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closeReliability fo anthropometry in children
Posted by jayberkley on 04 Jun 2012 at 09:05 GMT
We congratulate Mr Ayele and colleagues on publication of a well-conducted and informative study on the reliability of anthropometry undertaken within a rural community in Ethiopia [1].
We recently undertook a similar study in a rural community in Kenya, amongst infants under 6 months old [2]. We also found that the individual measures of mid upper arm circumference (MUAC), weight and length had high intra- and inter-observer reliability. However, for the assessment of acute malnutrition amongst children aged 6 months to 5 years, the WHO recommend the use either weight for height represented as a z-score or percentile, or unadjusted MUAC [3]. When we examined the z-score values, we found reliability to be markedly reduced, especially for weight for height. These findings concord what was observed amongst 12 to 59 month old children in Guatemala (Velzeboer et al., 1983). It appeared that any small errors in the two measures were multiplied when the two measures were combined in calculating the z-scores. This may be a problem that is specific to younger infants, especially in the light of the findings in Ethiopia that measurements of smaller children are less reliable.
For practical use in detection of acute malnutrition, it is important that the reliability of weight for height is known under field conditions, and whether this is affected by age. We would therefore be very grateful if Mr Ayele and colleagues would present reliability estimates for the z-score indices, calculated from their study data using the WHO growth reference standards [4]. Although this would not address potential errors in using the look-up tables for combining weight and height [5], it would be enormously valuable to programmes deciding how best to undertake community-based interventions.
Martha Mwangome MSc
James A Berkley FRCPC
KEMRI/Wellcome Trust Research Programme, PO Box 230, Kilifi, 80108, Kenya.
1. Ayele B, Aemere A, Gebre T, Tadesse Z, Stoller NE, See CW, Yu SN, Gaynor BD, McCulloch CE, Porco TC et al: Reliability of measurements performed by community-drawn anthropometrists from rural Ethiopia. PLoS One 2012, 7(1):e30345.
2. Mwangome M, Fegan G, Mbunya R, Prentice A, Berkley J: Reliability and accuracy of anthropometry performed by community health workers among infants under 6 months in rural Kenya. Tropical Medicine & International Health 2012, 7(5):622-629.
3. WHO child growth standards and the identification of severe acute malnutrition [http://www.who.int/nutrit...]
4. The WHO Child Growth Standards [http://www.who.int/childg...]
5. Poustie VJ, Watling RM, Ashby D, Smyth RL: Reliability of percentage ideal weight for height. Arch Dis Child 2000, 83(2):183-184.
RE: Reliability fo anthropometry in children
jeremykeenan replied to jayberkley on 28 Jul 2012 at 03:41 GMT
We thank Ms. Mwangome and Dr. Berkley for their interest in our paper. We agree that WHZ measurements should have increased variability compared to weight or height measurements, due to multiplication of measurement error. We converted weight and height measurements from our study into weight-for-height Z-scores (WHZ) using the 2006 WHO Growth Standards, with software provided by the WHO.[1, 2] We assessed the reliability of WHZ measurements by calculating the technical error of measurement (TEM), reliability (or intraclass correlation coefficient [ICC]), and repeatability, using the same analyses as described in our paper.[3]
We found that in general, indices of reliability were lower for the WHZ outcome than for the weight or height measurements themselves. For example, the overall intra-anthropometrist coefficient of reliability for weight, height, and WHZ were 0.999, 0.998, and 0.983, respectively (see Table 2 in original paper, and Table 1 below). The inter-anthropometrist coefficient of reliability for weight, height, and WHZ were 0.999, 0.997, and 0.904, respectively (see Table 3 in original paper, and Table 2 below). These coefficients of reliability for WHZ are still within a reasonable range of accuracy, and higher than the ICCs of 0.60 and 0.71 reported in Mwangome et al.[4] However, we measured older children (ages 0-5 years) compared with the Mwangome study (infants). To address this, we stratified the analysis based on whether height was measured as a length versus a standing height. We found that the coefficient of reliability for WHZ was lower for children who had a length measured (ICC=0.798) compared with children who had a height measured (ICC=0.978; see Table 3 below, and compare with Table 5 in original paper). This suggests an increased level of measurement error in young children (children with a length measured had a median age of 1 year, interquartile range [IQR] 10 months to 2 years, whereas children with a height measured had a median age of 4 years, IQR 2 to 4), likely because younger children cannot cooperate with the measurements as well as older children can. This finding is consistent with the Mwangome study, which found lower ICCs for WHZ in children <90 days compared with children >90 days.
