowel Syndrome

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  • A 20-year-old woman presents to the clinic complaining of intermittent abdominal pain. She reports that the pain has been present for as long as she can remember and would improve after she defecates or when she is relaxed. It is especially worse now as she is feeling stressed because of final exams. Her past medical history is significant for an extensive gastrointestinal workup (e.g., fecal leukocytes, stool culture, fecal RBCs, and colonoscopy) with no abnormalities. A physical examination demonstrates an anxious patient with unremarkable findings.
  • Clinical definition
    • idiopathic functional gastrointestinal disorder characterized by chronic abdominal pain and altered bowel habits
    • patients often present as teens or in their 20’s
  • Epidemiology
    • demographics
      • prevalence in North America estimated to be around 10-15%  
      • higher prevalence in women (14%) compared to men (9%)
  • Pathogenesis
    • pathophysiology is uncertain as it is likely a disorder resulting from interaction from several factors
      • gastrointestinal tract motor abnormalities (e.g., abnormal transit time) are detectable in some patients
      • hypersensitization of visceral afferent nerves in the gut
      • alterations in particular immune cells and markers
      • suggested genetic susceptibility
      • psychosocial factors may influence the expression of symptoms
  • Associated conditions
    • 50% of patients have comorbid psychiatric disease (e.g., major depression)
    • fibromyalgia
    • chronic fatigue syndrome
    • GERD and functional dyspepsia
  • Symptoms
    • chronic abdominal pain with periodic exacerbations 
      • can be exacerbated with stress or meals
      • may be relieved with defecation
    • non-bloody diarrhea  
    • constipation
  • Physical exam
    • abdominal distension
    • mild abdominal tenderness
  • Diagnostic testing
    • diagnostic approach
      • diagnosis is based on clinical symptoms and the fulfillment of symptom-based diagnostic criteria with a limited workup to exclude organic disease
      • Rome IV criteria for IBS
        • recurrent abdominal discomfort occurring at least 1x/week for 3 months associated with at least 2 of the following
          • related to defecation
          • change in stool frequency
          • change in stool form (appearance)
    • imaging
      • abdominal imaging (e.g., radiograph)
        • done in patients with constipation
        • assess for stool accumulation and determine severity
    • studies
      • laboratory testing
        • done primarily to exclude alternative diagnosis
        • CBC, CRP, and fecal calprotein
        • serologic testing for celiac disease if diarrhea present  
      • colonoscopy  
        • indicated if patients have alarm features
          • age of onset > 50 years
          • rectal bleeding/melena
          • weight loss
          • abnormal CBC
          • family history of inflammatory bowel disease (IBD) or colorectal cancer
  • Celiac disease   
    • distinguishing factors
      • positive serologic tests for celiac disease
  • Inflammatory bowel disease 
    • distinguishing factors
      •  lesions present on colonoscopy
  • First-line
    • lifestyle and dietary modification (e.g., avoidance of gas-producing foods)
      • high fiber and low fat diet
    • psychiatric support with continuity of care
    • laxatives (e.g., polyethylene glycol) are indicated in patients with moderate to severe IBS with constipation
    • antidiarrheal agents (e.g., loperamide) are indicated in patients with moderate to severe IBS with diarrhea
    • antispasmodics (e.g., dicyclomine) are used on a as needed basis for IBS-related abdominal pain
  • Second-line
    • bile acid sequestrants in patients with moderate to severe IBS with diarrhea
    • tricyclic antidepressants for patients with coexisting depression and persistent abdominal pain despite antispasmodic use
  • Dehydration
  • Electrolyte imbalance
  • Bowel impaction
  • Hemorrhoids  

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Questions (1)
Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2

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