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Biomedical subjects

N J Birkett

Publications and source records attributed to N J Birkett.

15 recordsLinked to original sources
PubMed1992

Effect of nondifferential misclassification on estimates of odds ratios with multiple levels of exposure.

Nondifferential misclassification of exposure status with a dichotomous exposure will produce biased estimates of odds ratios such that the misclassified odds ratio is always biased toward the null value. However, when an exposure classification has more than two levels, empirical data indicate that the direction of bias is less predictable. Analysis of an algebraic model of multi-level exposure misclassification reveals that all odds ratios based on the misclassified data are constrained between the nonmisclassified odds ratio for the most extreme category and the inverse of this value. This implies that the misclassified odds ratio for the most extreme exposure level will be biased toward the null but that odds ratios for intermediate levels of exposure could be biased away from the null value. Further, the amount of bias depends not only on the misclassification rates but also on the distribution of subjects across exposure levels. If it is assumed that misclassification is likely to occur only between adjacent categories, the range of possible misclassified odds ratios is reduced but is still sufficient to permit serious distortion of an exposure-response relation. In general, biases away from the null occur only for intermediate levels of exposure. Reversal of an exposure-response relation is more likely to occur when misclassification rates are high (especially between nonadjacent levels) and when the number of exposure levels is low.

Bias↗
PubMed1989

A tool for assessing inadequate dialysis.

Blood urea nitrogen (BUN) is thought to be a useful marker of the concentration of other solutes that underly the manifestation of uremia. By monitoring quality assurance parameters, 8 out of 57 dialysis patients were found during monthly blood work exams to have BUN greater than 100 mg/dl. An "Inadequate Dialysis Checklist" was designed for hemodialysis and peritoneal dialysis and used to assess all eight patients' treatments. The dialysis was corrected or integrally modified. As a result, the number of patients with BUN greater than 100 mg/dl decreased. These checklists can be useful for case managers to assess and evaluate dialysis variables.

Blood Urea Nitrogen↗
PubMed1989

Payment for deliveries in Sierra Leone.

The type and amount of payment for deliveries were investigated in 1982 during a survey on health status in two districts. Data on the payments made for 83.5% of the 2591 deliveries in 535 randomly selected study villages showed that the most common method of payment was in cash only. Payments in kind were mostly given to trained traditional birth attendants (TBAs) (for 38.1% of their deliveries) and rare for professional staff (2.9% of deliveries). The total amount paid for a delivery differed significantly with the type of birth attendant (P less than 0.00001) and the place of delivery (hospital, peripheral health unit or home) (P less than 0.00001). The total average payment for a delivery was highest for professional birth attendants (Le 16.60) and lowest for untrained TBAs (Le 4.85) (Le 2 = approx. US+ 1 at the time of the study). The outcome of a delivery had a significant effect on the amount paid. Payments were significantly higher for stillbirths than for live births among professional and auxiliary birth attendants (P less than 0.0001). However, the trained and untrained TBAs received less payment for stillbirths (Le 2.25) than for live births (Le 4.89) (P = 0.0146). The results show that there are several levels of financial disincentives for pregnant women requiring the services of trained auxiliary or professional health workers at the time of delivery.

Cross-Sectional Studies↗
PubMed1988

Evaluation of diagnostic tests with multiple diagnostic categories.

The evaluation of diagnostic tests attempts to obtain one or more statistical parameters which can indicate the intrinsic diagnostic utility of a test. Sensitivity, specificity and predictive value are not appropriate for this use. The likelihood ratio has been proposed as a useful measure when using a test to diagnose one of two disease states (e.g. disease present or absent). In this paper, we generalize the likelihood ratio concept to a situation in which the goal is to diagnose one of several non-overlapping disease states. A formula is derived to determine the post-test probability of a specific disease state. The post-test odds are shown to be related to the pre-test odds of a disease and to the usual likelihood ratios derived from considering the diagnosis between the target diagnosis and each alternate in turn. Hence, likelihood ratios derived from comparing pairs of diseases can be used to determine test utility in a multiple disease diagnostic situation.

Bayes Theorem↗
PubMed1988

Computer-aided personal interviewing. A new technique for data collection in epidemiologic surveys.

