The WHO estimates that on any given day, approximately 7 per cent of patients in high-income countries and 15 per cent in low-to-middle-income countries have at least one HAI. In the EU alone, HAIs cause approximately 4 million episodes and 37,000 attributable deaths per year. The most common HAIs are healthcare-associated pneumonia (including ventilator-associated pneumonia), urinary tract infections (catheter-associated), bloodstream infections (central line-associated), and surgical site infections.
Understanding how each of these infections develops, which organisms cause them, which risk factors predispose patients, how to prevent them through evidence-based bundles, and how to investigate and control clusters is core knowledge for everyone working in acute healthcare.
Catheter-Associated Urinary Tract Infection (CAUTI)
CAUTI is the most common HAI in most healthcare settings. A urinary catheter provides a direct pathway for bacteria to ascend from the urethral meatus and perineal skin into the bladder. Within days of catheterisation, bacteria adhere to the catheter surface and form biofilms. By 30 days, nearly all long-term catheterised patients have bacteriuria.
Risk factors: female sex (shorter urethra), prolonged catheterisation (greatest modifiable risk factor), diabetes, immunosuppression, poor insertion technique, breaks in the closed drainage system (e.g., disconnecting the catheter from the drainage bag).
Prevention: the most effective prevention is not inserting a catheter unless genuinely indicated (incontinence alone is not an indication), removing it as soon as it is no longer necessary, using aseptic insertion technique, maintaining a closed drainage system, and securing the catheter to prevent traction-induced trauma. Catheter care bundles incorporating these elements reduce CAUTI rates by 50 to 70 per cent in settings where they are consistently implemented.
Common causative organisms: E. coli (most common), Klebsiella pneumoniae, Pseudomonas aeruginosa, Enterococcus faecalis, Candida species (particularly in ICU patients receiving broad-spectrum antibiotics).
Central Line-Associated Bloodstream Infection (CLABSI)
CLABSI is one of the most deadly HAIs, with a mortality rate of 12 to 25 per cent per episode. A central venous catheter (CVC) provides a direct access route for bacteria from skin at the insertion site or from the catheter hub to the bloodstream.
Pathogenesis: bacteria colonise the skin around the insertion site or the catheter hub and migrate along the outer or inner surface of the catheter. Biofilm formation on the catheter surface protects bacteria from antibiotics and from immune clearance. Bacteria shed from the biofilm enter the bloodstream continuously, causing sustained bacteraemia.
Common causative organisms: coagulase-negative staphylococci (most common overall), Staphylococcus aureus (MRSA), gram-negative rods (E. coli, Klebsiella, Pseudomonas), Candida species.
Prevention bundle (the "Central Line Bundle"): maximal sterile barrier precautions (full body drape, sterile gown, mask, cap, gloves) during insertion, chlorhexidine skin antisepsis at insertion site, optimal catheter site selection (subclavian preferred over femoral for infection risk), daily review of line necessity (remove as soon as no longer needed), use of antimicrobial-impregnated catheters in high-risk settings.
Ventilator-Associated Pneumonia (VAP)
VAP is pneumonia that develops more than 48 hours after the patient is intubated and mechanically ventilated. It occurs in 10 to 30 per cent of intubated patients and carries a mortality attributable fraction of approximately 5 to 15 per cent above baseline.
Pathogenesis: oropharyngeal bacteria colonise the mouth and trachea around the endotracheal tube cuff. Microaspiration of contaminated secretions past the cuff into the lower respiratory tract is the primary route. The endotracheal tube also impairs normal mucociliary clearance and cough reflexes.
Common early VAP causative organisms (occurring before 5 days of ventilation): Streptococcus pneumoniae, Haemophilus influenzae, MSSA. Late VAP (after 5 days): Pseudomonas aeruginosa, MRSA, Acinetobacter baumannii, Klebsiella pneumoniae (often ESBL or carbapenem-resistant).
Prevention bundle: head of bed elevation to 30 to 45 degrees (reduces aspiration), daily sedation holds and spontaneous breathing trials (to expedite extubation), oral decontamination with chlorhexidine, subglottic secretion drainage (catheters with a suction port above the cuff), hand hygiene before any airway manipulation.
Surgical Site Infection (SSI)
SSI occurs within 30 days of surgery (or within 1 year for procedures involving implants) at the site of the surgical incision. SSI is classified by depth: superficial incisional (involving skin and subcutaneous tissue), deep incisional (involving fascia and muscle layers), and organ/space (involving body cavities or organs opened during surgery).
SSI rate is a key performance metric for surgical departments. In the UK, SSI after hip arthroplasty occurs in approximately 1 per cent of procedures. After colonic surgery, SSI rates are 10 to 15 per cent without adequate prophylaxis, falling to 2 to 5 per cent with optimal prophylaxis and technique.
