Measurement error of waist circumference: gaps in knowledge

Abstract

Objective

It is not clear whether measuring waist circumference in clinical practice is problematic because the measurement error is unclear, as well as what constitutes a clinically relevant change. The present study aimed to summarize what is known from state-of-the-art research.

Design

To identify the magnitude of the measurement error of waist circumference measurements from the literature, a search was conducted in PubMed from 1975 to February 2011.

Results

The measurement error may vary between 0·7 cm and 15 cm. Taking a realistic range of measurable waist circumference into account (60–135 cm), we argue that a short-term clinically relevant change in waist circumference of 5 % may lie between 3·0 and 6·8 cm and a maintained clinically relevant change of 3 % between 1·8 and 4·1 cm.

Conclusions

Based on these results, we conclude it may be difficult to distinguish clinically relevant change from measurement error in individual subjects, due to the large measurement error and unclear definition of clinically relevant change. More research is needed to address these gaps in knowledge. To minimize measurement error, we recommend using a uniform measurement protocol, training and repeated measurements.

Clinical guidelines recommend physicians to base their obesity-related cardiovascular risk management on abdominal as well as general obesity^{( 1 , 2 )}. Abdominal obesity (as measured by waist circumference) has been conveyed as a better independent predictor of obesity related-disorders than general obesity (as measured by BMI)^{( 3 – 5 )}. The presence of abdominal obesity can thus indicate the need for interventions in subjects who would otherwise not be considered at risk based on general obesity alone^{( 1 )}. Measuring abdominal obesity using the waist circumference has been marked as feasible because it is easy to learn, takes no more time than measuring body height and weight, and requires minimal costs^{( 6 )}.

Waist circumference is now used more often to monitor changes as a result of interventions, not only by trained researchers but also by clinicians in primary care settings as well^{( 7 – 9 )}. However, physicians report they find it hard to measure^{( 10 – 12 )}. Moreover, studies showing good reliability of waist circumference measurements are mainly performed by health professionals trained in anthropometrics (1·3 to 6·5 cm)^{( 13 – 15 )} while studies in which measurements are conducted by physicians show larger variability (0·7 to 12 cm)^{( 16 – 18 )}.

The consequences of variability for clinical practice are not yet clear. This depends on whether the variability is so large that clinically relevant changes within subjects or clinically relevant differences between subjects cannot be measured reliably. Three problems can be identified here. First, only a few reliability studies are available. Second, many studies report reliability (e.g. the intra-class correlation coefficient), but not an absolute measurement error (e.g. in centimetres). This information is needed to interpret change scores in individual subjects in clinical practice^{( 19 )}. Third, it is not clear what a clinically relevant change in waist circumference is, because there is insufficient evidence on the dose–response relationship between reductions in waist circumference and obesity-related morbidity and mortality^{( 11 , 20 , 21 )}. Thus, it is necessary to summarize what is known from state-of-the-art research and identify gaps in knowledge. The aims of the present study were therefore to: (i) explain the difference between reliability and measurement error and highlight why it is important to determine the measurement error of waist circumference; (ii) discuss what is known about the measurement error of waist circumference and which factors may cause this; (iii) discuss what is known about clinically relevant changes in waist circumference; (iv) discuss how knowledge about clinically relevant changes can help interpret the magnitude and importance of the measurement error of waist circumference; and finally (v) provide recommendations for future research and clinical practice on the measurement error of waist circumference.

The difference between reliability and measurement error

The terms ‘reliability’ and ‘measurement error’ are part of the concept term ‘reproducibility’, as both address whether measurement results are reproducible in test–retest situations^{( 19 )}. Reliability refers to how well subjects can be distinguished from each other in populations, despite the measurement error. This information is required for instruments that are used for discriminative purposes (e.g. to characterize individual differences between subjects in order to establish their clinical status and therapeutic needs, such as for discriminating between overweight and obese subjects). Measurement error assesses exactly how close values of repeated measurements within subjects are. This information is required for instruments that are used for evaluative purposes (e.g. to register change over time). The difference between reliability and measurement error is important for clinical practice because when studies present the reliability (e.g. intra-class correlation coefficient) of waist circumference measurements, a clinician is informed whether the instrument is able to discriminate between (e.g. overweight and obese) subjects in a sample. The clinician is not informed whether the instrument is suitable for monitoring waist circumference of individual subjects over time. In the latter case, the absolute measurement error around a single measurement of a single change score is important^{( 19 )}. This measurement error is expressed in for example the standard error of measurement or the limits of agreement^{( 22 )}. Moreover, measurement error provides an important advantage over reliability for clinical interpretation as it is expressed on the actual scale of measurement (e.g. centimetres), and not as a dimensionless value between 0 and 1. While information on both reliability and measurement error is necessary for clinical practice, reliability is generally high^{( 15 )} but the magnitude of measurement error is not clear. In the present paper we focus on this absolute measurement error, which influences measurements in individual persons^{( 22 )}.

