Summary
Our findings indicate that GPs’ weight, height, BMI and HC measurements are more accurate than their measures of WC and WHR, with errors becoming increasingly more important in higher BMI subgroups.
Comparison with existing literature
Several authors showed that general obesity measurements were more reliable than abdominal obesity measurements and we recently confirmed these findings when these measurements were taken by GPs within their own practice [12, 21, 22]. As suggested in our previous paper, [13] these results are probably explained by the fact that weight and height measurements are universally known and performed using a relatively simple procedure. In comparison, abdominal obesity measurements are newer concepts and require specific manipulation. We discussed the role of GPs’ knowledge and their usual practice in anthropometrics in our previous paper [13]. We showed that, compared to weight, height and BMI, a majority of GPs hardly ever used the abdominal obesity measures and their knowledge regarding these measurements was relatively low.
We showed statistically significant results regarding the mean relative differences between GPs’ weight measurement and the gold standard. However, these differences are not clinically relevant (0.36% measurement error for normal group (absolute difference: 0.22 kg), 0.44% for overweight group (absolute difference: 0.30 kg) and 0.37% for obese group (absolute difference: 0.34 kg)).
We did not record the gender of the volunteers; therefore, we cannot compute the percentage of misclassification, as the definition of abdominal obesity differs by gender. However, in a previous study, [12] we showed that only 1% of the volunteers were misclassified when the measurements were based on the BMI, compared to 6% when using WC measurement, and 23% when using WHR determination.
Our study suggests that the proportions of error increase across BMI subgroups, except for height. These results slightly contrast with a small study, in which Wang examined the association between BMI subgroup and intrarater reliability. Two unexperimented research assistants received training and together measured WC and HC on 76 participants, twice within a 10-min interval: one was responsible for placing the tape and the other for reading the tape and recording the data. The reliability of these measurements was found to be high for all subgroups, without significant differences across BMI subgroups [14]. The design of this study did not include a gold standard, thus precluding the assessment of measurement error and direct comparison with our findings. In addition, the authors used Asian definitions of overweight and obesity to define BMI subgroups, with lower BMI cut-offs. In another study by Nordhamn [15], WC and HC were measured by two raters in 26 overweight and 25 lean participants. Each participant was measured four times, three times on the first occasion and once on the second. The authors concluded that reliabilty decreased in overweight participants for WC and WHR determination, but not for HC [15]. This study also had important limitations, which makes comparisons with our results difficult: measurement error could not be calculated (no gold standard), WC and HC were measured with the participant in supine position, the definitions of BMI subgroups were unusual and only two groups were included (lean: BMI < 26 kg/m2, overweight: BMI ≥ 26 kg/m2). To our knowledge no such studies have been carried out in primary care settings.
Limitations
First, the GPs were not selected at random (convenient sample); as a consequence, our findings may be too conservative, as these GPs may have been more concerned with the subject covered by our study, and therefore may take the anthropometric measurements more frequently and/or more carefully. Second, the study was carried out only in the Geneva area and the study sample may not be representative of all GPs in Switzerland, or Europe more broadly. Third, the study sample was biased towards a higher proportion of overweight (33.2%) and obese (21.6%) volunteers than in the general population in Switzerland [23]. Fourth, we did not record the age and gender of the volunteers, so we cannot provide adjusted mean differences.
Strengths
The study was undertaken within the normal conditions of day-to-day clinical practice and involved GPs with no particular training in anthropometric measurements. There were only minimal differences between the two research assistants in their mean measurements; this added strength to the value of our gold standard. Misclassification was unlikely, as we used the research assistants’ measurements to stratify the subjects into BMI subgroups. Finally, the BMI subgroups were defined using the usual WHO definitions of overweight and obesity [24].