From the Primary Care Sciences Research Centre, Keele University, Keele, Staffordshire, United Kingdom.
Received for publication July 21, 2003; accepted for publication December 16, 2003.
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ABSTRACT |
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bias (epidemiology); consent forms; epidemiologic methods; informed consent; longitudinal studies; medical records; selection bias
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INTRODUCTION |
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There are a number of potential effects of "consent" regulations on epidemiologic research. First, asking study participants for consent to review their medical records, for example, may affect survey response rates. Investigators who have examined this have reported conflicting results (25). Second, there are implications for study design, particularly in increasing sample sizes to allow for nonconsent (6). Third, concerns have been raised about whether consent will be obtained uniformly among survey responders and, if some form of selection bias (or "consent bias") exists, how this affects the external validity of study findings (79).
Several studies have examined patterns of consent, but only a few studies have been carried out in samples considered to be representative of the general population, and these have only examined consent to review of medical records (2, 7, 10) or consent to medical examination (11). Other studies have evaluated patients using health services in the United States, either at hospitals (12, 13) or at family practice centers (9). Two studies in the United Kingdom evaluated specific samples of primary care patients (6, 14). A range of types of consent have been requested in these studies, the most common being consent to examination of medical records (2, 6, 7, 9, 10, 12, 13, 15). Other studies have requested consent to being videotaped for research purposes (16), consent to medication review (14), and consent to medical examination (11).
There is limited information available on patterns of consent in general population samples, particularly on consent to be followed up as part of a study. In this paper, we aim to describe and evaluate patterns of consent to follow-up and consent to review of primary-care medical records among patients responding to a number of large-scale epidemiologic surveys in the United Kingdom.
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MATERIALS AND METHODS |
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The studies had a mailing policy of a postcard sent to nonresponders 2 weeks after the baseline mailing and a reminder questionnaire sent to people who still had not responded 4 weeks after baseline. Two exceptions to this policy were study A, where reminder questionnaires were mailed at 4 weeks but postcards were not sent, and study C, where the reminder questionnaire was sent 5 weeks (instead of 4 weeks) after baseline.
The number of questions on the questionnaires used in the studies ranged from 36 to 225 (see table 1), but all contained demographic, disease-specific, and generic items. Two studies surveyed subjects aged 50 years or more (studies B and F); one assessed young adults aged 1825 years (study G); and one studied only females aged 1854 years (study D).
Analysis
Information on age, gender, response, and consent to follow-up and/or consent to review of medical records was collated from all studies. Information on reporting of the symptom under investigation for each study (as specified in table 1) was also collated for survey responders.
The proportion of the total sample responding, the proportion of responders consenting to follow-up, and the proportion of responders consenting to review of medical records were calculated for each study separately and then across all studies. For each study, odds ratios for the effect of age (categorized into age groups) on response and consent were calculated separately for females under age 50 years, males under age 50 years, females aged 50 years or more, and males aged 50 years or more. The youngest age group in each category was used as the reference group (1829 for those under age 50 years and 5059 for those aged 50 years or more). This split by gender and age was used because of the nature of the studies, two of which only considered people aged 50 years or more (studies B and F) and one of which only considered females (study D). It also allows for easier interpretation by researchers considering studies of only one gender or only older or younger populations, but it gives the overall age-gender picture as well. We then pooled these odds ratios across the studies to obtain overall odds ratios with 95 percent confidence intervals for age within each of the four age-gender divisions given above. This was performed using random-effects models (26).
The effect on consent of reporting the symptom under investigation was assessed among survey responders in two ways. First, we performed separate multiple logistic regression analyses for follow-up and review of medical records for each study with age, gender, and self-report of the symptom under investigation entered jointly as predictors of consent. The effects on consent of interaction between age, gender, and the symptom under investigation were then assessed by the addition of three two-way interactions (age x gender, age x symptom, and gender x symptom) to these models. Finally, we pooled the age- and gender-adjusted odds ratios for the symptoms under investigation for each study to obtain an overall summary (a pooled odds ratio with 95 percent confidence interval) of the effect of self-reporting of the symptom under investigation on consent rates, again using a random-effects model.
Analyses were carried out using SPSS for Windows 11.0 (27) and Stata 7.0 (28).
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RESULTS |
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The consent figures for responders pooled across all studies are shown in table 3. The odds ratios for consent to follow-up (pooled across studies C, D, E, F, and G) suggest an increase in the age group 3049 years in comparison with the age group 1829 years, though the figures are not significantly different. However, consent fell significantly with increasing age for both male and female responders over the age of 50 (when compared with the 5059 age group). The proportions of responders consenting in the under-50 group were similar but slightly higher in females than in males, but in the 50-and-over group, higher proportions of male responders consented to follow-up. In particular, there were age-gender interactions in studies E (p = 0.005) and F (p = 0.07), where there were steeper falls in consent to follow-up among older female responders than among male responders. For example, consent fell to below 50 percent among women aged 80 years or more in study F.
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Table 3 also shows the proportions of the total sample (i.e., responders and nonresponders) consenting for each age and gender category, with figures ranging from 22 percent of men under age 30 consenting to follow-up to 67 percent of women aged 4049 consenting to record review. The proportions consenting are higher for females than for males under the age of 50, fairly similar among persons aged 5069 years, and higher for males in the oldest age groups.