Measurement error of waist circumference

To identify the magnitude of the measurement error of waist circumference measurements from the literature, a systematic search was conducted in PubMed from 1975 to February 2011. Search terms for measurement error were selected from a search filter that was developed for finding studies on measurement properties^{( 23 )} and combined with the text word ‘waist circumference’. Studies using self-reported measurements and those among children or adolescents were excluded because these are associated with higher measurement error^{( 15 )}. Data were extracted on the smallest detectable change (SDC) or smallest detectable difference (SDD), which reflect the smallest change or difference in waist circumference of an individual subject that can be detected beyond measurement error^{( 19 )}. The search resulted in 559 studies, of which nine reported on the intra- or inter-observer measurement error of waist circumference (e.g. repeated measurements on the same subjects by one observer or by different observers, respectively; Table 1). The methodological quality of studies was assessed by two authors (L.M.V. and C.B.T.) using the COSMIN checklist for grading studies on measurement properties (Box C)^{( 24 )}. An overall methodological quality score was obtained by taking the lowest rating of the eleven items (‘worst score counts’) from the following ratings: excellent, good, fair or poor. For example, if for a study one item scored poor, the overall methodological quality of that study was rated as poor.

Table 1.

Studies reporting intra- or inter-observer measurement error of waist circumference measurements

