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Definition, epidemiology, and general management of nosocomial infection 

Definition, epidemiology, and general management of nosocomial infection
Definition, epidemiology, and general management of nosocomial infection

Caroline Landelle

and Didier Pittet

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date: 18 May 2022

Key points

  • A nosocomial infection or health care-associated infection (HAI) is defined by the World Health Organization (WHO) as an infection occurring in a patient during the process of care in a hospital or other health care facility, which was not present or incubating at the time of admission. This includes infections acquired in the health care facility, but appearing after discharge, and also occupational infections among health care workers (HCWs) of the facility.

  • The prevalence of nosocomial infections generally exceeds 25% in intensive care units (ICUs) worldwide. Although ICU beds account for only 5% of all hospital beds and care for less than 10% of patients admitted, ICU-acquired HAIs account for more than 20% of all nosocomial infections.

  • Nosocomial infection rates tend to be higher in surgical than in medical ICUs, and higher in adult than in paediatric ICUs (except neonatal ICUs).

  • In the adult ICU, the most common site of infection is the lower respiratory tract, whereas bloodstream infections are more prominent in the paediatric ICU.

  • Despite accumulated evidence and expert opinion for the effectiveness of hand hygiene in preventing pathogen cross-transmission, HCWs compliance with recommendations remains unacceptably low worldwide, usually 30% to 50%. Successful multimodal interventions have been associated with decreasing HAI rates.


Nosocomial or ‘hospital’ infection were formerly so named to mark their hospital origin. The term ‘health care-associated infection’ (HAI) is now used to take into account more appropriately the patient care process in today’s complex health care systems, often involving long-term, rehabilitation, ambulatory, and home care. HAI is one of the most common medical complications affecting patients in intensive care units (ICUs). In general, HAI prevalence exceeds 25% in ICUs worldwide. Although ICU beds account for only 5% of all hospital beds and care for less than 10% of patients admitted, ICU-acquired HAIs account for more than 20% of all HAI overall. Associated costs are equally tremendous. An assessment of mortality attributable to HAI in the ICU setting is difficult as it shares common risk factors with both the underlying disease and acuity of illness. Crude mortality rates are estimated to vary between 10% and 80% and attributable morbidity and mortality due to ICU-acquired HAI may be in excess of corresponding rates for the infections that initially led to the patient’s hospitalization.


HAI is defined by the World Health Organization (WHO) as an infection occurring in a patient during the process of care in a hospital or other health care facility, which was not present or incubating at the time of admission. This includes infections acquired in the health care facility, but appearing after discharge, and also occupational infections among health care workers (HCWs) of the facility. The diagnosis of infection at specific body sites, e.g. urinary tract, surgical site, respiratory tract, and blood, should rely on standardized criteria based on clinical, laboratory, microbiological, and imaging parameters. The most commonly used definitions are those of the United States Centers for Disease Control and Prevention [1]‌, although several European countries use also the definitions of the ‘Hospital Infection Link for Infection Control through Surveillance’ (HELICS) network for ICUs [2]. In particular, work is being conducted on the definition of ventilator-associated pneumonia (VAP) to decrease the complexity and subjectivity of the definition and to allow a more reliable assessment and comparison of quality of care for ventilated patients [3].

Host colonization is a prerequisite for the development of infection, particularly in critical care. Although factors favouring the progression from colonization to infection are not completely understood, it is estimated that almost 50% of ICU-acquired HAI are preceded by colonization with the same microorganism. Factors associated with colonization are similar to infection (duration of hospitalization, high exposure to invasive devices, prolonged antibiotic therapy). Several studies have shown that severity of illness and ICU admission contributed to rapid colonization with gram- negative bacteria. Often these are endemic in the ICU and imply that the physiological flora of a patient can be substituted by the local endemic flora after some days in this setting.


In 1992, the European Prevalence of Infection in Intensive Care (EPIC) study [4]‌ included data from 1417 ICUs in 17 West European countries and provided valuable information on the prevalence and epidemiology of infection in critically-ill European patients. Fifteen years later, the Extended Prevalence of Infection in Intensive Care (EPIC II) study was conducted to provide a picture of the extent and patterns of infection in ICUs worldwide [5]. In 2007, 14,414 patients in 1265 ICUs from 75 countries participated in a one-day point prevalence survey. Of 13,796 patients aged >18 years, 7087 (51%) were considered to be infected. Patients who had longer ICU stays prior to the study day had higher infection rates. Infection prevalence varied markedly by geographical region, ranging from 46% to 60%. There was a significant relationship between the percentage of infected patients and the hospital mortality rate. This likely reflects differences in critical care practices between countries and underlines the importance of controlling for case mix when interpreting and comparing HAI rates between hospitals or countries.