Most epidemiologic studies involve the collection of data directly from selected respondents. Traditionally, interviewers are provided with the interview in booklet form on paper and answers are recorded therein. On receipt at the study office, the interview results are coded, transcribed, and keypunched for analysis. The author's team has developed a method of personal interviewing which uses a structured interview stored on a lap-sized computer. Responses are entered into the computer and are subject to immediate error-checking and correction. All skip-patterns are automatic. Data entry to the final data-base involves no manual data transcription. A pilot evaluation with a preliminary version of the system using tape-recorded interviews in a test/re-test methodology revealed a slightly higher error rate, probably related to weaknesses in the pilot system and the training process. Computer interviews tended to be longer but other features of the interview process were not affected by computer. The author's team has now completed 2,505 interviews using this system in a community-based blood pressure survey. It has been well accepted by both interviewers and respondents. Failure to complete an interview on the computer was uncommon (5 per cent) and well-handled by paper back-up questionnaires. The results show that computer-aided personal interviewing in the home is feasible but that further evaluation is needed to establish the impact of this methodology on overall data quality.

Blood Pressure↗
PubMed1988

Caffeine-containing beverages and the prevalence of hypertension.

Caffeine can produce a mild hypertensive effect for a few hours after use. Some epidemiological data relate chronic coffee intake to an increased risk of cardiovascular disease. We explored the possibility that a regular intake of caffeine-containing beverages (tea, coffee, cola) might produce a chronic increase in blood pressure and increase the likelihood of developing sustained hypertension. A random population survey of 2436 adults was made, with standardized blood pressure readings and a food-frequency questionnaire to determine the regular intake of various nutrients. We found evidence that caffeine intake was positively related to an increased diastolic blood pressure but the effect was small (less than 1 mmHg at usual caffeine intake). There was no evidence that a regular caffeine intake increases the risk of being classified as hypertensive.

Age Factors↗
PubMed1987

Assessing hypertension control in the community: the need for follow-up measurements to ensure clinical relevance.

In community surveys of hypertension control the diagnosis is often based on blood pressure measurements taken on only one visit. The clinical diagnosis of hypertension requires demonstration of sustained blood pressure elevation. We conducted a survey that contrasted the results of these two approaches to determining the prevalence of hypertension and the extent to which hypertension is detected and treated. A multistage random sample of 2737 people was selected, examined and interviewed on up to three occasions. Rates of hypertension prevalence and control were computed from data from one, two and three visits. The prevalence of hypertension was overestimated by 30% when the diagnosis was based on data from one rather than three visits, the rates being 149 and 115/1000. The prevalence of undetected hypertension was overestimated by 350%, the rates being 27 and 6/1000. The proportion of subjects with controlled hypertension was underestimated by 23%, at 56%, compared with 73%. These results confirm the need for follow-up measurements to provide a valid assessment of hypertension control in the community.

Adolescent↗
PubMed1987

Factor VIII concentrate for hemophilia: comparison of two heat-treated products.

In a controlled trial of heat-treated factor VIII concentrate from Cutter Laboratories and from Connaught Laboratories Limited, 25 patients with hemophilia received Cutter factor VIII and 24 patients with hemophilia received Connaught factor VIII. The mean increase in factor VIII level was similar for the two products (1.88% and 1.98% per unit/kg of body weight respectively [p greater than 0.1], or 89% and 95% respectively of the expected increase [p greater than 0.1]). In-vivo survival (the duration of factor VIII in the circulation) was compared after seven infusions of each product. The mean half-disappearance times were 10.7 and 9.1 hours respectively (p = 0.1), and the mean biologic half-lives were 11.6 and 9.9 hours respectively (p = 0.04); the clinical significance of the latter difference is dubious. Both products could be reconstituted satisfactorily from the dried state. Two subjects experienced relatively minor side effects following infusion of one lot of Cutter factor VIII. Individual responses to both products showed considerable variation. Therefore, when it is critical to attain predetermined levels of factor VIII, such as before and after surgery, factor VIII assays should be performed to check the patient's response.

Acute Disease↗
PubMed1986

Hypertension control in two Canadian communities: evidence for better treatment and overlabelling.

