Dorit Sohr

A modified ROC analysis for the selection of cut-off values and the definition of intermediate results of serodiagnostic tests

A total number of 50 sera from clinically confirmed cases of canine Borrelia (B.) burgdorferi infection and 44 negative control sera were tested with a B. burgdorferi specific antibody ELISA. The data were submitted to the two-graph receiver operating characteristic (TG-ROC) analysis which is a plot of the test sensitivity (Se) and specificity (Sp) against the threshold (cut-off) value assuming the latter to be an independent variable. Thus, in contrast to the conventional ROC analysis, valid pairs of Se and Sp can be read for pre-assigned threshold values directly from the TG-ROC plots. A cut-off that realises equal test parameters (Se = Sp = theta 0 (theta-zero)) can be obtained as the intersection point of the two graphs. Since the value for theta 0 is below a preselected accuracy level (95% or 90%), two cut-off values are selected that represent the bounds of an intermediate range (IR). IR can be considered as a borderline range for the clinical interpretation of test results. The proportion of the measurement range (MR) that gives unambiguous test results can be expressed using IR as the valid range proportion (VRP = (MR-IR)/MR). VRP and theta 0 are useful parameters for test comparison since they do not depend upon the selection of a single cut-off point. In addition, the selection of cut-off values is supported by graphical displays of efficiency, Youdens index and likelihood ratios which can be considered as functions of the pre-assigned cut-off value. TG-ROC was derived as a user-defined template for a commercially available spreadsheet programme (MS-EXCEL, Microsoft).

Operating Room Ventilation With Laminar Airflow Shows No Protective Effect on the Surgical Site Infection Rate in Orthopedic and Abdominal Surgery

Objective:To evaluate whether operating room (OR) ventilation with (vertical) laminar airflow impacts on surgical site infection (SSI) rates. Design:Retrospective cohort-study based on routine surveillance data. Patients and Methods:Sixty-three surgical departments participating voluntarily in the German national nosocomial infections surveillance system “KISS” were included (a total of 99,230 operations). Active SSI surveillance was performed according to the methods and definitions given by the US National Nosocomial Infection Surveillance system. Surgical departments were stratified according to type of OR ventilation used: (1) turbulent ventilation with high-efficiency particulate air-filtered air, and (2) HEPA-filtered (vertical) laminar airflow ventilation. Multivariate analyses were performed by the generalized estimating equations method to control for the following variables as possible confounders: (a) Patient-based: wound contamination class, ASA score, operation duration, patients’ age and gender, endoscopic operation; (b) Hospital-based: the number of beds in the hospital, its academic status, operation frequency, and long-term participation in KISS. Results:The risk for severe SSI after hip prosthesis implantation was significantly higher using laminar airflow OR ventilation (1.63 < 1.06; 2.52>), as compared with turbulent ventilation. The adjusted odds ratios for the other operative procedures analyzed were: knee prosthesis 1.76 < 0.80, 3.85>; appendectomy 1.52 < 0.91, 2.53>; cholecystectomy 1.37 < 0.63, 2.97>; colon surgery 0.85 < 0.49, 1.49>; and herniorrhaphy 1.48 < 0.67; 3.25>. Conclusions:Unexpectedly, in this analysis, which controlled for many patient and hospital-based confounders, OR ventilation with laminar airflow showed no benefit and was even associated with a significantly higher risk for severe SSI after hip prosthesis.

Nosocomial infection and antibiotic use: a second national prevalence study in Germany.

BACKGROUNDnIn 2011, seventeen years after the first national study on the prevalence of nosocomial infections and antibiotic use in German hospitals, a second national prevalence study was carried out according to the specifications of the European Centre for Disease Prevention and Control (ECDC).nnnMETHODSnThe ECDC protocol, containing uniform surveillance definitions and ascertainment methods, was implemented. The only infections counted were those that were active or under treatment with antibiotics on the day of the study. In addition to the representative sample required by the ECDC, which consisted of 46 hospitals, further hospitals participated on a voluntary basis.nnnRESULTSnData on 41 539 patients in 132 hospitals were analyzed. The prevalence of infections that had arisen during the current hospital stay was 3.8% in the overall group and 3.4% in the representative sample of 9626 patients in 46 hospitals. The prevalence of all nosocomial infections, including those acquired before the current hospital stay and still present upon admission, was 5.1% in both the overall group and the representative sample. The prevalence of antibiotic use on the day of the study was 25.5% and 23.3% in the two groups, respectively.nnnCONCLUSIONnThe prevalence of nosocomial infection has not changed since 1994, but the prevalence of antibiotic use has increased. In interpreting these findings, one should bear in mind that confounders may have influenced them in different directions: The mean length of hospital stay is now shorter than in 1994, but the mean age of hospitalized patients is higher.

Early- and Late-Onset Pneumonia: Is This Still a Useful Classification?

