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,1
* Institute of Cancer Epidemiology, The Danish Cancer Society, Copenhagen, Denmark;
Danish Epidemiology Science Centre and Research Unit for Dietary Studies at the Institute of Preventive Medicine, Copenhagen University Hospital, Copenhagen, Denmark; and
** Department of Clinical Epidemiology, Aalborg Hospital, and Aarhus University Hospital, Aalborg, Denmark
1To whom correspondence should be addressed. E-mail: j.bigaard{at}dadlnet.dk.
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ABSTRACT |
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 TOP ABSTRACT SUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • waist circumference • self-reported • umbilicus • middle age
The worldwide obesity epidemic (1) has led to a demand for simple obesity indicators to monitor the development and prevalence of obesity. The WHO recommended the use of the BMI (weight divided by height squared) as an indicator of obesity (1). BMI does not directly reflect the distribution of fat in the body (2). In the mid 1990s, waist circumference was introduced as an independent risk indicator identifying individuals with need for weight management (3) and an accumulation of cardiovascular risk factors (4,5). Waist circumference was implemented in clinical guidelines for the treatment of obesity (1,6,7). The evidence for waist circumference as a risk indicator is accumulating (8–12).
Earlier validation studies of self-reported vs. technician-measured waist circumference were published in the early 1990s, before identification of the obesity epidemic; most studies used correlation and comparison of the mean differences (13–21), although this approach may be misleading. A high correlation between 2 methods occurs if 1 method always measures twice as much as the other because correlation assesses only the closeness to a linear relation between the methods and not agreement or whether the methods can be used interchangeably (22,23). The degree of correlation does indicate how well 1 method works as a proxy for the other as a linear covariate or as an outcome in a regression analysis aimed at establishing statistical significance. In this case, a constant displacement does not matter, but a slope different from 1 in the linear relation between the 2 measurement methods would bias the estimated association in future regression analyses accordingly. In categorical analyses using prespecified limits for the categories, a constant displacement would lead to some misclassification.
Follow-up data of waist circumference and weight were collected for the Danish Diet, Cancer and Health cohort to evaluate the importance of changes in body size and fat distribution for the development of cancer and other diseases. At the 5-y follow up, we collected self-reported values of weight and waist circumference at the level of the umbilicus. At baseline, laboratory technicians measured weight, height, circumference at the natural waist, and hip circumference. We changed the body site for measurement of waist circumference to the level of the umbilicus at follow-up to simplify the measurement instructions for participants using umbilicus as a body mark for measurement.
The aim of this study was to investigate agreement 1) between self-reported and technician-measured waist circumference at the level of the umbilicus, 2) between waist circumference measured at the level of the umbilicus and at the narrowest part between the lower rib and the iliac crest (the natural waist) and 3) between self-reported waist circumference at the level of the umbilicus and technician-measured waist circumference at the natural waist. Further, we investigated the influence of the body size and the body shape on the possible differences. We considered technician-measured BMI and baseline body shape for differences 1) and 2). Difference 3) is the important difference in future studies evaluating the association between changes in waist circumference and different exposures in the total cohort. We therefore investigated body size measurements available for the total cohort, namely, baseline technician-measured BMI and baseline body shape measurements.
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SUBJECTS AND METHODS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Follow-up.
All participants received a postal questionnaire between September 1999 and August 2002, 
5 y after baseline data collection. In the questionnaire, participants were instructed to report their weight in kilograms and their waist circumference in centimeters measured at the level of the umbilicus using an enclosed paper measuring tape.
Validation study.
In connection with the data collection for the 5-y follow-up, participants were recruited for a validation study. Potential participants were recruited in 2 rounds (Fig. 1). In round 1 of the recruitment (June–August 2001), potential participants returned their follow-up questionnaire between October 1, 1999 and June 1, 2001 and 44% participated (256 of the 582 invited). All participants were inhabitants of Copenhagen County. In round 2 of the recruitment (November–December 2001), potential participants returned their follow-up questionnaire between August 1 and October 1, 2001 and 62% participated (160 of the 258 invited). All participants were inhabitants of Copenhagen County. The study was conducted in accordance with the Helsinki Declaration II and approved by the Ethical Committees on Human Studies in Copenhagen and Aarhus municipalities.
