Study guide
4.6 Clostridioides difficile & Catheter-Related Bloodstream Infection
ABIM Internal Medicine Exam
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1 Chapter 0: The ABIM Exam, Blueprint & Study Strategy
0.1 Exam Format, Question Types & the 2026 Blueprint 0.2 Eligibility, Registration, Fees & Exam-Day Logistics 0.3 Scoring, the Passing Standard & Score Reports 0.4 Building a Blueprint-Weighted Study Plan
2 Chapter 1: Cardiovascular Disease (14% of Exam)
1.1 Ischemic Heart Disease & Acute Coronary Syndromes 1.2 Antithrombotic Therapy, Secondary Prevention & Mechanical Complications of MI 1.3 Atrial Fibrillation, Atrial Flutter & Stroke Prevention 1.4 Supraventricular Tachycardias, Ventricular Arrhythmias & Bradycardias 1.5 Heart Failure: Classification, Staging & Guideline-Directed Medical Therapy 1.6 Device Therapy, HFpEF & the Cardiomyopathies 1.7 Valvular Heart Disease & Endocarditis Prophylaxis 1.8 Pericardial Disease & Acute Aortic Syndromes 1.9 Hypertension: Classification, Diagnosis & Secondary Causes 1.10 Hypertensive Crises & Dyslipidemia 1.11 Adult Congenital Heart Disease & Coarctation of the Aorta 1.12 Syncope: Reflex, Orthostatic & Cardiac Causes 1.13 Cardiac Physical Examination, Murmurs & Antithrombotic Selection
3 Chapter 2: Endocrinology, Diabetes & Metabolism (9% of Exam)
2.1 Diabetes Mellitus: Diagnosis, Glycemic Targets & Pharmacotherapy 2.2 Insulin Therapy, Hyperglycemic Emergencies & Chronic Complications 2.3 Thyroid Function Testing & Hypothyroidism 2.4 Thyrotoxicosis, Thyroid Storm & Thyroid Nodules 2.5 Adrenal & Pituitary Disorders 2.6 Hypercalcemia, Hypocalcemia & Disorders of Mineral Metabolism 2.7 Osteoporosis & Paget Disease of Bone 2.8 Ovarian, Testicular & Reproductive Endocrine Disorders 2.9 Hypoglycemia, Endocrine Tumors, Polyglandular Syndromes & Severe Hypertriglyceridemia
4 Chapter 3: Gastroenterology & Hepatology (9% of Exam)
3.1 Esophageal Disease 3.2 Stomach & Duodenal Disease and Gastrointestinal Hemorrhage 3.3 Inflammatory Bowel Disease 3.4 Celiac Disease & Irritable Bowel Syndrome 3.5 Diverticular Disease & Colorectal Cancer Screening 3.6 Viral Hepatitis, Steatotic & Alcohol-Associated Liver Disease 3.7 Cirrhosis & the Complications of Portal Hypertension 3.8 Acute Pancreatitis 3.9 Chronic Pancreatitis & Biliary Tract Disease 3.10 Inherited, Autoimmune & Drug-Induced Liver Disease 3.11 Small Intestinal Disease, Ischemia & Gastrointestinal Complications of HIV
5 Chapter 4: Infectious Disease (9% of Exam)
4.1 Lower Respiratory Tract Infections & Sepsis Syndromes 4.2 Viral Respiratory Pathogens & Tuberculosis 4.3 Central Nervous System Infections 4.4 Skin, Soft Tissue, Bone & Joint Infections 4.5 Infective Endocarditis & Cardiovascular Infection 4.6 Clostridioides difficile & Catheter-Related Bloodstream Infection 4.7 HIV Diagnosis, Antiretroviral Therapy & Prevention 4.8 Opportunistic Infections & Sexually Transmitted Infections 4.9 Fever of Unknown Origin, Neutropenic Fever & Toxic Shock 4.10 Travel-Related Illness, Vector-Borne Disease & Outbreaks 4.11 Infection Prevention, Nosocomial Infection & Postexposure Prophylaxis 4.12 Rheumatic Fever, Poststreptococcal Disease, Enteric & Hepatic Infections
6 Chapter 5: Pulmonary Disease (9% of Exam)
