Biomedical subjects
Publications and source records attributed to W van Pelt.
Simpson's paradox was first recognized at the beginning of the 20th century, but few examples with real data have been presented. In this paper we present an example of this phenomenon from a multicenter study on nosocomial infections, and we try to explain intuitively this type of extreme confounding.
OBJECTIVE: To study the spread of methicillin-resistant Staphylococcus aureus (MRSA) in hospitals, especially secondary MRSA infections in relation to the origin of the MRSA strain and the measures taken regarding contact isolation. DESIGN: Secondary data analysis. METHODS: As part of the national MRSA surveillance of the National Institute of Public Health and the Environment, data were collected using questionnaires. The analysis covered the period July 1994-June 1996 and was performed for index cases of secondary infections versus sporadic cases. Possible risk factors were determined. RESULTS: In the study period 30 index cases of (clusters of) secondary infections and 191 sporadic cases were found. The size of the clusters was limited. Strict contact isolation as described in the guidelines of the Dutch Working Group on Infection Prevention prevented secondary infections in most cases. Patients for whom no relation could be found between the MRSA infection and a stay abroad were found to have caused more secondary infections, even when the data were corrected for contact isolation measures.
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OBJECTIVE: To assess the relative importance of risk factors for surgical-site infections (SSIs) in orthopedic patients and thereby determine which risk factors to monitor in the national surveillance of SSI in The Netherlands. DESIGN: Reanalysis of data on SSI and associated risk factors from two surveillance projects on nosocomial infections, carried out in 1992 and 1993 in The Netherlands: Project Surveillance Nosocomial Infections in the region of Utrecht (PSZU) and the first Project Surveillance Surgical Wound Infections (SWIFT-1). Odds ratios (ORs) were calculated for age, gender, preoperative stay, and the number of operations. In addition, in PSZU, other nosocomial infections, and, in SWIFT-1, prophylactic antibiotics, acute surgery, and wound contamination were studied. PARTICIPANTS: The study was confined to hospitalized orthopedic patients (PSZU, 4,872; SWIFT-1, 6,437). RESULTS: In PSZU, the following ORs were significant in a multivariate model: age 0-44 years, 1.0; 45-64 years, 1.6; 65-74 years, 4.7; and 75-99 years, 6.0. For a preoperative stay over 4 days, the OR was 3.3 (95% confidence interval [CI95], 2.5-4.0), and for multiple surgery, 2.5 (CI95, 1.9-3.0). For females, the OR was 0.8 (not significant). The same model applied to SWIFT-1 gave similar ORs. Adjustment for additional nosocomial infections (PSZU) decreased the ORs for ages over 65 years remarkably. The OR for additional nosocomial infections in patients under 65 years of age was 15.6 (CI95, 4.3-57.4). Adjustment for prophylactic antibiotics, acute surgery, and wound-contamination class (SWIFT-1) did not influence the ORs of the original model, but showed that wound-contamination class was an important risk factor. CONCLUSIONS: Age, additional nosocomial infections, wound-contamination class, preoperative stay, and the number of operations were identified as important risk factors for SSI in Dutch orthopedic patients.
We studied circadian variation in FVC, FEV1, PEF, TLC, VC, and RV between 9:00 A.M. and 9:00 P.M. and analyzed how this variation affected estimated longitudinal change. Data from 876 adults were obtained in a longitudinal survey of samples from two Dutch areas. Subjects participated in four surveys held at 3-yr intervals between 1975 and 1985. FVC, FEV1, PEF, and VC increased from 9:00 A.M. until noon and decreased afterwards. TLC was constant over the day, whereas RV decreased from 9:00 A.M. to noon. Average variation in FVC, FEV1 and PEF, expressed as percentages of average level, was 4. 8% (200 ml), 2.8% (86 ml), and 3.1% (250 ml/s), respectively. These results are compatible with circadian changes in airway size. No differences in variability were found between men and women. Significantly larger changes between 9:00 A.M. and noon were found in young adults, smokers, and those with respiratory symptoms than in other subgroups. Adjustment for diurnal variation reduced, albeit slightly, residual standard deviations of estimated longitudinal declines. Average diurnal change was large relative to underlying longitudinal change. Its effect on longitudinal change within an individual can therefore be large depending on age, smoking habits, symptomatology, number of visits, time of measurement, and difference in time between measurements. However, when people are measured at random times during the day for at least three visits, or when measurements are made after 11:00 A.M., effects of diurnal variation in pulmonary function on estimated average longitudinal decline are minimal.