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Measurements were performed in a study clinic situated in Copenhagen. Measurements were performed in accordance with instructions for the baseline data collection. Height was measured with the participants standing without shoes and was recorded to the nearest 0.5 cm. Weight was measured using a digital scale, with the participants wearing light clothing or underwear, and was recorded to the nearest 100 g. Waist circumference was measured at the narrowest part between the lower rib and the iliac crest (the natural waist) or, in the case of an indeterminable waist narrowing, halfway between the lower rib and the iliac crest, and was recorded to the nearest 0.5 cm. Additionally, waist circumference was measured at the level of the umbilicus and recorded to the nearest 0.5 cm. Two investigators, the first and the second authors of this paper, performed the anthropometric measurements, except for 7 participants measured by a third technician. The interobserver variation of the waist circumference measured at the level of umbilicus by the 2 investigators was investigated in 47 participants (22 men and 25 women).
Statistical methods.
The different measurements of waist circumferences were compared by paired t tests. For illustrative purposes, we calculated Spearman’s correlation between measurements. Agreements between measurements were illustrated in Bland-Altman plots (22,23) in which the difference between the 2 measurements are plotted against the mean of the 2 measurements. Limits of agreements were calculated as the mean difference ± 2 SD, and the 95% CI was calculated using Bland and Altman’s method to assess the precision of these limits (23).
Multiple regression was used to identify variables explaining difference 1: self-reported minus technician-measured waist circumference at the level of the umbilicus; difference 2: waist circumference measured at the level of the umbilicus minus circumference measured at the natural waist; and difference 3: self-reported waist circumference at the level of umbilicus minus technician-measured circumference at the natural waist. We investigated the following variables as covariates for the agreement between measurements; difference 1: months between measurements, body size as current BMI (current weight divided by current height squared, weight and height measured by the technicians in the validation clinic), and body shape (waist and hip circumference measured at baseline); difference 2: the same variables except "months between measurements," which was omitted because measurements took place the same day; and difference 3: months between measurements, current BMI (weight and height measured by the technicians in the validation clinic), baseline BMI (baseline weight divided by baseline height squared), and body shape (waist and hip circumference measured at baseline). The analyses were performed using SAS (SAS Institute, version 8).
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RESULTS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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65 y) among 9647 potential participants who had returned their follow-up questionnaire (Fig. 1). Of the 582 invited, 256 (44%) visited the validation clinic. In round 2 of the recruitment (November–December 2001), 258 people were invited after random selection stratified according to sex, age group, and BMI (<18.5, 18.5–24.9, 25–29.9, 30 kg/m2) among the 1927 potential participants who had returned their follow-up questionnaire within the previous 2 mo (Fig. 1). However, to recruit a group of women with BMI < 18.5 kg/m2, we allowed a time interval of up to 6 mo after return of their follow-up questionnaire. It was not possible to recruit a similar group among men, because very few had a BMI < 18.5 kg/m2. Of the 258 invited, 160 (62%) visited the validation clinic. A total of 416 (176 men and 240 women) were recruited for the validation study; however, 8 were excluded from analyses using self-reported waist circumference because they did not report waist circumference in the follow-up questionnaire, leaving 408 (173 men and 235 women) eligible for those analyses. Descriptions of age and body measurements at baseline and follow-up, the variables used in the analyses, are reported in Table 1. The number of months between the return of the questionnaire (self-reported measurement of waist circumference) and the visit to the validation clinic (technician-measured waist circumference) was <6 mo for 53% of the men and 75% of the women (Table 2). In round 2 of invitation, all men and 89% of the women had time intervals < 3 mo.
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0.18), but not both for women (P = 0.001), showing that the 2 circumferences captured the same underlying aspect equally well for women. When current BMI and baseline body shape measurements, waist and hip circumference, were mutually adjusted, current BMI was most important in men but not in women. The included variables explained only 10% of the variation in men and 7% in women.
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0.12 cm (95% CI: 0.06, 0.18) larger for men and 0.11 cm (95% CI: 0.02, 0.24) larger for women. When mutually adjusted (column 2), body size (current BMI) was not significant, whereas body shape remained significant for the difference in both men and women. The variables included explained only 10% of the variation in men and 5% in women.
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DISCUSSION |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Studies frequently use correlation coefficients to evaluate the agreement between the measurements, but the correlation depends on the range and the distribution of the variables; furthermore, correlation ignores any systematic bias between the 2 measurements (23). Bland and Altman defined limits of agreement as the interval including 95% of the observed differences (22,23). It is a clinical, not a statistical decision, how narrow these limits of agreement should be to consider the agreement between the 2 methods to be good (23); our findings are examples of high correlation between measurements but poor agreement because of systematic errors.