5.1 COPD: Diagnosis, GOLD Classification & Initial Pharmacotherapy 5.2 Interventions That Reduce COPD Mortality & Exacerbation Management 5.3 Asthma: Diagnosis, Stepwise Therapy & Status Asthmaticus 5.4 Interstitial Lung Disease & Idiopathic Pulmonary Fibrosis 5.5 Sarcoidosis, Hypersensitivity Pneumonitis & Occupational Lung Disease 5.6 Pulmonary Embolism: Diagnosis, Risk Stratification & Anticoagulation 5.7 Pulmonary Hypertension & Sleep-Disordered Breathing 5.8 Acute Respiratory Distress Syndrome & Mechanical Ventilation 5.9 Pleural Disease, Empyema & Pneumothorax 5.10 Bronchiectasis, Cystic Fibrosis & Restrictive Lung Disease 5.11 Evaluation of Cough, Dyspnea, Hemoptysis & the Solitary Pulmonary Nodule
7 Chapter 6: Rheumatology & Orthopedics (9% of Exam)
6.1 Rheumatoid Arthritis: Diagnosis & Systemic Manifestations 6.2 Antirheumatic Pharmacotherapy & the Spondyloarthropathies 6.3 Gout: Pathophysiology & Acute Management 6.4 Urate-Lowering Therapy & Calcium Pyrophosphate Deposition Disease 6.5 Osteoarthritis 6.6 Systemic Lupus Erythematosus 6.7 Systemic Sclerosis, Sjogren Syndrome & Inflammatory Myopathies 6.8 The Vasculitides 6.9 Regional Musculoskeletal Syndromes, Back Pain & Sports Injuries 6.10 Other Primary Rheumatic Disorders & Mixed Connective Tissue Disease 6.11 Osteonecrosis & Metabolic Bone Disease
8 Chapter 7: Hematology (6% of Exam)
7.1 Approach to Anemia & the Microcytic Anemias 7.2 Macrocytic, Normocytic & Hemolytic Anemias 7.3 Hemoglobinopathies, Thalassemias & Paroxysmal Nocturnal Hemoglobinuria 7.4 Platelet Disorders, Thrombocytopenia & the Bleeding Patient 7.5 Coagulation Factor Disorders, Thrombophilia & Anticoagulant Reversal 7.6 Acute Leukemias, Chronic Leukemias & Myeloproliferative Neoplasms 7.7 Lymphomas & Plasma Cell Disorders 7.8 Myelodysplastic Syndrome, Myeloproliferative Neoplasms & Porphyria 7.9 Transfusion Medicine & Late Complications of Transplantation
9 Chapter 8: Medical Oncology (6% of Exam)
8.1 Breast, Lung, Colorectal & Prostate Cancer 8.2 Oncologic Emergencies 8.3 Genitourinary & Gynecologic Malignancies 8.4 Head and Neck, Bone & Soft Tissue Cancer and Cancer of Unknown Primary 8.5 Complications of Cancer & Its Treatment 8.6 Cancer Prevention, Survivorship & Psychosocial Care
10 Chapter 9: Nephrology & Urology (6% of Exam)
9.1 Acute Kidney Injury: Staging, Etiology & Urine Microscopy 9.2 Glomerular Disorders & Nephrotic Syndrome 9.3 Nephritic Syndromes, RPGN & Indications for Renal Replacement Therapy 9.4 Chronic Kidney Disease: Staging & Progression Retardation 9.5 Complications of Chronic Kidney Disease: Anemia, CKD-MBD & Acidosis 9.6 Hyponatremia, Hypernatremia & Water Balance 9.7 Potassium Disorders & Systematic Acid-Base Analysis 9.8 Renal Tubular Acidosis & Nephrolithiasis 9.9 Prostate Disorders, Urinary Incontinence & Other Urologic Conditions 9.10 Cystic Kidney Disease, Obstructive Uropathy, Tubulointerstitial Disease & Hematuria
11 Chapter 10: Neurology (4% of Exam)
10.1 Acute Ischemic Stroke: Syndromes & Reperfusion 10.2 Transient Ischemic Attack, Secondary Prevention & Hemorrhagic Stroke 10.3 Headache & Facial Pain 10.4 Seizures, Epilepsy & Movement Disorders 10.5 Multiple Sclerosis & Neuromuscular Junction Disorders 10.6 Motor Neuron Disease & Acute Peripheral Neuropathy 10.7 Cranial Neuropathies & Neuro-Ophthalmology 10.8 Peripheral Neuropathies, CNS Tumors & Other Neurologic Disorders
12 Chapter 11: Psychiatry & Behavioral Health (4% of Exam)
11.1 Major Depressive Disorder & Medical Mimics 11.2 Antidepressant Pharmacotherapy & Treatment-Resistant Depression 11.3 Bipolar Disorder, Mood Stabilizers & Lithium Toxicity 11.4 Anxiety, Stress-Related & Obsessive-Compulsive Disorders 11.5 Alcohol Use Disorder & Withdrawal 11.6 Pharmacotherapy for Alcohol & Opioid Use Disorder 11.7 Toxidromes, Psychiatric Emergencies & Suicide Risk Assessment 11.8 Psychotic, Somatic Symptom, Factitious & Personality Disorders 11.9 Eating Disorders, Sleep Disorders & Sexual Health
