Incidence and Completeness of Notification of Legionnaires' Disease
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Epidemiol. Infect. (2008), 136, 540–550. f 2007 Cambridge University Press doi:10.1017/S0950268807008977 Printed in the United Kingdom Incidence and completeness of notification of Legionnaires’ disease in The Netherlands: covariate capture–recapture analysis acknowledging regional differences N. A. H. VAN HEST 1,2*, C. J. P. A. HOEBE 3, J.W. DEN BOER4,J.K.VERMUNT5, 6 7 1,2 E. P. F. IJZERMAN , W.G. BOERSMA AND J. H. RICHARDUS 1 Department of Infectious Disease Control, Municipal Public Health Service Rotterdam-Rijnmond, Rotterdam, The Netherlands 2 Department of Public Health, Erasmus MC, University Medical Centre Rotterdam, Rotterdam, The Netherlands 3 Department of Infectious Disease Control, South Limburg Public Health Service, Geleen, The Netherlands 4 Department of Infectious Disease Control and the Environment, Kennemerland Public Health Service, Haarlem, The Netherlands 5 Department of Methodology and Statistics, Tilburg University, Tilburg, The Netherlands 6 Regional Public Health Laboratory Kennermerland, Haarlem, The Netherlands 7 Department of Pulmonary Diseases, Medical Centre Alkmaar, Alkmaar, The Netherlands (Accepted 14 May 2007; first published online 22 June 2007) SUMMARY To estimate incidence and completeness of notification of Legionnaires’ disease (LD) in The Netherlands in 2000 and 2001, we performed a capture–recapture analysis using three registers: Notifications, Laboratory results and Hospital admissions. After record-linkage, 373 of the 780 LD patients identified were notified. Ascertained under-notification was 52.2%. Because of expected and observed regional differences in the incidence rate of LD, alternatively to conventional log-linear capture–recapture models, a covariate (region) capture–recapture model, not previously used for estimating infectious disease incidence, was specified and estimated 886 LD patients (95% confidence interval 827–1022). Estimated under-notification was 57.9%. Notified, ascertained and estimated average annual incidence rates of LD were 1.15, 2.42 and 2.77/100 000 inhabitants respectively, with the highest incidence in the southern region of The Netherlands. Covariate capture–recapture analysis acknowledging regional differences of LD incidence appears to reduce bias in the estimated national incidence rate. INTRODUCTION false-positive cases and often incomplete for true- positive cases, as described for Legionnaires’ disease Any surveillance system is concerned with the quality (LD) [2, 3]. of the data collected, including the degree of ascer- LD is a serious, possibly fatal, pneumonia caused tainment of affected individuals [1]. A conventional by Legionella spp., occurring in sporadic cases and surveillance system is notification, possibly containing outbreaks [4, 5]. Under the present legislation regard- ing infectious diseases in The Netherlands, LD is placed in category B. This group of infectious diseases * Author for correspondence: N. A. H. van Hest, M.D., M.Sc., Tuberculosis Control Physician/Epidemiologist, Division of has to be notified within 24 h to the Municipal Public Infectious Disease Control, Municipal Public Health Service Health Service by the diagnosing physician. The Rotterdam-Rijnmond, PO Box 70032, 3000 LP Rotterdam, The Netherlands. Municipal Public Health Service forwards this in- (Email: [email protected]) formation to the Register of Notifiable Infectious Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.226, on 29 Sep 2021 at 17:27:06, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0950268807008977 Estimating Legionnaires’ disease rates 541 Diseases at the Office of the Health Care Inspectorate of registered cases plus the estimated number of non- where national data are aggregated for analysis, moni- registered patients. Capture–recapture methods have toring, public health intervention or policy making. been used to estimate the total number of patients Since 1999 an average of 230 LD patients were with LD and other infectious diseases [2, 3, 17]. notified in The Netherlands annually. The average The validity of capture–recapture analysis depends national annual incidence rate was 1.4 LD patients on possible violation of the underlying assumptions per 100 000 inhabitants, almost three times higher and one focus is to establish which method is most than the average annual incidence rate in the United appropriate for specific datasets [15]. Usually, log- States and the United Kingdom [6, 7]. However, the linear modelling of data from at least three linked incidence rate based on notifications varies consider- registers is the preferred capture–recapture method ably per province [8]. Under-diagnosis