First author, year, reference and country	Design and population	Outcome assessor and training	Number of measurements and protocol	Measurement site	Intra- and inter-observer measurement error (SDC and SDD)	Quality
Bosy-Westfahl, 2010( 25 )	Cross-sectional. 16 lean and obese adults	4 nutritionists. Well-educated and trained, regular comparison of results	3 non-consecutive measurements, one measurement per site. Protocol: refs (1, 38–40).	(i) Lowest rib (distal border) (ii) Iliac crest (superior border)	Intra-observer: (i) 3·3 cm (ii) 6·1 cm	Poor
Germany			Horizontal tape, after normal expiration, non-elastic	(iii) Midway between both	(iii) 5·5 cm
			plastic tape, standing, no		Inter-observer:
			compression of skin		(i) 6·7 cm
					(ii) 15·0 cm
					(iii) 14·1 cm
Dhaliwal 2009( 47 )	Cross-sectional.	2 observers (n unknown at 8 survey sites).	2 consecutive measurements.	Narrowest point between the rib cage and iliac	Intra-observer:	Fair
Australia	9279 subjects from the Australian Risk Factor Prevalence Study (1989).	Trained	Protocol: refs (41, 42).	crest	1·8 cm (men)
	Aged 20–69 years, 93 % Australian, UK, European and 5 % Asian or African		Horizontal tape, after full expiration, metal tape, no compression of skin, no clothing around the waist, front of subjects		1·7 cm (women)
Geeta 2009( 43 )	Cross-sectional.	2 public health nurses.	3 consecutive measurements.	Midway between lowest	Intra-observer:	Fair
Malaysia	130 working adults at selected office setting (2005).	Trained, unaware of objective, previous measurement blinded	Protocol: clinical manual of NHMS III( 45 )	rib margin and iliac crest	1·2 cm (lower, upper limit: −1·1, 1·3 cm)
	Aged 18–64 years, mean age 36 (sd 11) years, 67 % female, 83 % Malaysian.		Horizontal tape, after normal expiration, Seca S201 tape, front of subjects		Inter-observer:
	Non-pregnant, ≥2 months postnatal, no physical disability or body deformation to stand upright				2·1 cm (lower, upper limit: −1·9, 2·3 cm)
Nordhamn 2000( 14 )	Cross-sectional.	5 observers from the metabolic unit.	3 consecutive measurements, 1 measurement after 1–3 weeks.	Midway between lower rib and iliac crest	Intra-observer:	Fair
Sweden	25 lean and 26 overweight (BMI <26 and ≥26 kg/m2) students, university staff and company employees, 50 % men	Trained	Protocol: unknown.		≈5·3 cm (all)
					≈3·8 cm (lean)
			Supine position		≈6·4 cm (overweight)
					Intra-observer:
					≈2·5 cm (all)
					≈3·2 cm (lean)
					≈2·1 cm (overweight)
Panoulas 2008( 17 )	Cross-sectional.	Preliminary study: 4 doctors, 4 nurses, 2 physiotherapists.	Preliminary study: 3 measurements by the 9 other participants.	Midway between the palpated lowest rib margin and iliac crest in the midaxillary lines	Preliminary study	Poor
UK	102 patients in an outpatient	Trained.	Clinical study: ?		Intra-observer:
	department of a hospital for the preliminary study, 28 new patients	Clinical practice study: 3 medical students.	Protocol: ref. (39).		0·7 cm
	matched to 28 original patients for the clinical practice study (2006)	Untrained and after written instructions	During expiration, standard measuring tape, no clothing around the waist		Inter-observer:
					1·4 cm
					Clinical study, untrained
					Inter-observer:
					2·6 cm (n 102), 2·5 cm (n 28)
					Clinical study, trained
					Inter-observer:
					3·3 cm (n 28)
Sebo 2008( 16 )	Cross-sectional.	12 doctors.	2 consecutive measurements by all 12 doctors, repeated after 1 week.	Midpoint between lowest rib and iliac crest	Inter-observer:	Poor
Switzerland	24 healthy adult volunteers.	Untrained and after a 1 h	Protocol: refs (3, 44–46).		12·0 cm (untrained)
	Mean age 41 (sd 14) years, 54 % women, 21 % overweight, 42 % obese	training session (theory, demonstration, practice)			7·2 cm (trained)
			Horizontal tape, end of normal expiration, standard measuring tape, standing, no clothing around the waist, no compression of skin
Sicotte 2010( 18 )	Cross-sectional.	1 doctor, 2 health professionals, without prior experience.	2 times on 2 consecutive days by the same observer on 2 study occasions.	Uppermost limits of the ileum	Intra-observer:	Poor
Canada	12 (at 3 months) and 7 other (at 18 months) HIV-patients in Mali (year unknown)	Trained, written instructions, practice every 2 weeks during 3 months	Protocol: unknown.		3·4–9·2 cm
			Marks on location, horizontal tape, non-stretchable Gulick tape		Inter-observer:
					5·5–6·5 cm
Ulijaszek 1999( 15 )	Review:	Health professionals	Not specified	Not specified	Inter-observer:	Fair
UK	(i) 2 studies between 1987 and 1995				(i) 3·6 cm (range: 2·8–4·4 cm)
	(ii) 8 studies between 1987 and 1997				Intra-observer:
					(ii) 6·5 cm (range: 1·7–11·7 cm)
Wang 2010( 28 )	Cross-sectional.	2 research assistants, without prior experience.	2 times with 10-min intervals.	Midpoint between lowest rib and iliac crest	Intra-observer:	Fair
Taiwan	76 participants from in-patient wards	Trained, instructions, 20-min	Protocol: unknown.		3·0 cm (all)
	(year unknown).	practice	Horizontal tape, end of normal		1·7 cm (underweight)
	33 % men, mean age 47 (sd 14)		expiration, inelastic plastic		1·7 cm (normal),
	years, underweight (n 5),		measuring tape, standing, feet		2·2 cm (overweight)
	normal (n 27), overweight (n 21), obese (n 23)		apart and arms hanging freely		4·7 cm (obese)

ref./refs = reference number(s) in the References list.

The selected studies included between seven and 9279 participants, consisting of healthy adults to employees or patients. The outcome assessors were physicians in three studies and other health professionals in six studies. The outcome assessors were trained in advance in seven studies or between repeated measurements in two studies. All followed a standard (although different) protocol. Participants were measured in standing position, except for one study that measured participants in supine position. These measurements were carried out midway between the lower rib and the iliac crest in five studies, at the narrowest point between the rib cage and the iliac crest in one study, and at the uppermost limit of the ileum in another study. One study examined the effect of measurement site (lower rib, iliac crest or midway) on the measurement error^{( 25 )}. Finally, the overall methodological quality of the studies was fair or poor.