ABSTRACT The choice of empirical treatment of nosocomial pneumonia in the intensive-care unit (ICU) used to rely on the interval after the start of mechanical ventilation. Nowadays, however, the question of whether in fact there is a difference in the distribution of causative pathogens is under debate. Data from 308 ICUs from the German National Nosocomial Infection Surveillance System, including information on relevant pathogens isolated in 11,285 cases of nosocomial pneumonia from 1997 to 2004, were used for our evaluation. Each individual pneumonia case was allocated either to early- or to late-onset pneumonia, with three differentiation criteria: onset on the 4th day, the 5th day, or the 7th day in the ICU. The frequency of pathogens was evaluated according to these categories. A total of 5,066 additional cases of pneumonia were reported from 2005 to 2006, after the CDC criteria had been modified. From 1997 to 2004, the most frequent microorganisms were Staphylococcus aureus (2,718 cases, including 720 with methicillin [meticillin]-resistant S. aureus), followed by Pseudomonas aeruginosa (1,837 cases), Klebsiella pneumoniae (1,305 cases), Escherichia coli (1,137 cases), Enterobacter spp. (937 cases), streptococci (671 cases), Haemophilus influenzae (509 cases), Acinetobacter spp. (493 cases), and Stenotrophomonas maltophilia (308 cases). The order of the four most frequent pathogens (accounting for 53.7% of all pathogens) was the same in both groups and was independent of the cutoff categories applied: S. aureus was first, followed by P. aeruginosa, K. pneumoniae, and E. coli. Thus, the predictabilities of the occurrence of pathogens were similar for the earlier (1997-to-2004) and later (2005-to-2006) time frames. This classification is no longer helpful for empirical antibiotic therapy, since the pathogens are the same for both groups.

Converting incidence and prevalence data of nosocomial infections : Results from eight hospitals

OBJECTIVEnTo investigate the use of the formula of Rhame and Sudderth for the interconversion of prevalence and incidence data on the frequency of nosocomial infections.nnnDESIGNnComparison of observed and calculated incidence data and prevalence data.nnnSETTINGnOne 8-week incidence investigation in the surgical and intensive care units of eight medium-sized hospitals; three separate point-prevalence studies in the same units.nnnRESULTSnThe overall prevalence observed after the three prevalence studies in 2,169 patients was 6.8% (95% confidence interval [CI95], 5.7-8.0). In 2,882 discharged patients observed during the incidence study, the mean hospitalization was 9.8 days; patients with one or more nosocomial infection had a mean hospitalization time of 22.3 days and a mean interval of 8.2 days from admission to the first day of infection. Based on these data, the overall calculated incidence was 4.7%, whereas the observed incidence was 4.3% (CI95, 3.6-5.2). Vice versa, an overall prevalence of 6.2% was found when calculated from the observed incidence data. The incidence data calculated from prevalence data also were within the confidence interval of the incidences observed for urinary tract infections and surgical-site infections. (However, it was not possible to convert the data for two of the eight hospitals.)nnnCONCLUSIONnThe approximate mathematical relationship between the prevalence and incidence data of nosocomial infection is confirmed by this study. However, although it is theoretically possible, we would not recommend the conversion of prevalence into incidence data or vice versa.

Laminar airflow ceiling size: no impact on infection rates following hip and knee prosthesis.

OBJECTIVEnLaminar airflow (LAF) systems are widely used, at least in orthopedic surgery. However, there is still controversial discussion about the influence of LAF on surgical site infection (SSI) rates. The size of the LAF ceiling is also often a question of debate. Our objective is to determine the effect of this technique under conditions of actual rather than ideal use.nnnDESIGNnCohort study using multivariate analysis with generalized estimating equations method.nnnSETTINGnData for hip and knee prosthesis procedures from hospitals participating in the German national nosocomial infection surveillance system (KISS) from July 2004 to June 2009 were used for analysis.nnnPATIENTSnA total of 33,463 elective hip prosthesis procedures due to arthrosis (HIP-A) from 48 hospitals, 7,749 urgent hip prosthesis procedures due to fracture (HIP-F) from 41 hospitals, and 20,554 knee prosthesis (KPRO) procedures from 38 hospitals were included.nnnMETHODSnThe data were analyzed for hospitals with and without LAF in the operating rooms and by the size of the LAF ceiling. The endpoints were severe SSI rates.nnnRESULTSnThe overall severe SSI rate was 0.74 per 100 procedures for HIP-A, 2.39 for HIP-F, and 0.63 for KPRO. For all 3 prosthesis types, neither LAF nor the size of the LAF ceiling was associated with lower infection risk.nnnCONCLUSIONSnThe data demonstrate consistency and reproducibility with the results from earlier registry studies. Neither LAF nor ceiling size had an impact on severe SSI rates.

How to Survey Nosocomial Infections

Many surveillance methods for nosocomial infections (NIs) have been put forward in the literature, and all have their advantages and disadvantages. Different surveillance methods are useful, depending on whether the objective of surveillance is only to increase sensitivity to infection control problems and to identify areas with possible infection control problems; to confirm a possible infection control problem through comparison with other units or departments; or to use surveillance data for identifying the sources of infections. Furthermore, time effectiveness is a major point in selecting the most appropriate method, particularly the method for case identification. In units or departments with a high level of NI, even highly time-consuming surveillance methods may be ultimately time-effective; in units or departments with a lower level of NI, the time-effectiveness depends on the time necessary for case identification. Close liaison with staff in the units is a sine qua non for the success of all surveillance activities.