These findings highlight the need for detailed anthropometry training of community health workers, and special attention when measuring the smallest children.
Berhan Ayele, MSc
The Carter Center Ethiopia
Jeremy Keenan, MD MPH
Francis I. Proctor Foundation, University of California, San Francisco
Table 1. Intra-anthropometrist reliability of WHZ for repeated measurements of 84 children in rural Ethiopia
METRIC OBSERVER WHZ
Mean 1 -0.84 (-1.05 to -0.64)
2 -0.73 (-0.98 to -0.48)
3 -0.70 (-1.49 to 0.09)
All -0.78 (-0.93 to -0.62)
TEM 1 0.11 (0.08 to 0.13)
2 0.08 (0.06 to 0.10)
3 0.06 (0.03 to 0.09)
All 0.09 (0.08 to 0.11)
Reliability (ICC) 1 0.969 (0.949 to 0.989)
2 0.989 (0.982 to 0.996)
3 0.995 (0.987 to >0.999)
All 0.983 (0.976 to 0.990)
Repeatability 1 0.30 (0.23 to 0.37)
2 0.23 (0.18 to 0.28)
3 0.17 (0.08 to 0.26)
All 0.26 (0.22 to 0.30)
Notes: Reliability calculations are shown separately for each of the 3 measurers in the study, and also using aggregated data from all 3 measurers.
TEM = technical error of measurement; ICC = intraclass correlation coefficient
Table 2. Inter-anthropometrist reliability of WHZ for repeated measurements of 89 children in rural Ethiopia
METRIC WHZ
Mean -0.61 (-0.76 to -0.45)
TEM 0.23 (0.20 to 0.27)
Reliability (ICC) 0.904 (0.867 to 0.942)
Repeatability 0.65 (0.55 to 0.75)
Notes: TEM = technical error of measurement; ICC = intraclass correlation coefficient
Table 3. Inter-anthropometrist reliability of weight and WHZ measurements, stratified by whether children had height or length measured
Estimate, % (95% Confidence Interval)
Measurement Height Measured Length Measured
(N=61) (N=28)
Weight
Mean 13.47 kg (12.90 to 14.05) 8.93 kg (8.12 to 9.75)
TEM 0.08 kg (0.07 to 0.10) 1.28 kg (0.91 to 1.64)
Reliability (ICC) 0.999 (0.998 to 0.999) 0.997 (0.995 to 0.999)
Repeatability 0.23 (0.19 to 0.27) 0.32 (0.23 to 0.41)
WHZ
Mean -0.58 (-0.76 to -0.40) -0.67 (-0.99 to -0.35)
TEM 0.10 (0.08 to 0.12) 0.40 (0.29 to 0.51)
Reliability (ICC) 0.978 (0.967 to 0.989) 0.798 (0.660 to 0.935)
Repeatability 0.28 (0.23 to 0.34) 1.11 (0.81 to 1.42)
Notes: TEM = technical error of measurement; ICC = intraclass correlation coefficient
1. World Health Organization. WHO Anthro (version 3.2.2, January 2011) and macros. [accessed September 18, 2011]; Available from: http://www.who.int/childg...
2. World Health Organization. Growth reference 5-19 years. [accessed September 18, 2011]; Available from: http://www.who.int/growth...
3. Ayele B, Aemere A, Gebre T, et al. Reliability of measurements performed by community-drawn anthropometrists from rural Ethiopia. PLoS ONE 2012; 7:e30345.
4. Mwangome MK, Fegan G, Mbunya R, Prentice AM, Berkley JA. Reliability and accuracy of anthropometry performed by community health workers among infants under 6 months in rural Kenya. Trop Med Int Health 2012. doi: 10.1111/j.1365-3156.2012.02959.x.