Overall, the intra-observer measurement error varied from 0·7 cm to 9·2 cm. The inter-observer measurement error varied from 1·4 cm to 15 cm (Table 1). In most studies that measured both, the intra-observer measurement error was smaller^{( 15 )}. Moreover, smaller intra- and inter-observer measurement errors were found in larger studies. No notable differences in relation to the measurement error were observed according to participant characteristics, outcome assessor, measurement protocol, effects of training or methodological quality. However, greater measurement error was reported from measuring at the iliac crest or midway, compared with the lower rib, possibly because the latter is most easily located^{( 25 )}.

Based on the small number of studies and the many differences between the studies, we conclude that it is difficult to draw conclusions on the magnitude of measurement error. Moreover, the variation in measurement error may be caused by a number of other factors not mentioned in Table 1 such as muscle mass, bone structure, lean tissues, looseness of abdominal muscles, posture, phase of respiration and time since the last meal^{( 26 , 27 )}. Additionally, measurement error may be larger among overweight and obese subjects compared with normal-weight subjects due to difficulty in locating anatomical landmarks^{( 14 , 15 , 27 , 28 )}.

Clinically relevant change in waist circumference

Whether the measurement error is problematic in clinical practice can only be judged if there is a clear conception of the magnitude of change in, or the difference between, waist circumference that is considered important. In other words, we need to identify a minimal important change (MIC) within subjects or a minimal important difference (MID) between subjects in waist circumference^{( 29 )}. While several studies suggest that a reduction of waist circumference may be associated with benefits across a wide range of health outcomes, there is limited evidence for what constitutes a minimal important change or difference in waist circumference^{( 30 – 33 )}. The National Institutes of Health stated in 1998 that a sustained reduction of 4 cm may be clinically relevant^{( 34 )}. More recently it has been suggested that, similar to body weight and BMI, a reduction in waist circumference of >5 % may be considered a clinically relevant change for individual subjects in the short term and a maintained waist circumference of >3 % from initial waist circumference may be considered clinically relevant for individual subjects in the long term^{( 35 )} (I Lemieux and R Ross, personal communications). No clear definitions were provided on what short-term change and long-term maintenance are^{( 35 )}. Following that recommendation, for an overweight woman with a waist circumference of 80 cm this corresponds to a waist reduction of at least 4·0 cm and a maintained reduction in waist circumference of at least 2·4 cm. For an obese woman with a waist circumference of 110 cm, this corresponds to a waist reduction of at least 5·5 cm and a maintained reduction in waist circumference of at least 3·0 cm. This shows that for subjects with a larger waist circumference, a larger reduction in waist circumference is necessary for change to be clinically relevant. Taking a realistic range of measurable waist circumference, for example 60–135 cm, this implies that a short-term change between 3·0 and 6·8 cm and a maintained change between 1·8 and 4·1 cm may be clinically relevant.

The relationship between measurement error and clinically relevant change

In order to distinguish clinically relevant change from measurement error, the measurement error (SDC) should be smaller than the clinically relevant change (MIC; see Fig. 1a). In this case, changes as large as the clinically relevant change will be statistically significant^{( 29 )}. Thus the smaller is the measurement error, the smaller the change that can be detected beyond the measurement error. But if the measurement error (SDC) is larger than the clinically relevant change (MIC), this change cannot be distinguished from measurement error (see Fig. 1b).

Fig. 1.

Interpretation of change: (a) the smallest detectable change (SDC) as a parameter of measurement error is smaller than the minimal important change (MIC); and (b) the smallest detectable change (SDC) as a parameter of measurement error is larger than the minimal important change (MIC). Adapted from Terwee et al.^{( 29 )}

The range of measurement error presented in Table 1 (0·7–15 cm) indicates that we are probably able to detect a short-term clinically relevant change of 4·0 cm (5 % for a women of 80 cm) or 5·5 cm (5 % for a women of 110 cm), as the intra-observer measurement error is smaller than 4 cm in all but one study. However, the probability to detect a long-term clinically relevant change of 2·4 cm (3 % for a women of 80 cm) or 3·0 cm (3 % for a women of 110 cm) is much lower, as the intra-observer measurement error is larger than 3 cm in more than half of the studies. Across the realistic range of waist circumference measurements (60–135 cm), many relevant short-term changes (between at least 3·0 and 6·8 cm) and maintained changes (between at least 1·8 and 4·1 cm) probably cannot be distinguished from measurement error. Interestingly, the measurement error of waist circumference seems equally problematic for normal-weight, overweight or obese subjects. Although the measurement error is larger among overweight or obese subjects, a larger reduction in waist circumference is also necessary to obtain a clinically relevant change.