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BACKGROUND: Lyme disease is caused by Borrelia burgdorferi which is transmitted in Europe by the tick ixodes ricinus. Erythema migrans is a skin lesion which is pathognomonic of Lyme disease. A retrospective study was carried out to determine the geographical distribution of the occurrence of tick bites and erythema migrans in the Netherlands and to identify ecological risk factors. METHODS: In April 1995, all general practitioners (GPs) in the Netherlands were asked to complete a postal questionnaire on the number of tick bites and erythema migrans case-patients seen in 1994 and the size of the practice. Reminders were sent to non-responders. Information on ecological risk factors by local government area was obtained from a geographical information system. RESULTS: The response rate was 79.9%. In 1994, GPs reported seeing approximately 33,000 patients with tick bites and 6500 with erythema migrans. The incidence rate of erythema migrans was estimated at 4.3 per 10,000 population. Ecological risk factors for both tick bites and erythema migrans were the proportion of the area covered by woods, sandy soil, dry uncultivated land, the number of tourist-nights per inhabitant and sheep population density. The cattle population density was a risk factor for erythema migrans. CONCLUSIONS: Using simple methods, a crude estimate of the incidence rate of erythema migrans was obtained rapidly, and high risk areas were identified. Lyme disease appears to be an important problem in the Netherlands.
OBJECTIVE: To determine the feasibility of standardized surveillance of nosocomial infections (NI) in The Netherlands, using local data on patients with NI collected by infection control practitioners (ICPs) and denominator data on all patients under surveillance obtained from the Dutch National Medical Registry (LMR). DESIGN: A prospective, multicenter study. SETTING: Eight hospitals in the Utrecht region, and the National Institute of Public Health and the Environment. PATIENTS: ICPs traced NI in gynecological and orthopedic patients for 9 to 16 months. Denominator data on all patients under surveillance were obtained from the LMR. RESULTS: Data from 8,922 patients were collected; the ICPs registered 470 patients with 526 NI. Overall, the NI incidence was 5.9 per 100 patients, or 6.3 per 1,000 patient days. Urinary tract infections (UTI) were most frequent (3.3%), followed by surgical-wound infections (SWI; 2.0%) and bloodborne infections (0.12%). The incidence of both SWI and UTI differed markedly between hospitals, only partially on account of differences in patient mix (age, type of operations), antibiotic prophylaxis, and intensity of tracing methods for NI. Delay in the availability of denominator data hampered the timely feedback of incidence figures. CONCLUSIONS: Surveillance of NI in a network of sentinel hospitals offered valuable information on the occurrence of NI and on factors influencing the incidence of NI. It revealed situations in which both NI surveillance and infection control methods in individual hospitals should be improved. Obtaining denominator data on all patients from electronically registered patient discharge data greatly reduces the workload of ICPs and enables surveillance on all types of NI (all body sites and all pathogens). For timely feedback, numerator and denominator data within hospitals must be linked.
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We studied interindividual variation in pubertal growth patterns from peak growth velocities (PGV) and peak growth ages (PGA) of ventilatory function, standing height, and weight in a selection of 144 boys from a longitudinal survey of 404 pupils in a Dutch secondary school. Measurements were made at intervals of approximately 0.5 yr between 1978 and 1985. Between 9 and 14 measurements were available for each selected individual. Average age on enrollment was 12.7 years. Ventilatory function was characterized by FVC, FEV1, peak expiratory flow (PEF), and maximal expiratory flow at 50% of the FVC (MEF50), derived from maximum expiratory flow volume (MEFV) curves. PGVs and PGAs were derived from monotonically increased regression splines, fitted to the data of each individual and each variable separately. The 90% percentile ranges of PGA were approximately 4.5 yr in all variables. In almost all boys, the PGA of height occurred earlier than that of ventilatory function, but the magnitude of the time lag varied considerably. Median PGAs agreed well with peak growth ages derived from average growth velocity curves fitted on exactly the same data. However, median PGVs were 1.25 to 1.40 times higher than the corresponding estimates from the average curves. The latter finding implies that in almost all cases, individual development deviates considerably from development suggested by average growth profiles. No differences in PGA and PGV were found between subjects with a prepubertal history of respiratory symptoms and those without. The large interindividual variations in PGA and PGV, and in the time lag between growth of height and of ventilatory function, are not accounted for in cross-sectional reference equations. These equations are therefore not suitable to predict individual development during adolescence.
OBJECTIVE: Evaluation of the effect of vaccination against Haemophilus influenzae type b (Hib) on the occurrence of invasive Hib infections in children since its introduction into the national immunization programme in April 1993. DESIGN: Observational study. SETTING: Nationwide investigation. METHOD: Data collected through active surveillance of invasive Hib infections by paediatricians for the period from October 1993 to September 1994 (11 months) were compared with data from the meningitis surveillance by the Netherlands Reference Laboratory for Bacterial Meningitis. RESULTS: A total of 139 paediatric reports of invasive disease by H. influenzae concerned 57 cases of only meningitis, 35 of meningitis with sepsis, 2 of meningitis with arthritis, one of meningitis with arthritis and osteomyelitis, 34 of epiglottitis including one case with sepsis, 8 of only sepsis and 2 of only arthritis. All proven infections by Hib occurred in children who had not or incompletely been vaccinated. One child with sepsis had had three vaccinations and became ill five months later; the isolated bacterial strain was not serotyped. Typing was performed in only 80% of the isolates, of which 98% were of type b. Appropriate culturing and typing was often omitted in case of epiglottitis. CONCLUSION: The effect of vaccination against Hib became apparent in a small number of cases of invasive Hib disease reported by paediatricians; the peak incidence of meningitis no longer occurred in children under one year of age but in children aged one year. The paediatric surveillance described offers possibilities for monitoring Hib epidemiology.
