The authors describe and empirically demonstrate a form of bias that results from deriving subjects for clinical studies from available patients currently being followed in specific disease clinics instead of inception cohorts (patients enrolled at a uniform and early point in their disease). They label this effect "clinic patient bias." It is a variation of prevalence-incidence (Neyman) bias in that it also results from the time gap between the onset of a specific characteristic (a risk factor, exposure or disease) and enrollment in the study, causing selective exclusion of fatal or short episodes, or mild or silent cases. Clinic patient bias may distort an estimate of relative risk in either direction. The empirical example is derived from a study of risk factors for developing complications such as peritonitis among end-stage renal disease patients treated with continuous ambulatory peritoneal dialysis (CAPD). The use of available clinic patients rather than an inception cohort (patients newly beginning CAPD) resulted in the demonstration of false apparent risk relationships for two variables: the calendar date when patients began CAPD (with those enrolled at an earlier time appearing to be at lower risk), and serum albumin level at the start of CAPD (with those having lower albumin levels appearing to be at higher risk). This example demonstrates one of the potential hazards of using active or available clinic patients as a source of subjects for clinical studies.

Cross-Sectional Studies↗

PubMedDate not supplied

Water nitrates and CNS birth defects: a population-based case-control study.

The relation between maternal exposure to nitrates in drinking water and risk of delivering an infant with a central nervous system (CNS) malformation was examined by means of a case-control study in New Brunswick, Canada. All cases of CNS defects for a "high" and a "low" prevalence area of New Brunswick, for the years 1973-1983, were included in the study. Controls were selected randomly from the livebirth files for the province, matched on county of maternal residence and date of birth. One hundred and thirty (130) cases were identified and individually matched with two controls each. Individual water samples were collected from the case and control mother's address given on the birth or stillbirth records. The study revealed that the effect of nitrate exposure in water was modified by whether the source of the drinking water was a private well or a public municipal distribution system. Compared to a baseline nitrate level of 0.1 ppm, exposure to nitrate levels of 26 ppm from private well water sources was associated with a moderate, but not statistically significant, increase in risk (risk odds ratio = 2.30; 95% confidence interval = 0.73-7.29). If the source of drinking water was a municipal distribution system or a private spring, an increase in nitrate exposure was associated with a decrease in risk of delivering a CNS-malformed infant; however, these effect estimates were not statistically significant. The positive increase in risk with nitrate exposure from well water sources requires further study using a larger case series and a larger proportion of exposures to nitrate levels exceeding 5 ppm.

Central Nervous System↗

PubMedDate not supplied

An evaluation of 24-hour urinary creatinine excretion for use in identification of incomplete urine collections and adjustment of absorbed dose of pesticides.

The accurate quantification of the absorbed dose of pesticides following occupational exposures generally requires complete 24-hour urine collections, often over extended periods of time. Difficulty in obtaining volunteer cooperation may result in incomplete urine collections. Traditionally, 24-hour urinary creatinine has been used to identify incomplete urine samples and has been used to standardize pesticide and other chemical dose estimates. More recently, the use of creatinine to standardize dose estimates has been questioned, as has its utility in the identification of incomplete urine collections. This research evaluates the use of personal observation, statistical methods, and published models to predict creatinine excretion to identify and adjust for incomplete urine collections. Based on the use of published creatinine prediction models, an evaluation of the day-to-day creatinine excretion within subjects, and personal observation, a small number of suspected urine samples were identified. Although it is likely that these samples were incomplete, correction of these urine volumes based on the published models did little to improve pesticide dose prediction. Further, results indicate that subjects who report missed urine samples may be able to estimate the missing volumes with some accuracy. In future pesticide exposure studies, the use of self-reported missed volumes may help to increase the accuracy of dose prediction when there is strong cooperation with collection procedures. A statistical model to predict creatinine excretion in professional turf applicators was developed to provide a preliminary screening for urinary completeness for future studies in which compliance with urinary collection is thought to be insufficient.

Adult↗

PubMedDate not supplied

A study to validate the Canadian Aerobic Fitness Test.

Our purpose was to assess the validity of VO2 max values predicted from The Canadian Aerobic Fitness Test (CAFT) by comparing them with peak VO2 values measured during a maximal treadmill protocol. Male and female subjects, 15-69 yr (n = 129), performed two submaximal exercise tests (CAFT protocol), and one maximal treadmill test. There was no significant heart rate habituation between the first two CAFT protocols. Peak VO2 values measured during the treadmill test (TM) were significantly higher than those predicted from the CAFT whether the sample was analyzed overall or categorized by sex. When the sample was categorized by age group, all but groups 1 (15-19) and 6 (60-69) had treadmill peak VO2 values significantly higher than those predicted using Jetté's equation. Using treadmill measured peak VO2 scores as the gold standard, VO2 max predictions using the CAFT protocol and Jetté equation placed individuals of lower fitness levels more accurately than highly fit individuals, into one of five fitness categories.

Adolescent↗