1.3 Infection Types & Clinical Syndromes

1.3 Infection Types & Clinical Syndromes

Healthcare-associated infections (HAIs) represent major causes of morbidity, mortality, and extended hospital stay. Infection preventionists (IPs) perform standardized surveillance using Centers for Disease Control and Prevention (CDC) National Healthcare Safety Network (NHSN) criteria to identify clinical syndromes, track infection rates, and direct prevention bundles across acute care and long-term care settings.

1. Primary Bloodstream Infections & CLABSI

A primary bloodstream infection is an intravascular infection originating directly within the bloodstream without an identifiable secondary site of localized infection.

Central Line-Associated Bloodstream Infection (CLABSI) Criteria

• NHSN Surveillance Definition: A Laboratory-Confirmed Bloodstream Infection (LCBI) where an eligible central venous catheter (CVC) or umbilical catheter was in place for >2 calendar days on the date of event (with day of line placement being Day 1), and was present on the date of event or the day before.
• Pathophysiology: Pathogens gain access to the bloodstream via two primary routes:
  1. Extraluminal Migration: Microorganisms on skin migrate along the external surface of the catheter at the insertion site (dominant route for short-term catheters).
  2. Intraluminal Migration: Contamination of hub connections, blood tubing, or infusates allows microbial entry into the internal lumen (dominant route for long-term catheters).
• Common Etiologies: Coagulase-negative Staphylococci (S. epidermidis), Staphylococcus aureus, Enterococcus species (including VRE), Candida species (C. albicans, C. glabrata, C. auris), and Gram-negative bacilli (Pseudomonas aeruginosa, Klebsiella pneumoniae, Enterobacter spp.).

2. Healthcare-Associated & Ventilator-Associated Pneumonia

Healthcare-associated respiratory tract infections represent a primary cause of ICU mortality.

Syndromic Classifications

• Healthcare-Associated Pneumonia (HAP): Pneumonia occurring >= 48 hours after hospital admission in a non-intubated patient.
• Ventilator-Associated Pneumonia (VAP): Pneumonia occurring >48 hours after endotracheal intubation or mechanical ventilation placement.

Diagnostic & Clinical Diagnostic Criteria

Diagnosis requires a combination of clinical, radiological, and microbiological findings:

1. Radiological: New, progressive, or persistent infiltrate, consolidation, or cavitation on chest radiography or CT scan.
2. Clinical Criteria: Fever (>38.0°C or hypothermia <36.0°C), leukocytosis (>= 12,000/uL) or leukopenia (<= 4,000/uL), and new onset of purulent sputum or worsening gas exchange (PaO2/FiO2 ratio decline).
3. Microbiological Sampling: Quantitative or semi-quantitative endotracheal aspirate, bronchoalveolar lavage (BAL), or protected specimen brush (PSB) cultures.

Key Pathogens & Prevention Bundles

VAP and HAP are frequently caused by multidrug-resistant pathogens: Pseudomonas aeruginosa, MRSA, Acinetobacter baumannii, and ESBL-producing Enterobacterales. Prevention relies on the mechanical ventilation bundle: elevation of the head of the bed (30–45 degrees), daily sedation vacations and spontaneous breathing trials, subglottic secretion drainage, and oral care with chlorhexidine or sterile water.

3. Catheter-Associated Urinary Tract Infections (CAUTI)

Urinary tract infections account for a significant proportion of HAIs, almost all attributable to indwelling urethral catheters.

NHSN CAUTI Surveillance Definition

A CAUTI requires meeting all three of the following criteria:

1. Patient had an indwelling urinary catheter in place for >2 calendar days on the date of event.
2. Patient exhibits at least one localized or systemic sign or symptom: fever (>38.0°C), suprapubic tenderness, costovertebral angle pain or tenderness, urinary urgency, frequency, or dysuria.
3. Positive urine culture with >= 10^5 colony-forming units (CFU)/mL of no more than two species of microorganisms.

Asymptomatic Bacteriuria (ASB) vs. True CAUTI

Asymptomatic bacteriuria is the presence of >= 10^5 CFU/mL of bacteria in urine without clinical signs or symptoms of UTI. ASB is extremely common in catheterized patients (occurring at a rate of 3–8% per catheter-day). Critical Clinical Rule: Screening for or treating ASB with antibiotics is strongly contraindicated—except in pregnant women or prior to invasive urologic procedures. Treating ASB does not prevent CAUTI, increases drug resistance, and triggers C. difficile colitis.

Pathogens & Risk Factors

Dominant pathogens include Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis (associated with encrustation and struvite stone formation due to urease production), Enterococcus species, and Pseudomonas aeruginosa. The single most effective CAUTI prevention intervention is reducing catheter use and minimizing catheter duration.

4. Surgical Site Infections (SSI)

SSIs are classified according to the anatomical depth reached by the infection within specified post-operative timeframes.

Prophylaxis & Risk Mitigation

Prophylactic antimicrobial administration must achieve therapeutic tissue levels at incision. Cefazolin is the agent of choice for most procedures, administered within 60 minutes prior to surgical incision (vancomycin and fluoroquinolones require infusion initiation within 120 minutes prior to incision). Prophylaxis should be discontinued within 24 hours post-operatively.

5. Clostridioides difficile Infection (CDI) & CNS Infections

Clostridioides difficile Infection

CDI is a toxin-mediated colonic mucosal disease. Broad-spectrum antimicrobial therapy (especially fluoroquinolones, 3rd/4th generation cephalosporins, clindamycin, and carbapenems) disrupts normal colonic microflora, allowing ingested or resident C. difficile endospores to germinate, proliferate, and produce Toxin A (enterotoxin) and Toxin B (cytotoxin).

• Laboratory Diagnosis: Requires testing only unformed, liquid stools (matching Bristol Stool Chart types 5–7). Diagnostic testing uses a two-step algorithm: a highly sensitive Nucleic Acid Amplification Test (NAAT/PCR) or Glutamate Dehydrogenase (GDH) screening test, paired with a highly specific Enzyme Immunoassay (EIA) for Toxins A/B to differentiate active disease from asymptomatic carriage.
• Infection Control: Requires Contact Precautions in a single-patient room, hand hygiene using soap and water (alcohol sanitizers are ineffective against spores), and environmental disinfection using EPA-registered sporicidal agents (List K).

Central Nervous System (CNS) Infections

Healthcare-associated CNS infections occur primarily following neurosurgical procedures, head trauma, or external ventricular drain (EVD) placement.

• Pathogens: Staphylococcus aureus, Coagulase-negative Staphylococci, Acinetobacter baumannii, and Pseudomonas aeruginosa.
• CSF Analysis: Bacterial meningitis displays neutrophilic pleocytosis (high WBCs), elevated protein (>100 mg/dL), and markedly decreased CSF glucose (<40% of serum glucose).