13 Chapter 12: Dermatology (3% of Exam)
12.1 Dermatitis, Papulosquamous & Inflammatory Dermatoses 12.2 Acne, Rosacea & Cutaneous Infections 12.3 Herpes Zoster & Autoimmune Blistering Diseases 12.4 Nodules, Tumors & Cutaneous Oncology 12.5 Dermatologic Emergencies & Severe Cutaneous Adverse Reactions 12.6 Pigment Disorders, Alopecia, Nail Disease & Photosensitivity 12.7 Vascular Dermatoses, Environmental Injury & Cutaneous Signs of Systemic Disease
14 Chapter 13: Obstetrics & Gynecology / Women's Health (3% of Exam)
13.1 Contraception, Preconception Care & Teratogenicity 13.2 Hypertensive Disorders of Pregnancy & Gestational Diabetes 13.3 Thyroid Disease & Venous Thromboembolism in Pregnancy 13.4 Menopause & Menopausal Hormone Therapy 13.5 Abnormal Uterine Bleeding, Endometrial Disease & Gynecologic Cancer Screening 13.6 Breast Disorders & Diseases of the Vulva and Vagina 13.7 Menstrual Disorders, Fibroids, Endometriosis & Early Pregnancy Complications
15 Chapter 14: Geriatric Syndromes (3% of Exam)
14.1 Comprehensive Geriatric Assessment & Frailty 14.2 Falls, Gait Assessment & Polypharmacy 14.3 Urinary Incontinence & Pressure Injuries in Older Adults 14.4 Mild Cognitive Impairment, Dementia & Its Subtypes 14.5 Delirium & the Pharmacotherapy of Cognitive Disorders 14.6 Palliative Care, Hospice & Advance Care Planning 14.7 Chronic Wounds, Nutrition, Swallowing, Constipation & Hip Fracture 14.8 Sensory Impairment, Dizziness, Elder Mistreatment & Care Settings
16 Chapter 15: Allergy & Immunology (2% of Exam)
15.1 Anaphylaxis: Recognition & Emergency Management 15.2 Urticaria & Angioedema 15.3 Drug Allergy, Desensitization & Adult Immunodeficiency 15.4 Food Allergy, Insect Venom Hypersensitivity, Serum Sickness & Contrast Reactions 15.5 Rhinitis, Conjunctivitis, Eosinophilic & Mast Cell Disorders
17 Chapter 16: Preventive Medicine, Immunization & Clinical Epidemiology
16.1 The USPSTF Framework & Evidence-Based Cancer Screening 16.2 Cardiovascular & Metabolic Screening and Primary Prevention 16.3 Adult Immunization & Vaccines in Special Populations 16.4 Clinical Epidemiology, Biostatistics & Study Design
18 Chapter 17: Miscellaneous - Ethics, Patient Safety & Quality Improvement (2% of Exam)
17.1 Principles of Bioethics & Decisional Capacity 17.2 Informed Consent, Treatment Refusal & Special Scenarios 17.3 Surrogate Decision-Making, Advance Directives & Confidentiality 17.4 Patient Safety Science, Medical Error & Diagnostic Reasoning 17.5 Quality Improvement Methods, Health Disparities & Health Equity
19 Chapter 18: Perioperative Medicine & Medical Consultation
18.1 Preoperative Cardiovascular Risk Assessment 18.2 Preoperative Pulmonary Risk & Perioperative Medication Management 18.3 Perioperative Anticoagulation, Bridging & Postoperative Complications
20 Chapter 19: Ophthalmology (1% of Exam)
19.1 The Acute Red Eye & Ophthalmic Emergencies 19.2 Painless Vision Loss & Chronic Ophthalmic Disease 19.3 Uveitis, Scleritis & Orbital Disease
21 Chapter 20: Otolaryngology & Dental Medicine (1% of Exam)
20.1 Dizziness, Vertigo & Epistaxis 20.2 Ear Disorders, Hearing Loss & Upper Respiratory Infections 20.3 Oral Cavity, Salivary Gland, Laryngeal & Taste and Smell Disorders
4.6 Clostridioides difficile & Catheter-Related Bloodstream Infection
Key Takeaways Explain This Topic AI
Antibiotic colitis is enumerated under colonic and anorectal disease, while nosocomial and procedure- or device-associated infections are separate blueprint subsections.