and under- because it can reduce bias due to inter-dependencies notification are likely. This can obscure the true between two registers [13, 17] Stratified capture– burden of LD, hamper the detection of clusters of LD recapture analysis according to categorical covariates patients and hinder good investigations into the poss- associated with the probability of capture in a register ible source of legionella infections. The Dutch Health can further reduce bias [11, 12, 14, 16]. An alternative Council estimated an annual number of 800 LD is to include these covariates, e.g. demographic, diag- patients. This number is based on the annual number nostic or prognostic variables, in a log-linear co- of cases of pneumonia in The Netherlands (110 000) variate capture–recapture model but these models of whom 15% require hospital admission (16 000) of have rarely been used to estimate human disease which 5% is caused by Legionella spp. (800) [9]. incidence [18, 19]. Record-linkage is important for assessing the This study aims to estimate incidence and com- quality and completeness of infectious disease regis- pleteness of notification of LD in The Netherlands ters, i.e. comparing patient data across multiple in 2000 and 2001 using record-linkage of three data registers [10]. Completeness of notification can be sources and capture–recapture analysis. assessed by comparison with case-ascertainment, i.e. the total number of patients observed in at least one MATERIALS AND METHODS register, or the estimated total number of patients obtained by capture–recapture analysis. The total Data sources and patient identifiers number of individuals present in one or more regis- Three LD data sources were used: trations does not necessarily reflect a reliable approxi- mation of the true number of cases. The purpose of (1) Notification. Patients notified by their physician capture–recapture analysis is to assess the number to the Health Care Inspectorate. A uniform ques- of cases that are not registered. In an article published tionnaire collected additional information from in 1972, Stephen Fienberg demonstrated how this local Public Health Services processing the noti- number of unobserved cases could be estimated, using fications. log-linear analysis [11]. For capture–recapture analy- (2) Laboratory. Patients with a specified positive sis, according to Fienberg, the availability of data from laboratory test result reported by the clinical at least three different, possibly incomplete, partially microbiologists in a survey among all clinical overlapping and preferably, but not necessarily, microbiology laboratories after obtaining per- independent sources is needed [12–16]. The data can mission for this survey from the Dutch Society for be put in a 2r2r2 contingency table, indicating the Microbiology and supported by the Inspector- absence or presence of a case in each of the registers. General for Infectious Diseases of the Health This table has one empty cell, corresponding to the Care Inspectorate. Positive laboratory test results number of cases never registered. Based on certain were classified as either confirmed (culture, urine assumptions, which will be discussed later, capture– antigen test or a fourfold rise in antibody titre recapture analysis aims at obtaining an estimate of [o128 IU] against Legionella spp. in paired acute the unregistered number of patients in the empty and convalescent serum samples) or probable cell from the available data in the other cells. This [PCR, a high titre (o256 IU) against Legionella estimate can be found under the best fitting and most spp. in one serum sample or direct fluorescent parsimonious log-linear model, as explained later. antibody staining], according to the European Finally, the total number of individuals is the number Working Group for Legionella Infections Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.226, on 29 Sep 2021 at 17:27:06, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0950268807008977 542 N. A. H. Van Hest and others (EWGLI) definitions. Patients with LD only known to the Hospital register were classified as cases with unknown laboratory verification. (3) Hospital. Hospitalized patients recorded in the National Morbidity Registration by Prismant, covering all hospitals in The Netherlands with: (a) an International Code for Diseases (ICD-9 code) for all forms of pneumonia (ICD-9 codes 480.0–487.0) for individuals known to Notification and/or Laboratory. (b) An ICD-9 code 482.8 for individuals only known to Hospital. ICD-9 has no specific code for LD and, as reported from other countries, in The Netherlands ICD-9 code 482.8 (pneumonia due to other specified bacteria) is used for LD patients [20]. Hospital records coded as 482.8 can therefore