This survey was conducted to determine the annual incidence of severe sepsis during the first 24 hours of ICU admission in The Netherlands. The number of severe sepsis patients in 47 ICUs (42% of total national ICU capacity) was captured in the 24-hour period between 08:00 on 11 December 2001 and 08:00 on 12 December 2001; 134 patients were present in the ICUs during the study period for treatment of severe sepsis, and 18 of these arrived during the study period. We calculated that the annual number of patients who need ICU treatment for severe sepsis in The Netherlands is 8643 ± 929, which was estimated to represent 0.61% of hospital admissions and 11% of ICU admissions. Some considerations regarding the interpretation of this number are addressed here.
In 1986, Verbrugh and coworkers [6
] found that 0.54% of all hospital admissions in two general hospitals resulted in sepsis (defined as bacteraemia with clinical symptoms), and Kieft and coworkers [7
] concluded, from a prospective study conducted in a university hospital, that sepsis syndrome accounted for 1.36% of all hospital admissions. The incidences of sepsis and severe sepsis per hospital admission, as calculated from our survey, were 0.65% and 0.61%, respectively, based on the prevalence series. These percentages are in the region of the findings reported by Verbrugh and coworkers, but they are lower than the incidence found by Kieft and colleagues, which may be because the latter group used a different definition of sepsis and admissions in a single university hospital only. The incidence of severe sepsis, at 11% of ICU admissions, was similar to the rate reported by Alberti and coworkers [16
], who surveyed 14,364 ICU patients for more than 1 year in ICUs in several European countries, Canada and Israel. They identified 1634 patients with either severe sepsis or septic shock at admission. Finfer and coworkers [17
] found that 571 ICU admissions were due to severe sepsis in 5878 consecutive ICU patients admitted to Australian and New Zealand ICUs. Padkin and coworkers [18
] reported a much higher incidence (27.1%) within the first 24 hours of ICU admission in England, Wales and Northern Ireland from a retrospective database analysis, but at a population level the incidence was similar (51 per 100,000 inhabitants). This inconsistency suggests that ICU admission policy is an important issue to consider when comparing findings between different institutions or countries. The median ICU stay in their survey was 3.6 days, which is low compared with our and previously reported findings [3
]. Two recent retrospective analyses of discharge databases in the USA [3
] also found higher incidence rates by population (240–300 per 100,000) and by hospital admission (2.6%), but in these studies severe sepsis patients were not defined by American College of Chest Physicians/Society of Critical Care Medicine guidelines criteria but by ICD-9-CM (International Classification of Diseases, Ninth revision, Clinical Modification) codes, and all hospital patients were considered, rather than just ICU patients.
The protocol of the present survey specifically asked participating ICUs to include only those patients admitted to the ICU with an infection, which implied that patients with ICU-acquired severe sepsis were excluded. Although a true estimate of severe sepsis should have included these patients, the diagnosis of severe sepsis in patients admitted to the ICU with problems other than infection is very difficult. Current parameters of severe sepsis are probably not sufficiently accurate. In addition, this would have required consensus about new infections and newly developed organ dysfunction or worsening organ dysfunction in all ICUs, which was not possible for the present study. From the results of the EPIC (European Study of Prevalence of Infection during Intensive Care) study [21
], it appeared that in 78 Dutch ICUs 17% of the patients had an ICU infection but only 0.4% of the patients had sepsis resulting from an ICU infection. A Dutch surveillance study conducted among 16 ICUs [22
], however, found that 8% of patients who were admitted to the ICU (with or without prior infection) developed sepsis due to an ICU-acquired infection. Also, Alberti and coworkers [16
] reported that 9.2% of ICU patients developed ICU-acquired sepsis, severe sepsis, or septic shock, whereas the rate for community-acquired and hospital-acquired sepsis, severe sepsis, or septic shock was 17.4%. However, that study did not report the exact methodology for determining organ failure developed in the ICU, or the time frame within which the symptoms of severe sepsis occurred. Nevertheless, by ignoring sepsis due to ICU-acquired infections, the present survey probably underestimates the national incidence of cases of severe sepsis.
Moreover, severe sepsis can occur outside ICUs; such cases were not picked up in the present survey. Angus and coworkers [3
] concluded from their retrospective analysis that only 51.1% of severe sepsis patients (defined using ICD-9-CM codes) received ICU care in the USA, although an additional 17.3% received organ replacement therapy in CCUs or intermediate care units. Because severe sepsis is defined by the presence of organ failure, ICU admission is almost always required for organ-replacing therapeutic intervention and specific treatment for severe sepsis.