Only diarrheal stool should be tested for Clostridioides difficile, because testing formed stool detects colonization rather than disease.
Fulminant disease with hypotension, ileus or megacolon requires combined oral and intravenous therapy plus urgent surgical consultation.
Fecal microbiota-based therapy is used for multiply recurrent rather than first-episode infection.
Catheter removal is mandatory for bloodstream infection due to Staphylococcus aureus, Candida species, Pseudomonas, or when there is tunnel infection, septic thrombosis or persistent bacteremia.
Last updated: August 2026
1. Clostridioides difficile Infection (CDI)
Clostridioides difficile is an anaerobic, spore-forming, toxin-producing Gram-positive bacillus that is the leading cause of healthcare-associated infectious diarrhea.
Pathogenesis & Risk Factors
Microbiota Disruption: Exposure to broad-spectrum antimicrobials ( Clindamycin, Fluoroquinolones, 3rd/4th generation Cephalosporins, Carbapenems ) disrupts normal anaerobic colonic flora, allowing ingested spores to germinate into vegetative bacilli.
Toxins: Production of Toxin A (enterotoxin) and Toxin B (cytotoxin) causes actin depolymerization, mucosal inflammation, epithelial cell necrosis, and the formation of characteristic inflammatory pseudomembranes composed of neutrophils, mucin, and cellular debris.
Risk Factors: Recent antibiotic use (within past 90 days), advanced age (>=65 years), hospitalization, proton pump inhibitor (PPI) therapy, chronic kidney disease, and gastrointestinal surgery.
Diagnostic Testing Strategy
Who to Test: Only test patients with unexplained, new-onset diarrhea (>=3 unformed stools in 24 hours) . Never test asymptomatic patients (high rate of asymptomatic colonization).
Laboratory Algorithm:
Multi-step testing combines Glutamate Dehydrogenase (GDH) antigen screening (high sensitivity) or NAAT / PCR for toxin genes with Enzyme Immunoassay (EIA) for Toxins A and B (high specificity).
GDH (+) / Toxin EIA (+) confirms active, toxin-producing C. difficile disease.
GDH (+) / Toxin EIA (-) represents either low toxin levels or asymptomatic colonization; clinical correlation or reflex PCR is required.
Staging and Management (IDSA/SHEA Guidelines)
Clinical Category Clinical & Laboratory Criteria Preferred First-Line Treatment Alternative Regimens
Initial Episode: Non-Severe • WBC count <= 15,000/mcL AND ; • Serum Creatinine < 1.5 mg/dL • Oral Fidaxomicin 200 mg PO BID for 10 days (preferred due to lower recurrence rates) • Oral Vancomycin 125 mg PO QID for 10 days.; (Oral Metronidazole is no longer recommended as first-line therapy).
Initial Episode: Severe • WBC count > 15,000/mcL OR ; • Serum Creatinine >= 1.5 mg/dL • Oral Fidaxomicin 200 mg PO BID for 10 days • Oral Vancomycin 125 mg PO QID for 10 days.