Occurrence of Methicillin-Resistant Staphylococcus aureus Infections in German Intensive Care Units

AbstractBackground: This study aims to describe the occurrence of nosocomial methicillin-resistant Staphylococcus aureus (MRSA) infections and the relation between endemic and epidemic nosocomial MRSA infections in intensive care units (ICUs) in Germany.n Methods: The ICU component within the German national nosocomial infections surveillance system (KISS) was established in January 1997. The number of participating ICUs increased from 25 in 1997 to 178 (June 2000). In every case of nosocomial infection (NI), the pathogens identified were referred to the surveillance system. To identify clusters and outbreaks and distinguish them from mere single events, the following simple definitions were applied: a cluster was two nosocomial MRSA infections within 3 months; an outbreak was three or more nosocomial MRSA infections within 3 months; all other MRSA infections were classed as single events.n Results: A total of 10,261 NIs were identified during the observation period in the 139 ICUs which had been participating for at least 6 months. Among the 1,535 nosocomial S. aureus infections, 219 MRSA infections were identified (14.3%). Only 51 (36.7%) of the 139 KISS ICUs identified nosocomial MRSA infections. In 12 ICUs (8.6%) however, more than 50% of all nosocomial S. aureus infections were caused by MRSA. The mean incidence density was 0.31 nosocomial MRSA infections/1,000 patient days (range 0-3.6). Outbreaks were registered in 13 ICUs, clusters in 12 further ICUs and only single events in the remaining 26.64.0% of alt MRSA infections were registered during outbreaks and 12.8% in cluster situations. Thus, only 23.2% of MRSA infections were single events.n Conclusion: These definitions of clusters and outbreaks can only provide an estimate of the real number of cases. However, through the targeted identification of ICUs with endemic and epidemic MRSA problems and subsequent strict measures to prevent the spread of MRSA within these ICUs, it may be possible to retain, or even reduce, the present level of MRSA infections in Germany.

Are nosocomial infection rates in intensive care units useful benchmark parameters

SummaryBackground: The objectives of this study were to determine to what extent the German national nosocomial infection surveillance system (Krankenhaus Infektions Surveillance System, KISS) can take into account the circumstances prevailing in various intensive care units (ICUs) and to establish whether KISS-ICU infection rates can serve as useful benchmark parameters.nMethods: The investigation focused on three major factors: microbiological monitoring, severity of illness and the duration of surveillance. For each of these factors separate infection rates were calculated for various ICU groups and the differences compared.nResults:Significant differences were found for catheter-associated urinary tract infections (CAUTI) with routine monitoring, but not for ventilator-associated pneumonia (VAP). Significant differences were assessed for central venous catheter-associated bloodstream infections (CVC-BSI), considering the average ventilator utilization rate in the ICU as a surrogate parameter for the average severity of illness in its patient group. Surveillance periods of about 1 year were necessary to confirm definite outlier and nonoutlier positions for the majority of the ICUs.nConclusion: Using KISS data for internal orientation, it is possible to note important differences between ICUs when interpreting infection rates; some initial examples of successful use of surveillance data for the reduction of infection rates are already available. However, the use of such data for external assessment is not possible, because external observers are often unable to fully consider important factors in the interpretation of infection rates.

Impact of department volume on surgical site infections following arthroscopy, knee replacement or hip replacement

Objective To examine the association between surgical department volume and the risk of surgical site infections (SSI) after orthopaedic procedures. Background A minimum volume regulation of at least 50 knee replacements per year was implemented in 2006 in German surgical departments. Methods SSI rates were obtained from Krankenhaus-Infektions-Surveillance-System, the German national nosocomial infections surveillance system (January 2003–June 2008). The authors analysed the data by linear regression models. The adjusted ORs were estimated based on general estimating equation models to assess the independent effect of department volume (low, ie, ≤50, medium, ie, >50 and ≤100, and high, ie, >100 procedures annually). Results A total of 206 surgical departments performed 14u2008339 arthroscopies, 63u2008045 knee replacements and 43u2008180 hip replacements during the 5.5-year study period. SSI rates were significantly higher in departments with a procedure volume of ≤50 arthroscopies and knee replacements. A higher threshold of 100 procedures per year did lead to a significant decrease in SSI rates for all three procedures in the univariate analysis. The multivariate analysis showed that the risk of SSI in low volume departments was sevenfold higher for arthroscopies and twofold higher for knee replacement than in medium volume departments. SSI risk after hip replacement was significantly lower in high volume centres. Conclusion The authors findings offer some support for recommendations to concentrate arthroscopy and knee replacement in surgical departments with more than 50 procedures and hip replacement in departments with more than 100 procedures per year in order to reduce SSI.