Several risk factors for severe sepsis have been identified in previous studies, including age, sex, comorbidities, and causative pathogen [3
]. In extrapolating data from a cross-sectional prevalence survey such as the present one, consideration of the representativeness of the ICU patients in the participating ICUs relative to the national case mix of ICU patients, especially in terms of these risk factors, is important. Unfortunately, demographic and clinical information was only collected for the included patients, rather than for all ICU patients present during the study period. It was therefore not possible to determine how representative the case mix of ICU patients in our survey was. On the other hand, the participating ICUs were estimated to be representative of The Netherlands. Participating ICUs were distributed evenly over The Netherlands, and the representation of both university and general hospitals was similar (4/9 university hospitals [44%] and 43/109 general hospitals [39%]). ICUs in Dutch general hospitals are almost always mixed; only some university hospitals have separate ICU wards for surgical and medical patients. Therefore, no bias is expected to have occurred by, for example, including a disproportionate number of medical ICUs. We knew the number of ICU beds in our survey and based the estimate of national representation on this number; there was no national information on the number of beds in general and university hospitals separately, and therefore we could not reliably derive a specified incidence of severe sepsis by type of hospital.
Although this prevalence survey did not address the length of stay in the ICU directly, we estimated this duration on the basis of the duration to date by means of fitting the data to a geometrical distribution, as described by Freeman and Hutchison [12
]. We found a length of stay of 13.3 ± 1.1 days for severe sepsis patients. This result is consistent with the literature.
Angus and coworkers [3
] found a duration of ICU stay of 13.8 ± 20.0 days in teaching hospitals and 10.0 ± 13.8 days in nonteaching hospitals. Edbrooke and coworkers [19
] found a median ICU stay of 16.5 days for patients who developed sepsis in the ICU of a university hospital in the UK. We expected to find increasing lengths of stay with increasing severity of sepsis, but this was only true for severe sepsis versus sepsis (13.3 versus 12.7 days). For shock patients, however, we found a duration (Di
) of 11.6 days, but the prevalent population of shock patients was rather small to make a powerful fit of the duration to date (P
= 0.69). The method of Freeman and Hutchison [12
] is an indirect method with which to estimate duration of stay, and data on length of stay should ideally be extracted from prospective studies.
From the relationship between prevalence and incidence, as expressed in Eqn 4, we concluded that the ratio of incident and prevalent patients deviated from theoretical expectation. An explanation for this trend could be associated with the study design. In order to identify prevalent patients, the participating ICUs were required to go through records for all patients on their ward to determine whether they had an infection at the time of their admission, and to search the registration for the presence of SIRS criteria and organ failure during the first 24 hours of admission. This might have been too time consuming in some cases, or the specific data may not have been available. This introduces a negative bias in the number of prevalent patients found in the survey. On the other hand, newly admitted patients are likely to be monitored more closely for infection, SIRS criteria and organ failure when members of the medical staff are aware that a survey for severe sepsis is being conducted that day. This makes it conceivable that the number of incident severe sepsis patients is an overestimation of the actual daily incidence. Moreover, the number of incident patients in the survey (i.e. 18) is small and very sensitive to daily variation, which is also expressed by a large confidence interval for the estimated daily and annual incidences. Because we only measured prevalence and incidence over 1 day, there is a good chance that the relatively high number of incident patients is attributable to coincidence. The annual incidence of severe sepsis based on the prevalence series of cases therefore forms a more reliable estimate for future reference.
The results of the survey were not corrected for any influence of time, such as the day of the week or the month of the year on which the survey was conducted. The main reason for this was the fact that severe sepsis patients are usually acute cases, and the majority of the population came to the hospital for acute surgery or acute infection (Table ). It is not likely that these acute events differ from weekdays to holidays or weekends. Moreover, data on the admission rates for severe sepsis patients are unavailable, and any correction of the results in the survey would be as arbitrary as not correcting at all.
The present point prevalence survey was conducted to gain insight into the current incidence of severe sepsis occurring within the first 24 hours of ICU admission in The Netherlands. The results indicate that the national incidence of severe sepsis is in the range of 9000 patients per year. This number is lower (by 50–70%) than the annual incidence of diseases such as coronary heart disease and cerebrovascular accident, but it is comparable with the incidence of breast cancer, lung cancer and Parkinson's disease in The Netherlands. This demonstrates the importance of severe sepsis in terms of public health, resource allocation in intensive care and, because of the high overall mortality, causes of death in The Netherlands [23