The frequency of the occurrence of infection may also differ among different sites in the ICU and within a hospital as well illustrated by the annual United States National Healthcare Safety Network (NHSN) report [6]‌. Urinary tract infections predominate in general wards, whereas the most common ICU-acquired HAIs are lower respiratory tract infections. The type of ICU also plays a role. Infection rates tend to be higher in surgical than in medical ICUs, and higher in adult ICUs than in paediatric units, with the exception of neonatal ICUs. In adult ICUs, the lower respiratory tract is the most common site of infection, whereas bloodstream infections are more prominent in paediatric and neonatal ICUs. High rates of pulmonary infections are unique to adult ICUs where patients are frequently admitted because of respiratory distress and require mechanical ventilation. Although primary bacteraemia and infections due to vascular devices are less common than lower respiratory tract infections, morbidity, and mortality associated with these infections are particularly high.

When HAI rates have been compared over shorter increments of time, i.e. monthly, wide variations can be noted. Observations in different ICUs suggested that the level of skilled nursing care may be an important determinant of this variation. Several studies show that overcrowding, understaffing, or an imbalance between workload and resources are important determinants of HAI and microorganism cross-transmission in ICUs. The higher the workload, the lower the compliance with preventive measures, and the higher the rate of HAI. Importantly, not only the number of staff, but also the level of training affects outcomes. The causal pathway between understaffing and infection is complex and several factors may contribute, including primarily lack of time to comply with infection control recommendations.

Additional reasons for high rates include selection pressure for resistant organisms induced by high antimicrobial use and extensive exposure to medical devices. The patient’s underlying conditions play also an important role. Several studies demonstrate a correlation between the number of active comorbidities, the HAI rate, and other medical complications. In addition, almost all ICU patients are equipped with at least one vascular access or device breaking the normal skin barrier, thus enabling a direct connection to the external environment.

HAI do not only occur individually, they can develop also as outbreaks. Epidemics are associated with specific organisms sometimes introduced from outside and remaining within the ICU because of the continuous selection pressure of antibiotics. These do not necessarily need to be virulent—it is sufficient to be resistant enough in order to persist. Leading pathogens are methicillin-resistant Staphylococcus aureus (MRSA), multiresistant non-fermentative Gram-negative rods, such as Pseudomonas sp., Enterobacter sp., Serratia sp., Stenotrophomonas maltophilia, and Acinetobacter sp., all of which can become long-lasting problems. Pathogens producing extended-spectrum beta-lactamases (ESBL), such as Klebsiella sp. or Escherichia coli, are equally a major concern in many countries worldwide. In some parts of the world, vancomycin-resistant enterococci (VRE) and carbapenem-resistant Enterobacteriae have completed recently this list of worrisome pathogens. Last, but not least, non-bacterial pathogens can become a problem, e.g. the steady increase of HAI caused by non-albicans Candida sp.


Although resource-demanding, correctly performed surveillance is a condition sine qua non for effective infection control. Surveillance may help to define and detect common or unusual sources of cross-infection or failures in care management. It summarizes rates and reports feedback for corrective actions and is best performed by dedicated, specifically-trained, infection control staff in close collaboration with the ICU team. Controversy exists if surveillance should be continued post-discharge. It is wise to prolong it for a brief period after the patient has left the ICU because ICU-acquired HAIs may become evident only in the following days while the patient is in the general ward. Of note, this approach is labour-intensive and may not always be justified as only few HAIs may be detected after discharge. Target-oriented, post-discharge surveillance could be a rational alternative.

Surveillance has a major impact on the incidence of infections. However, to understand the meaning of infection rates always implies comparison. On a micro-epidemiological level, this implies comparison of endemic rates over time within the same population, before and after an intervention or system change, or outbreak detection. Inter-institutional comparison, termed ‘benchmarking’, is increasingly common with the aim of improving the effectiveness of health care and promoting patient safety. Similar comparisons might soon be made between different health care settings. Benchmarking among health care structures requires meticulous adjustment for case-mix and failure to adjust adequately for infection-associated factors will erroneously punish commitment to more challenging medical tasks and hinder quality improvement [7]‌. This is a field of intense research and expanding knowledge. Standardization of the surveillance method is a second challenge that has to be addressed to make comparison possible [8]. This obviously clashes with the will to improve and adapt definitions to medical progress and local specificities. Adjustment should be implemented according to variations in the use of microbiological investigation within the different health care settings and type of diagnostic techniques applied. Diagnostic power and accuracy largely impact on infection rates. Voluntary participation in a surveillance network, confidentiality, and adequate feedback of results are the prerequisites for health care settings’ adherence to the method and dedication to data quality.