Fulminant CDI • Hypotension, septic shock, ileus, or toxic megacolon (colonic diameter >6 cm with systemic toxicity) • High-Dose Oral Vancomycin 500 mg PO (or via NG tube) QID PLUS ; • IV Metronidazole 500 mg IV every 8 hours.; • If ileus is present: Add Vancomycin Rectal Enema (500 mg in 100 mL normal saline per rectum q6h as retention enema).; • Emergent Surgical Consultation for diverting loop ileostomy with colonic vancomycin lavage or subtotal colectomy. N/A (medical emergency requiring dual/triple therapy plus surgical consult).
First Recurrence • Recurrence of symptoms within 2–8 weeks of completing therapy • If Vancomycin was used for initial episode: Oral Fidaxomicin 200 mg BID x 10 days.; • If Fidaxomicin was used initially: Tapered and pulsed Oral Vancomycin regimen.; • Consider adding Bezlotoxumab (monoclonal antibody against Toxin B; 10 mg/kg IV single infusion; use with caution in congestive heart failure). • Tapered/pulsed Vancomycin (125 mg QID x 10-14d, BID x 7d, daily x 7d, then every 2-3 days for 2-8 weeks).
Second or Subsequent Recurrence (>=2 Recurrences) • Multiple failed courses of standard antimicrobial regimens • Fecal Microbiota Transplantation (FMT) delivered via colonoscopy or retention enema OR ; • FDA-Approved Live Biotherapeutic Products: Vowst (oral fecal microbiota spores live-brpk capsules) or Rebyota (fecal microbiota live-jslm rectal suspension) administered after completing antibiotic course. • Prolonged oral Vancomycin taper followed by oral Rifaximin chaser.
2. Catheter-Related Bloodstream Infections (CRBSI)
Central line-associated bloodstream infections (CLABSI) and catheter-related bloodstream infections (CRBSI) represent major sources of hospital-acquired bacteremia and septic shock.
Pathogenesis & Microbiology
Extraluminal Route (<2 weeks of placement): Skin microorganisms migrate along the external catheter surface into the bloodstream.
Intraluminal Route (>2 weeks of placement): Contamination of the catheter hub, connectors, or IV infusate leading to intraluminal biofilm colonization.
Pathogens: Coagulase-Negative Staphylococci (30–40%), Staphylococcus aureus (MSSA/MRSA ~20%), Candida species (~10–15%), Enteric Gram-negative bacilli ( Klebsiella, E. coli ), and Pseudomonas aeruginosa .
Diagnostic Criteria for CRBSI
Differential Time to Positivity (DTP): Blood cultures drawn simultaneously from the central venous catheter and a peripheral venipuncture show bacterial growth detected >=2 hours earlier in the catheter sample compared to the peripheral sample.
Paired Quantitative Blood Cultures: A colony count >=3-fold higher from the catheter blood sample compared to the peripheral blood sample.
Central Venous Catheter Removal Indications
Pathogen-Specific Indications: Catheter removal is mandatory for infections caused by: Staphylococcus aureus , Candida species , Pseudomonas aeruginosa , multi-drug resistant Gram-negative bacilli, or Mycobacterium spp.
Complications of Infection: Septic shock, hemodynamic instability, suppurative thrombophlebitis, endocarditis, metastatic seeding (e.g., osteomyelitis), or persistent bacteremia >72 hours after initiation of active antimicrobial therapy.
Local Signs: Severe tunnel infection or port pocket abscess.
Management Rules for Specific Pathogens
Staphylococcus aureus Catheter Infection: Always remove the catheter. Minimum 2 weeks of IV bactericidal therapy (Cefazolin for MSSA, Vancomycin/Daptomycin for MRSA) is required for uncomplicated bacteremia (criteria: catheter removed, negative follow-up blood cultures at 48–72 hours, no prosthetic material, normal defervescence <72 hours, and transesophageal echocardiogram [TEE] negative for endocarditis ). If any criterion is unmet, treat for 4 to 6 weeks .
Candida Catheter Infection: Always remove the catheter. Initiate an Echinocandin (Caspofungin 70 mg load then 50 mg daily, Micafungin 100 mg daily, or Anidulafungin) for 14 days after the first negative blood culture. Mandatory step: Perform a dilated funduscopic ophthalmologic examination in all patients with candidemia within the first week to rule out Candida endophthalmitis.