We evaluated the prevalence and control of hypertension in two Canadian cities without university medical centre facilities. A stratified multistage probability sample was selected, and we interviewed 6258 adults between the ages of 30 and 69 inclusive. Blood pressure measurements were obtained during home interviews. Up to two further visits were made to people with untreated blood pressure elevation. By a diagnostic criterion of 90 mmHg, the hypertension prevalence was 114/1000. Six per cent of the hypertensives were undetected, 6% detected but untreated, 17% treated but uncontrolled and 70% were being treated and controlled. Control was better in females and older subjects. These findings show no disadvantages to hypertensives living away from university medical centres. We found a hypertension prevalence of 143/1000 among people who reported being diagnosed as hypertensive but who had normal blood pressure while not on medication. These results suggest a problem with over-labelling of hypertensives.

Adult↗
PubMed1985

Prevalence and control of hypertension in an Ontario county.

A survey of a representative population sample was carried out to evaluate the prevalence and control of hypertension in Middlesex County, Ontario. Of the 3067 subjects selected 2735 completed the initial interview. If the diastolic blood pressure was greater than 89 mm Hg in three readings, up to two further visits were made. The prevalence rate of hypertension in the sample was estimated to be 115/1000. Only 5.1% of the hypertensive subjects were unaware of their condition, and 5.4% were aware but not receiving treatment. In 16.9% the hypertension was treated but uncontrolled, while in 72.6% it was treated and controlled. The prevalence rate was significantly higher in the older subjects (p less than 0.0001). Control was better in the women and the older subjects. The results indicate that physicians in Middlesex County are detecting and treating most patients with hypertension; screening programs are thus not needed. Control of hypertension could be further improved by determining why the condition in those receiving treatment is not being controlled.

Adolescent↗
PubMed1985

Adaptive allocation in randomized controlled trials.

Adaptive allocation has been proposed as a procedure to reduce the risk of chance imbalance of important prognostic factors in randomized controlled trials when the number of prognostic factors is large. In this article, minimization, a type of adaptive allocation, is compared to simple randomization and stratified allocation in a series of Monte Carlo simulations. Three outcomes are studied: estimated treatment effect, size of the rejection region, and power. Minimization produced an unbiased estimate of treatment effect and increased power when compared to simple randomization. Student's t test was conservative for both minimization and stratified allocation. Minimization and stratification produced similar improvements in power but there was some evidence that minimization might produce higher power than stratification when some prognostic variables cannot be included in the stratified allocation scheme.

Analysis of Variance↗
PubMedDate not supplied

The role of health care practitioners in the detection and control of hypertension.

It is well established that pharmacologic treatment of sustained diastolic blood pressure elevation can reduce the risk of subsequent cardiovascular morbidity and mortality. One problem confronting health care workers is to ensure that the maximum number of people who would benefit from antihypertensive treatment actually are brought under treatment and good control. Because hypertension is asymptomatic, it has been suggested that routine blood pressure measurement by a wide variety of health care practitioners would help to increase coverage. A community blood pressure survey was done to estimate detection and control rates. Information also was collected on recency of attendance to seven categories of health care practitioners. Nearly 90% of the population had seen a physician within the previous two years. Although nearly all subjects had visited a dentist, only 75% had visited within the last two years. For the other categories, population coverage was much lower. There was no evidence that undetected or untreated hypertensive persons were more likely to be seen by nonphysicians. These results do not identify a role for nonphysicians in hypertension detection or monitoring. Rather, routine blood pressure measurement on all patients by physicians should be adequate to ensure high detection and treatment rates.

Adult↗
PubMedDate not supplied

Does a mailed continuing education program improve physician performance? Results of a randomized trial in antihypertensive care.

Evidence is sparse concerning the value of the "educational" materials that physicians receive in the mail. We conducted a randomized trial of a mailed continuing education program on hypertension for primary care physicians. Although formal pretesting documented that the program led to significant improvements in physician knowledge over the short term, the current study showed no lasting effect on physician knowledge (mean scores on an end-of-study questionnaire were 50% and 52% for study and control physicians, respectively) and no influence on performance in lowering the blood pressures of patients referred from screening (mean blood pressure drop for study patients, 12.2/10.4 mm Hg vs 13.0/10.6 mm Hg for control patients). The chance that we missed a difference in diastolic blood pressure as great as 3 mm Hg is less than 5%. Resources spent on instructional materials mailed to physicians may be wasted.

Adult↗