Prevention: appropriate timing of antibiotic prophylaxis (within 60 minutes before incision, not more than 1 hour before as levels peak and begin to fall), appropriate agent selection (targeting the most likely wound pathogens for the procedure type), wound management (sterile closure technique, redressing protocol), normothermia maintenance during surgery (hypothermia impairs neutrophil function and increases SSI risk), and tight glucose control in diabetic patients (hyperglycaemia impairs wound healing and immune function).
Common SSI organisms: Staphylococcus aureus (most common for skin surface procedures), coagulase-negative staphylococci (most common for implant-associated SSI), E. coli and other Enterobacteriaceae (most common for intra-abdominal procedures), Pseudomonas aeruginosa (burns and severely contaminated wounds).
Clostridioides difficile Infection (CDI) as an HAI
CDI is unique among HAIs in that it is caused not by the direct procedure-associated route of bacteria entering the body, but by antibiotic-disrupted gut microbiome allowing C. difficile spores (ubiquitous in the hospital environment) to germinate and colonise the colon. It is one of the most important antibiotic-associated consequences.
The link between antibiotic use and CDI makes it a primary target for antimicrobial stewardship: reducing the use of high-CDI-risk antibiotics (clindamycin, fluoroquinolones, broad-spectrum cephalosporins) reduces CDI incidence. UK data consistently shows that restriction of fluoroquinolone prescribing is associated with a significant reduction in CDI rates.
Frequently Asked Questions
What is a hospital-acquired infection?
A HAI (healthcare-associated infection) is an infection that develops in a patient at least 48 hours after hospital admission, and which was not present or incubating at the time of admission. The 48-hour threshold is used to distinguish infections acquired in hospital from infections present at admission.
What is CLABSI?
Central Line-Associated Bloodstream Infection: a bloodstream infection in a patient with a central venous catheter, where the catheter is the most likely source of infection and no other infection source is identified. CLABSI is defined by the CDC/NHSN criteria as a laboratory-confirmed bloodstream infection in a patient with a central line in place for at least 2 days.
What is the central line bundle?
The central line bundle is a set of evidence-based practices that, when implemented together and consistently, substantially reduce CLABSI rates. Components: hand hygiene before insertion, maximal sterile barrier precautions, chlorhexidine skin antisepsis, optimal catheter site selection (subclavian or jugular over femoral), daily review of line necessity and prompt removal.
What is VAP?
Ventilator-Associated Pneumonia: pneumonia developing more than 48 hours after intubation and mechanical ventilation in a patient with no prior pneumonia. It results from microaspiration of contaminated oropharyngeal secretions into the lower respiratory tract past the endotracheal tube cuff.
What is a surgical site infection?
An SSI is an infection at or near a surgical incision site, occurring within 30 days of the procedure (or 1 year for implant procedures). It is classified as superficial incisional, deep incisional, or organ/space infection based on tissue depth involved.
How do antibiotic bundles reduce HAI rates?
Care bundles apply multiple evidence-based interventions simultaneously, because each element alone reduces risk partially, while applying all elements together produces a synergistic risk reduction far greater than any single measure. CLABSI rates have been reduced by 60 to 80 per cent in units implementing care bundles consistently, compared to historical baseline rates.
What organisms cause the most HAIs?
The CDC and ECDC identify a core group of HAI pathogens: Staphylococcus aureus (including MRSA), coagulase-negative staphylococci, Enterococcus species (including VRE), Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Acinetobacter baumannii, Clostridioides difficile, and Candida species. These organisms share features of antibiotic resistance, biofilm formation, and ability to survive in the healthcare environment.
Why is the HAI rate a hospital quality metric?
HAI rates are a direct measure of the quality and safety of patient care. They reflect hand hygiene compliance, appropriate antibiotic stewardship, quality of insertion techniques, adequacy of environmental cleaning, and effectiveness of infection prevention programmes. HAI surveillance data allows hospitals to benchmark against national averages, identify areas for improvement, and measure the impact of improvement programmes.
What is a point prevalence survey?
A point prevalence survey (PPS) measures the proportion of patients in a healthcare facility (or a sample of facilities) who have an HAI or are receiving antibiotics on a specified survey date. The European PPS of HAI and antimicrobial use in acute care hospitals (coordinated by ECDC) is conducted every 5 years across EU/EEA hospitals and provides the most comprehensive European data on HAI burden.
What is the HAI attributed mortality?
Attributable mortality is the excess mortality due to HAI compared to matched patients without HAI. For CLABSI, attributable mortality is approximately 12 to 25 per cent. For VAP, approximately 5 to 15 per cent. For CDI, approximately 5 per cent for mild to moderate disease, rising to 30 to 40 per cent for severe complicated CDI. For HAI due to carbapenem-resistant organisms, mortality is substantially higher due to very limited treatment options.