Weight-for-height measures
MUAC
There are several body measurements and combinations of body measurements that may be useful in assessing the nutritional status of adolescents. Measurements for use in humanitarian emergencies must use inexpensive equipment, be simple to teach to health workers, and be easy to interpret in the field. In addition, the ideal index used to assess adolescent nutritional status in emergencies would allow compensation for differences between the survey and reference populations in age, sexual development, and ethnicity. The measurements most commonly considered for use in emergencies include weight, height and MUAC. Skinfold thickness may also be a useful measure of body protein and fat stores, but is not considered useful in emergencies as it is difficult to obtain accurate measurements, particularly in emergencies.
Theoretical problems with indices using weight and height
Practical problems with indices using weight and height
Indices which use weight and height are currently the most frequently used tools to assess adolescent undernutrition. As described above, WHO recommends the use of BMI. To screen adolescents for admission to nutrition programs, other organizations, including Médecins Sans Frontières (MSF) and Action Contre la Faim (ACF), use weight-for-height reference tables extended to older children and adolescents (Michael Golden, personal communication). This method uses existing reference tables of weight-for-age and height-for-age to calculate the median weight and 70% of median weight for adolescents of each sex and height category. These height-specific cut-off points are shown in Annex 1. Although demonstrated to be effective in screening adolescents for admission to therapeutic feeding programs, this table has not been evaluated for use in defining less severe degrees of undernutrition in order to estimate the prevalence of undernutrition in a population of adolescents. Moreover, many of the same drawbacks listed below apply to this use of weight-for-height.
Lack of data directly correlating measurements to health outcomes - there are no data directly correlating weight-for-height, BMI, or the Rohrer Index with functional or health outcomes in adolescents. Hence, as yet there are no validated cut-offs for these indices to define undernutrition in adolescents.
Age - indices using weight and height are correlated with age. In young children, it is assumed that the index weight-for-height is constant regardless of a child's age. That is, a well-nourished three year-old child is assumed to have the same "ideal" weight as a stunted five year-old child of the same height. Although this assumption may not be entirely true, it does not grossly interfere with the use of weight-for-height in children less than five years of age. In contrast, because adolescents add substantial muscle and fat, especially during sexual development, the normal weight for adolescents of a given height changes substantially depending on their age and pubertal development (figure 1). As a result, when using weight-for-height, it is necessary to collect and record accurate ages. Moreover, use of weight-for-height may also require adjustment for the difference in the age of sexual maturation between the survey population and reference population.
As with weight-for-height, BMI also changes with age 37 (figure 1). Although this association with age is not so strong as with weight-for-height, the use of BMI still requires the collection of accurate ages, and also ages of maturational landmarks from individual subjects.
The Rohrer Index is calculated as the weight in kilograms divided by the height in meters cubed (wt/ht3). As seen in figure 1, the Rohrer Index may be less age-dependent during adolescence than other indices combining weight and height.
Correlations with height - some studies have found that the Rohrer Index is correlated with height, especially among older adolescents 60-62. Moreover, there has been very little research on the correlation of the Rohrer Index with other measurements of body protein and fat stores, such as percent body fat. And finally, there is no widely available reference population or established cutoff points for this index.
Difficulties in obtaining the component measures during famine - the height and weight measurements required to assess weight-for-height, BMI, and Rohrer Index may be difficult to obtain during an emergency. During severe famines where adolescents are affected, many of the most severely undernourished requiring admission to therapeutic feeding centres cannot stand, making measurement of height impossible. Many studies have reported that gross weakness and flexor contractions prevented measurements of weight or height in a substantial proportion of severely undernourished adults. (See accompanying report on assessment of adult nutritional status for further details 59.) Moreover, the necessary equipment, including scales and height boards, may not be available.
Difficulties in the calculation of the indices - the calculation of BMI and Rohrer Index may be unfamiliar to field workers and therefore more difficult to use than other anthropometric indices.
Famine oedema - regardless of the specific index used, evaluation of nutritional status must take into account the presence or absence of oedema. Many adolescents and adults develop oedema when severely undernourished 1, 63. This leakage of fluid into tissues artificially increases an individual's weight, which may result in a weight-for-height, BMI, or Rohrer Index which appears more normal than would be expected given the degree of emaciation. In addition, because adolescents and adults with oedema have a poorer prognosis than those who are equally undernourished but do not have oedema, adolescents with oedema should be identified and admitted to appropriate therapy. Although oedema in both feet or legs may be due to other causes, in a situation with a high prevalence of undernutrition, adolescents with bilateral oedema may be severely undernourished and should be referred to a clinician for further diagnosis. They should then be admitted for therapeutic feeding if famine oedema is diagnosed 59, 63.
Pregnancy - indices comparing weight and height cannot be used to assess pregnant adolescents. Because of the extra weight of the foetus, other products of conception, and added maternal tissue, indices using weight and height may not accurately indicate the nutritional status of pregnant adolescents. During pregnancy, other measures, such as weight gain during pregnancy or MUAC, must be used to judge nutritional status.
Theoretical problems with the use of MUAC
Practical problems with the use of MUAC
MUAC is relatively simple and easy to measure and has recently been recommended for use in rapid screening of adults for undernutrition to determine the need for admission to a feeding programme19, 59, 64, 65. In many well-nourished populations, a reasonable correlation exists between MUAC and BMI in adults. A scheme using a combination of MUAC and BMI has been proposed to categorise the degree of undernutrition in adults 65. No MUAC cut-offs have been established for the diagnosis of adolescent undernutrition yet. The published results of several nutrition assessment surveys of adolescents in developing countries include MUAC data; however, because of the lack of a reference population and cut-off points, no estimate of the prevalence of undernutrition could be made 4, 7, 51, 53, 66-71.
Lack of data directly correlating measurements to measurements of body fat and protein stores - there are no data directly correlating MUAC with other measures of body fat and undernutrition, such as BMI, in adolescents. In addition, there are no data relating MUAC cut-off points with functional or health outcomes in adolescents. Hence, as yet there are no validated cut-off points to define undernutrition with MUAC measurements in adolescents.
Age - MUAC changes substantially with age during adolescence, as shown in several reference populations from industrialised countries 41, 72. As a result, a different cut-off point must be used for adolescents of different ages. This requires an accurate age for each survey subject in order to judge whether they fall above or below an age-specific cut-off point.
Sexual development - MUAC changes with sexual development. The rapid addition of soft tissue, predominantly muscle tissue in males and subcutaneous fat in females, which occurs with puberty results in a more rapid rise in MUAC at this time than prior to or following puberty. One study clearly demonstrates a greater MUAC in postmenarcheal female adolescents than premenarcheal female adolescents of the same age 53.
Ethnicity - ethnic differences in MUAC have not been sufficiently studied to determine if a single cut-off point for MUAC could be used for adolescents in all ethnic groups.
Measurement error - in spite of the convenience and ease of measurement, MUAC measurement requires careful training and supervision in order to prevent wrapping the measuring tape too tightly or too loosely, which results in an erroneous estimate. One study estimated that the smallest change over time detectable in MUAC was 8-10%, when measurements were taken by different observers73. A second study demonstrated that MUAC measurements show more inter-observer variability than weight and height measurements 74.
MUAC should be measured at the mid-point of the upper arm between the shoulder (lateral end of the clavicle) and elbow (inferior tip of the olecranon). Although this not critical in young children who often have little muscle contour in the upper arm, it becomes increasingly important in post-pubertal adolescents who have developed adult musculature. Therefore workers will have to be carefully trained to measure adolescent MUACs.