Recommendations for future research and clinical practice

To summarize, we have shown that there are two important gaps in knowledge. First, the assessment of measurement error identified a wide range (0·7–15 cm) of measurement errors, due to the small number of fair and poor quality studies and many differences between studies. Second, no clear definition of clinically relevant change could be extracted from the literature. Taking a realistic range of a measurable waist circumference (60–135 cm) into account, we argue that a proposed clinically relevant change in waist circumference of 5 % in the short term (approximately 3·0–6·8 cm) may be detectable, but a proposed maintenance of 3 % (approximately 1·8–4·1 cm) may not be detectable, because it cannot be distinguished from measurement error. Although the current paper does not provide practising clinicians with empirical insight into the application and interpretation of waist circumference measurements in the clinical setting, the results do highlight that more attention should be paid to reducing measurement error, in order for clinicians and researchers to accurately measure real change in waist circumference rather than measurement error.

Three ways to potentially reduce measurement error in clinical practice are: (i) adopting a standard protocol; (ii) training; and (iii) repeating measurements^{( 15 )}. Two papers studied the influence of using different measurement protocols on waist circumference measurements. The first found that using different measurement protocols influenced the association between waist circumference, all-cause and CVD mortality, CVD and diabetes^{( 36 )}. However these protocols were only compared on measurement site. The second study found that the type of protocol significantly influenced waist circumference measurements by comparing the measurement of waist circumference in eleven different ways (by anatomical site, posture, respiratory phase and time since the last meal)^{( 27 )}. However, as we have shown, other factors may also influence measurement error and smaller measurement errors are required in order to detect (smaller) changes beyond measurement error. For clinicians, no standard protocol was advised as best. To overcome this gap in knowledge, we support the worldwide request for a uniform measurement protocol, decided upon by an expert team^{( 10 , 25 , 37 , 47 )}.

A second way to reduce measurement error is by training. Measurement error is likely to be larger if measurements are carried out by poorly (often recently) trained individuals^{( 15 )}. Training may thus reduce measurement error by quality control across time and by minimizing the number of observers^{( 15 )}. Unfortunately, it is unclear how (much) training is needed to decrease measurement error, nor whether the effect of training is sustained over time^{( 16 , 17 )}.

A third way to reduce measurement error is to repeat waist circumference measurements. If the same measurement is repeated for example two or three times and the average value is taken, the measurement error of this average value is much smaller (by a factor , with k being the number of repeated measurements)^{( 48 )}. For example, taking the realistic short-term (approximately 3·0–6·8 cm) and long-term (approximately 1·8–4·1 cm) clinically relevant change, two repeated measurements would result in an average measurement error of 2·1–4·8 cm for short-term clinically relevant change and 1·3–2·9 cm for long-term clinically relevant change. Three repeated measurements would result in an average measurement error of 1·7–3·9 cm for short-term clinically relevant change and 1·0–2·4 cm for long-term clinically relevant change. Thus, two measurements seem to be sufficient for detecting short-term changes, but three measurements seem to be necessary to distinguish long-term change from measurement error.

Conclusions

Four gaps in knowledge have been identified. First, the magnitude of measurement error in waist circumference is unclear. Second, the definition of clinically relevant change in waist circumference is unclear. We therefore caution clinicians and researchers when interpreting individual changes in waist circumference, as clinically relevant changes in waist circumference may not be distinguished from measurement error. Third, consensus is needed on adopting a uniform protocol for measuring waist circumference. Fourth, there is a lack of knowledge on the effects of training on measurement error in waist circumference. Considering these gaps in knowledge, it is clear that there is a need for more good quality research and for action. Until then, we recommend consistently using one standard protocol, quality control as part of training and minimizing the number of observers, outsourcing measurements to well-trained clinicians and repeating measurements at least two, but preferably three times. Ultimately, by reducing measurement error, smaller changes in waist circumference may be detected by clinicians beyond measurement error. This is necessary for accurately monitoring changes in waist circumference of individual subjects over time.