Figures 1 and 2 display the effect of having the symptom under investigation on consent to follow-up and consent to review of medical records, respectively, among responders. The squares represent the odds ratio for each study, and the horizontal lines represent the corresponding 95 percent confidence intervals. The size of each square reflects the weight of that study in the pooled odds ratio. The diamonds represent the odds ratios pooled across studies, with associated 95 percent confidence intervals. The relation of age and gender with consent described above remained when it was included in the logistic regression models along with symptom reporting. The interactions of symptom reporting with age and gender were nonsignificant across all studies.
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Figure 2 displays the effect of having the symptom under investigation on consent to review of medical records among survey responders. The results are very similar to those for consent to follow-up, with a pooled odds ratio of 1.44 (95 percent confidence interval: 1.34, 1.55). This indicates that responders who had the symptom being investigated had approximately 1.5 times the odds of consenting to review of their medical records as responders not reporting the symptom. There is consistency in the effect of reporting the symptom across the studies (test for heterogeneity: p = 0.43).
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DISCUSSION |
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Among responders under 50 years of age, although consent was high (7595 percent), persons in the youngest age group and males were slightly less likely to consent. Among responders aged 50 years or more, consent was slightly lower (5080 percent), the proportions consenting declined with age, and male responders were more likely to consent than female responders. Another United Kingdom study also found that younger people were more likely to consent to medical examination (11), though two studies carried out in the United States found that older people were more likely to consent to record review (7, 9). The higher proportion of men consenting than women has been found in other studies evaluating consent to review of medical records (7, 9, 13), consent to medical examination (11), and consent to medication review (14). This pattern for consent contrasts with initial survey response, which this study and other studies (2, 25) have found to be lower in younger people and in men.
Survey responders reporting the symptoms under investigation were more likely to consent, even after adjustment for age and gender, than responders not reporting the symptom under investigation. Similar findings have been reported in other studies; for example, Woolf et al. (9) reported that people in poorer health were more likely to consent to review of medical records, and Petty et al. (14) found that people receiving more repeat prescriptions were more likely to consent to medication review.
These findings mean that people reporting the symptom under investigation could be overrepresented in follow-up samples or reviews of medical records. This may not matter in situations where the focus of any subsequent study is subjects with the condition, since most of those people are likely to consent. However, where a representative sample of the general population is required in follow-up surveys or studies using review of medical records, bias may be a problem, particularly if consent is low. In studies such as these, standardization for the baseline distribution of the symptom or condition should be considered. However, the effect of consent bias may not be important, because consent is generally highfor example, in this study, the overall prevalence of the symptom under investigation was 63 percent, but symptom prevalence among people consenting only increased to 64 percent.
The effect of bias caused by requesting consent is likely to be lesser than the effect of bias caused by selective survey response, since the proportion of the sample responding to the study is generally lower than the proportion giving consent to review of medical records or follow-up. The interaction between consent bias and nonresponse bias is unknown (8, 9). This study has shown that, among survey responders, younger people and men in certain age groups are more likely to give consent, although the percentage of the total sample consenting is still low for younger age groups because of their lower levels of response. Even with a respectable initial response and a high level of consent among responders, these results suggest that only 3060 percent of the original sample (depending on age and gender) will both respond and consent. This should be taken into account in sample size calculations for studies with a longitudinal or multistage design. However, consent is only one step in the study process. Not all people who consent to follow-up will actually go on to participate in follow-up, and biases related to attrition may still be present at later stages of longitudinal studies. Therefore, the percentages of the original sample agreeing to be contacted for follow-up can be viewed as the maximum achievable participation rate at later stages.
One limitation of this study is that only a few self-report symptoms were studied, namely knee, neck, upper limb, and joint pain, headache, and increased vaginal bleeding. However, the studies of these different symptoms all showed consistent patterns, and it seems likely that other questionnaire-based studies of self-reported symptoms would find similar results. In addition, all of the studies were carried out within one specific geographic area, and similar studies in other areas may help to confirm the generalizability of the results.
In this study, we evaluated seven large surveys of a number of symptoms and found consistent patterns of consent. Overall, consent was high among responders, but biases were found; the biases were consistent for different populations but largely different from nonresponse biases. In addition, the additive processes of response and consent mean that less than half of an initial sample may be included in follow-up samples, and not all of these persons will actually respond to follow-up. It is important for researchers conducting longitudinal studies or studies involving review of medical records to be aware that not everyone will consent, and consent may not be evenly distributed among study participants. This information is important in study planning and interpretation. For example, sample sizes should be increased to account for nonconsenters, and researchers may need to carry out sensitivity analyses for investigation of potential effects of bias. However, the potential for bias does not necessarily imply that bias will invalidate study results. Researchers need to investigate the impact that bias may have in their study and how this may alter interpretation of results. Further work is needed to explore methods of increasing the proportion consenting to follow-up or to review of medical records in surveys, particularly among groups in which consent is lower, in order to minimize this source of bias.
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ACKNOWLEDGMENTS |
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Individual study investigators included Dr. H. Boardman, Dr. C. D. Mallen, R. W. McCarney, Dr. E. Thomas, and R. Wilkie.
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NOTES |
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REFERENCES |
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