Prevention must be guided by the measurement of indicators that identify gaps and point to the most appropriate solutions. These indicators are composed of HAI rates, structure indicators (e.g. alcohol-based hand rub available at the point of care), process indicators (e.g. hand hygiene compliance), and audits using checklists to assess if correct procedures and equipment are in place. This is known as the ‘recognize-explain-intervene’ concept, which was validated for the first time on a large scale by the Study on the Efficacy of Nosocomial Infection Control (SENIC) project carried out in United States hospitals in the 1980s [9]‌. While demonstrating that 35–50% of HAI are preventable by a few fundamental practices (e.g. correct use of urinary catheters and vascular access lines, therapy and support of pulmonary functions, surveillance of surgical procedures, timely hand hygiene, and application of isolation precautions), the SENIC project identified key elements for the success of an infection control programme: one infection control nurse per 200–250 beds; one epidemiologist per hospital (1000 beds); organized surveillance for HAI; and systematic feedback of HAI rates to administrators and HCW. Facing today’s challenge in health care in general and in ICUs in particular, the respective needs are one infection control nurse per 100–150 acute care hospital beds and one per ~25–35 ICU beds.

Standard precautions

Standard precautions refer to the comprehensive set of recommendations that must be followed in each care process and across all health care settings, regardless of the presence of an infectious pathogen. These precautions represent the primary strategy to prevent pathogen transmission among patients and HCW. They include the performance of hand hygiene according to pre-specified guidelines, use of personal protective equipment, respiratory hygiene/cough etiquette, safe injection practices, use of masks for catheter insertion, and lumbar puncture procedures, safe handling of contaminated equipment, textiles and laundry, routine cleaning and disinfection of environmental surfaces, and protective measures related to building construction and renovation.

Hand hygiene: a transversal measure

HCWs’ hands are the principal instruments in the course of complete nursing and highly invasive care in the ICU. Although hand hygiene is the single most important measure to prevent cross-transmission and to reduce the rate of nosocomial colonization and infection, compliance among HCWs is unacceptably low worldwide. Explanations for such a low compliance include insufficient time due to high workload, inconvenient access to hand cleansing facilities, inferior priority compared with other patient needs, lack of institutional priority for hand hygiene, lack of institutional safety climate, lack of leadership of senior medical and nursing staff, allergy or intolerance to hand hygiene solutions, and lack of awareness of recommendations or scepticism regarding their effect on HAI. Not surprisingly, high workload is correlated with an increasing number of hand hygiene opportunities per hour of patient care. To overcome the time constraint factor, hand hygiene indications have been condensed into five moments when action is required during health care [10,11] (Fig. 283.1).

Fig. 283.1 The ‘My 5 moments for hand hygiene’ concept.

Fig. 283.1 The ‘My 5 moments for hand hygiene’ concept.

Reprinted from Journal of Hospital Infection, 67(1), Sax H et al., ‘“My five moments of hand hygiene”: a user-centred design approach to understand, train, monitor and report hand hygiene’, pp. 9–21, copyright 2007, with permission from Elsevier and Healthcare Infection Society.

During the past decade the strength of evidence in favour of alcohol-based hand antisepsis, unless hands are visibly soiled, has become simply overwhelming. If actively promoted, alcohol-based hand rub can improve compliance with hand hygiene recommendations and can reduce HAI and transmission rates [12]. In high-demand settings such as ICUs, an alcohol-based hand rub appears to be the only method that might allow reasonable compliance. In addition, alcohol-based hand rubs with gels, rinses, or foams containing emollients are less harmful to the skin than regular hand washing with soap and water.

Multimodal strategies, including ‘bundle’ approach

As defined by the US Institute for Healthcare Improvement (, ‘care bundles, in general, are groupings of best practices with respect to a disease process that individually improve care, but when applied together result in substantially greater improvement. The science supporting the bundle components is sufficiently established to be considered standard of care’.

The proportion of HAI potentially preventable under routine working conditions remains unclear. Several reports suggest a great potential, ranging from a decrease of 10% to a maximum of 70%, depending on the setting, study design, baseline infection rates, and type of infection. The most important potential was identified for catheter-related bacteraemia. Although the optimal approach to reducing ICU-acquired HAI is unclear, studies and large quality improvement initiatives have shown that multimodal strategies can decrease HAI rates [13,14]. Problems of multiresistant organisms and isolation strategies, antibiotic control, selective oral or digestive decontamination, and prevention of some key ICU-specific infections are addressed in other chapters.


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