A 7-year-old boy, who had returned to the United States in June 1991 after a 3-year stay in Malawi, was evaluated in October 1991 because of hematuria. He was excreting Schistosoma haematobium eggs and was treated with praziquantel (PZQ; approximately 40 mg/kg). He may have spit up < or = 30% of this dose, and a concomitant Giardia lamblia infection might have caused malabsorption of PZQ. Because of persistent excretion of viable eggs, he was retreated with PZQ in January and May 1992. Egg excretion was first quantified 2 months following his second course of PZQ; at that time it was 35 eggs per 10 mL of urine. He excreted viable eggs at least as late as October 1992, 5 months after his third PZQ course. Experimental administration of chemotherapy to hamsters infected with the S. haematobium strain demonstrated that it was susceptible to PZQ. Repeated courses of therapy with PZQ may be necessary to cure S. haematobium infection, and both parasite and host factors should be considered if infection persists.
We compared the age dependence of cross-sectional and longitudinal changes in ventilatory function. FEV1, FVC, and data on chronic respiratory symptoms were obtained from 4,395 adults in a longitudinal survey of normal populations in two different areas in the Netherlands. They participated in up to five surveys at 3-yr intervals between 1972 to 1973 and 1984 to 1985. The ventilatory function in the oldest cohorts is substantially lower than might have been expected from the longitudinal change in the youngest cohorts. This holds for males and females, smokers and nonsmokers, subjects with or without symptoms, and for both survey populations. The robustness of the findings is demonstrated by various data-analytic strategies or omitting one or two of the five surveys from the analysis. Selective loss to follow-up cannot explain the discrepancy. It is concluded that the main differences between our longitudinal and cross-sectional findings may be due to a cohort effect. The implication is that in longitudinal studies of populations at risk, reference equations based on cross-sectional surveys may overestimate longitudinal change and hence lead to underestimating effects of exposure. Similarly, in clinical studies, accelerated decline in ventilatory function may be underrated if it is compared with cross-sectional standards. In older people at any one age the ventilatory function seems to improve in successive birth cohorts.
OBJECTIVES: The effects of organic dust exposure on the configuration of the maximum expiratory flow volume (MEFV) curve was examined multivariately. METHODS: Data from 390 male workers in the animal feed industry in The Netherlands were analyzed. A multivariate technique called nonlinear canonical correlation analyses was used to study the relationship between a set of organic dust exposure variables and a set of ventilatory function variables. RESULTS: The results indicate an almost independent effect of the overall mean organic dust exposure and the number of years of organic dust exposure on ventilatory function. Increasing mean organic dust exposure was associated with a decrease in both forced expiratory volume in 1 s (FEV1.0) and forced vital capacity (FVC) and decreased flows at high lung volumes only. Increasing number of years of dust exposure was associated with a decrease in FEV1.0 and a decrease in flow at all lung volumes, while the FVC seemed relatively constant. These two distinct patterns of reduction in ventilatory function may represent two different pathological processes. Whereas workers with prolonged exposure showed reduced values for all of the MEFV curve variables, except the FVC, those with only a few years of exposure especially showed a decrease in FVC and peak expiratory flow. The effect of current organic dust exposure was more evident for nonsmokers than for ex-smokers and current smokers. CONCLUSION: The major finding of this study was an independent effect of overall mean organic dust exposure and the number of years of organic dust exposure on the MEFV curve.
Angiotensin-converting enzyme activity was measured in lumbar cerebrospinal fluid from patients with 'probable' Alzheimer's disease (n = 17) and age-matched controls (n = 19), using a spectrofluorimetric method. In contrast to a previous finding, no statistically significant difference in the mean (specific) angiotensin-converting enzyme activity was found between the two groups. No correlation existed between (specific) enzyme activity and severity of dementia in the Alzheimer's disease patients. We conclude that angiotensin-converting enzyme in cerebrospinal fluid does not appear to be useful as a potential antemortem marker for Alzheimer's disease.
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Analysis of the blood flow velocities in the middle cerebral artery by transcranial Doppler ultrasonography was performed in 158 healthy volunteers (aged 14-70 years; 82 men and 76 women). In a subgroup of 38 men and 21 women the end-tidal [CO2] was also measured. The influence of biological factors such as age, sex, end-tidal [CO2], and pulsatility and resistance indices on the mean blood flow velocity in normal ageing was investigated by multiple regression analysis. In both sex groups the measured mean blood flow velocity decreased significantly with age (P values less than 0.0003 for women and less than 0.0001 for men). Women had significantly higher blood flow velocities than men (P = 0.008) and the age-corrected sex difference of 5.2 cm s-1 did not significantly depend on age (P = 0.93). The age-related linear decline of the mean blood flow velocity could not be explained by a concomitant decrease of the end-tidal [CO2]. In a group of subjects older than 50 years, the decrease of the mean blood flow velocity was significantly related to the increase of the pulsatility or resistance index.