Several multicentre studies using large databases [18
] showed a consistent effect of the number and composition of the workforce on the incidence of nosocomial infections.
The study by Needleman et al.
] used administrative data from 799 US hospitals in 11 states to investigate the association between several patient outcomes and nurse staffing. Outcomes sensitive to nurse staffing among medical patients were urinary tract infection, pneumonia, length of hospital stay and upper gastrointestinal bleeding. The authors built several models with various assumptions to estimate the impact of modification in nurse staffing on the rate of specific patient outcomes. Urinary-tract infection was estimated to be reduced by 9% when the nurse staffing was shifted towards higher numbers of registered nurses. Similarly, a higher number of registered nurses was expected to reduce the rate of pneumonia by 6%. These findings are supported by the study by Cho et al.
], which showed that an increase of 1 h worked by registered nurses per patient day or a 10% increase in the proportion of registered nurses were associated with, respectively, 8.9 and 9.5% decreases in the chance of pneumonia. By knowing or estimating the attributable cost of an infection, this type of information gives some insight into economical issues related to staff shortage and interventions to reverse it.
In a prospective multicentre study involving over 4000 critically ill patients, Alonso-Echanove et al.
] documented the infection risk associated with nurse skill mix. Among patients not receiving total parenteral nutrition there was an increased risk (by over twofold) of catheter-associated bloodstream infection in patients cared for by ‘float’ nurses [nurses not assigned to the intensive care unit (ICU)] for more than 60% of the duration of the central vascular access. Of note, the authors did not find an association between the nurse-to-patient ratio and the risk of catheter-associated blood-stream infection. Compared to others, this study is remarkable by its prospective design and its quality in the risk adjustment. The importance of the qualification and level of training of nurses has been highlighted in other studies [11•
]. By use of a case-control study performed in a surgical ICU in an endemic setting, Robert et al.
] found that patients who experienced primary bloodstream infection were more likely to have been hospitalized at a time when the unit was staffed with fewer regular ICU nurses and more pool nurses (i.e. those not experienced in critical care).
In a large ecological study including 211 hospitals per year over 7 years, Unruh [19
] related staffing patterns to six adverse events (atelectasis, decubitus ulcers, falls, pneumonia, posttreatment infections and urinary-tract infections). The number of licensed nurses was inversely correlated with the risk of urinary tract infection, and the proportion of licensed nurses with the risk of pneumonia. One important limitation of that study was the poor adjustment for individual risk factors; only a few hospital characteristics (such as ownership status, number of certified doctors and capacity utilization) were accounted for in the analysis.
Fridkin et al.
] performed, as part of an outbreak investigation, a retrospective case-control and a cohort study to identify risk factors for central venous catheter-associated bloodstream infections in a surgical ICU. The investigators found that several surrogate markers of workload were unfavourable during the outbreak period as compared to the pre-outbreak period, and that high workload was an independent risk factor for infection. Interestingly, there was a temporal association between the number of infections and the patient-to-nurse ratio, with a threshold above which the number of infections was above the endemic level. Thus there may be a critical staffing threshold below which optimal patient care becomes difficult, causing suboptimal device management and increased infection rates.
Studying the association between staffing and nosocomial infection is not straightforward, as it requires a complex risk adjustment and standardized and valid case definitions. Moreover, the causal pathway between staff shortage and infection is not easy to grasp, due to the complexity of the infection process itself. Cross-transmission of microorganisms known to be transmitted mainly by health-care workers’ hands, such as methicillin-resistant Staphylococcus aureus
(MRSA), is another issue: each transmission event is a failure of basic infection-control measures, such as hand hygiene, regardless of the patient case mix or severity of illness. Consequently, cross-transmission rate might be sensitive to compliance to infection control measures and to workload. Vicca [30
] offered an excellent illustration of the contribution of understaffing to the spread of MRSA, as a surrogate marker for hand-hygiene compliance. In this study, the number of incident MRSA cases was closely associated in time with several markers of workload, and there was a weak but significant correlation between the MRSA attack rate and varying staffing levels. Recently, Andersen et al.
] reported another outbreak of MRSA in a neonatal ICU, but provided only a descriptive epidemiological investigation. Before and during the outbreak there was high activity, overcrowding and a large proportion of relatively untrained nurses. The outbreak stopped after these and other infection control measures had been improved. A recently published study investigated the transmission of MRSA in a British ICU [32
]. The first part of this well conducted investigation consisted of a risk factor analysis based on prospectively collected data. A relative staff deficit was an independent risk factor for belonging to a cluster of MRSA, whereas it was not for sporadic cases. The authors then fitted a stochastic model to their data to investigate the effectiveness of infection control measures, and found that improved compliance with hand hygiene would be effective to reduce the net reproductive number, i.e. the number of transmission events. Several other studies linking overcrowding, understaffing or nursing workload with cross-transmission of MRSA [33
], extended-spectrum β
-lactamase-producing Enterobacteriaceae [36
], Klebsiella pneumoniae
], Enterobacter cloacae
] or gastrointestinal virus [40
] have been published. Data from an outbreak investigation performed in our neonatal ICU showed the intermediate step between high workload and nosocomial infection: noncompliance with hand-hygiene recommendations [39
]. We experienced an outbreak of E. cloacae
involving eight patients over a 6-week period. During the outbreak period, the number of patients increased, the mean occupancy rate exceeded the standard by 50%, whereas staff on duty remained constant, and far below what was required. An observational survey of hand-hygiene practices performed in the middle of the outbreak showed that compliance with hand-hygiene recommendations was 38%. The outbreak was resolved after workload decreased and compliance with hand disinfection increased.