Earlier validation studies of the waist circumference based the evaluation of the agreement between the measurements on the correlation, whereas the limits of agreement were not calculated (13–21). The few studies using limits of agreement to evaluate agreement between measurements also found wide limits of agreement (poor agreement). In the Glasgow MONICA study (20), the mean difference between self-reported and technician-measured waist circumference was –6.7 cm in men and the limits of agreement were from –21.0 cm to +7.7 cm. In women, the mean difference was –4.3 cm with limits of agreement from –21.1 to +12.3 cm. In the same study, the use of a specially designed "Waist Watcher" measuring tape gave a closer agreement; in men the mean difference was –0.5 cm with limits of agreements from –6.2 cm to +5.2 cm, and in women the mean difference was –0.4 cm with limits of agreement from –5.0 cm to +4.2 cm (20). In the Health Professionals’ Follow-up Study, close agreement was found; the mean difference was +0.14 cm and the limits of agreement were from –6.02 cm to +6.30 cm (24). From the same cohort, an earlier publication found a mean difference of +0.36 cm in men and we calculated the corresponding limits of agreement to be from –5.1 cm to +6.9 cm; in women, the mean difference was –0.05 cm with limits of agreement from –10.2 cm to +10.0 cm (participants in the Nurses’ Health Study) (15). These findings are in accordance with our results considering that participants in both the Males Professionals’ Follow-up Study (15,24) and the Nurses’ Health Study (15) were not lay people and 1 study used a specially designed measuring tape (20).
Some studies included pictorial measuring instructions (16,18), or the participants were asked to seek help from another person (13). Our questionnaire included short, simple written instructions: "Measure the waistline at the level of the navel, exhale, and mark the circumference on the measuring tape. Note the circumference in rounded centimeters."
We had expected that these middle-aged individuals would not be so concerned about their body size and thus would report their waist circumference in an unbiased manner. This assumption was incorrect. An English validation study of weight in women aged 60–79 y had a similar hypothesis, but elderly women in that study also underestimated their weight (25).
The finding that waist circumference is larger at the level of the umbilicus agrees with another study showing that the waist circumference measured at the narrowest waist was smaller than the waist circumference measured immediately below the lower rib, which was again smaller than the waist circumference measured halfway between the lower rib and the iliac crest, and that the waist circumference measured immediately above the iliac crest was largest (26). Waist circumference at the level of the umbilicus probably corresponds to a measurement immediately above the iliac crest. The difference between the waist circumference measurements at the 2 body sites depended on body shape (waist and hip circumference). Measuring the waist circumference at the level of the umbilicus includes an aspect of the hip circumference and this may explain the poorer agreement between the 2 waist circumference measurements among the women even though measured by technicians.
The high correlation between self-reported waist circumference at the level of umbilicus and technician-measured waist circumference at the natural waist indicated that the self-reported waist circumference is valuable as a proxy for the technician-measured waist circumference in future studies of changes in waist circumference in the Danish Diet, Cancer and Health study. However, the mean change will be overestimated for women and slightly underestimated for men, which could lead to misclassification in categorical approaches. Perhaps more importantly, the association of the difference between the 2 measures with the baseline BMI (men) and baseline waist circumference (women) indicated that it is imperative that future analyses be adjusted for these variables because otherwise the measurement error may capture the association with baseline waist circumference and BMI, which would bias the results.
In conclusion, self-reported measurements underestimated waist circumference in both men and women, and the underestimation increased with increasing body size. The limits of agreement were very wide, indicating poor agreement between measurements. Waist circumference measured at the level of the umbilicus was larger than that measured at the natural waist; the limits of agreement were acceptable from a clinical point of view in men, but very wide in women, indicating poor agreement. If possible, the same body sites for measurements of the waist circumference at baseline and at follow-up would be preferred in longitudinal studies. The self-reported waist circumference seemed to be usable as a proxy for technician-measured waist circumference in regression analyses, but such analyses should be adjusted for baseline waist circumference and baseline BMI.
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ACKNOWLEDGMENTS |
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Manuscript received 2 February 2005. Initial review completed 8 March 2005. Revision accepted 1 July 2005.
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LITERATURE CITED |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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