|Nutrition Guidelines (MSF, 1995, 191 p.)|
|Part II: Rapid Nutrition Surveys|
Anthropometry is the measurement of the human body. Body parameters such as weight and height are used to assess nutritional status.
The various anthropometric indicators and the method of measurement
Many body parameters can be used to assess individual nutritional status. The weight, the height and the mid upper arm circumference are the most commonly used, but skin-fold thickness and various other measurements are sometimes used.
A 25 kg hanging spring scale, graduated by 0.100 kg, is used. The scale is hooked to a tree, a tripod or a stick held by two people.
The weighing pants are suspended from the lower hook of the scale, and the scale is readjusted to zero. The child's clothes are removed and the child is placed in the weighing pants. The pants then hang freely from the hook. In cold countries or in certain cultures it might be impossible to undress a child. The average weight of the clothes should be evaluated and deducted from the measure. When the child is steady, the weight is recorded to Adapted from How to weigh and measure children, UN, 1986 the nearest 100 grams - the scale should be read at eye-level.
Figure 6: Weight assessment
If the child is moving and the needle does not stabilize, the weight should be estimated by recording the value situated at the mid-point of the range of oscillations. The measurer announces the value read from the scale, the assistant repeats it for verification and records it on the questionnaire.
Every morning the scale should be checked against a known 10 kg weight. If the measure does not match the weight, the scale should be discarded or the springs must be changed.
Children aged more than 2 years old are measured standing up. Children less than 2 years old are measured lying down. If the age is difficult to assess, children of more than 85 centimetres are measured standing, those less than or equal to 85 centimetres, lying down.
· For children of more than 2 years, the measuring board is set up in a place where there is room for movement. The child's shoes are removed. The child is placed on the measuring board, standing upright in the middle of the board. The child's ankles and knees should be firmly pressed against the board by the assistant while the measurer positions the head and the cursor.
· The child's head, shoulders, buttocks, knees and heels should be touching the board. The measurer reads the measure to the nearest 0.1 centimetre. The assistant writes down the measurement and repeats it to the measurers to make sure it has been correctly heard and recorded.
· For children of less than 2 years old, the measuring board is placed on the ground. The child is gently placed, lying down the middle of the board. The assistant holds the sides of the child's head and positions the head until touching the foot board. The measurer places his hands on the child's ankles or knees. While positioning the child's legs, he positions the cursor up against the bottom of the child's feet, which should be at right angles. He reads the measure. The remaining procedures are the same as for standing children.
If birth dates have been recorded on a health card or immunization card, determination of age is simple. In such cases, the date of birth is directly recorded onto the questionnaire in order to avoid mistakes in calculating the age. If birth dates are not recorded, a local calendar of events is used. The mother is asked whether the child was born before or after certain major events until a fairly accurate age is pinpointed. If that is not possible, children are selected on the basis of height. Only children more than 65 centimetres and less than 110 centimetres tall should be included in the sample.
In order to determine the presence of oedema, normal thumb pressure is applied to the foot or the leg for three seconds (3 seconds is approximately the time necessary to say one thousand and one, one thousand and two, one thousand and three). If a shallow print or pit remains when the thumb is lifted, then the child has oedema. Nutritional oedema should be found on both feet or legs. Only children with oedema on both feet or legs are classified as having nutritional oedema.
Mid upper arm circumference (MUAC) (see Ref. 14, 15,18, 20)
Mid upper arm circumference is measured on the left arm, at the mid-point between the elbow and the shoulder.
The arm should be relaxed. A special measuring tape is placed around the arm. The measurement is read from the window of the tape without pinching the arm or leaving the tape loose. The mid upper arm circumference is recorded to the nearest 0.1 centimetre (see Annex 2).
Various indices and their meaning
None of these parameters, except mid upper arm circumference, give information about nutritional status when taken alone. They should be related to each other in order to define indices. The weight is related to age: weight/age index; the weight with the height: weight/height index; the height with the age: height/age index.
THE CONCEPT OF A REFERENCE POPULATION
The indices are compared to values for a reference population to see if they are worse than expected from the reference. For the same age, the height or the weight of a child from the sample is compared to the height or weight of the children of the reference population. For the same height, the weight of a child from the sample is compared to the weight of the children from the reference population. Reference tables have been drawn up for both sexes. For field use, sex combined tables have also been drawn up (see Annex 13).
These reference values for the various indices have been calculated from data collected by the National Centre for Health Statistics (NCHS) in the United States of America. This reference population, composed of young Americans, should not be considered as reflecting an <<ideal>> nutritional status, but should be used as a tool which allows comparison of data sets against a standard. It is then possible to compare the nutrition status from samples from two different countries, or the nutritional status of one population over a certain time period. Local reference curves exist in some countries. They can be used locally, but results should also be presented using international NCHS curves in order to allow international comparisons.
MODIFICATION OF THE WEIGHT AND THE HEIGHT
The weight of a child can change substantially in a short period of time. Hence, a child exposed to nutritional stress may lose up to 20% of his body weight within a few weeks. In contrast, height cannot change to the same degree. The height of a child cannot reduced, but the speed of growth may be slowed down. In the same way, a decrease in weight can be corrected rapidly if the nutritional situation improves, whilst the effected height can only be corrected in a small proportion of children. These are the reasons why each index has a different meaning.
MEANING OF THE INDICES
The weight / age index
The weight for age index expresses the weight of a child in relation to his age. However this index does not allow differentiation between two children of the same age and weight, one being tall and thin (wasted), the other shorter but not wasted. This index is mainly used during Maternal and Child Health clinic visits, since it is a good way of assessing the nutritional evolution of a child over time.
The height / age index
The height/age index expresses the height of a child in relation to his age. It reveals stunting at a given age, but does not allow discrimination between 2 children of the same age and height, one being thin (wasted) the other one being heavier. This index reflects the past nutritional history of a child rather than his current nutritional status. It is mainly used to identify chronic malnutrition.
The weight / height index
The weight/height index expresses the weight of a child in relation to his height. It reveals whether a child is thin or not but does not discriminate between 2 children of the same height and weight, one being older than the other, and possibly stunted. It is the index used to measure acute malnutrition called "wasting", meaning current or acute malnutrition at the time of the survey.
Mid upper arm circumference
The mid-arm circumference is almost stable from 6 to 59 month and hence does not need to be related to the age. But it is less reliable to measure and so it is only used for the rapid screening of populations to get an idea of the situation and for entry to nutrition programmes. We will not consider MUAC as a tool to assess nutritional status in this part.
In emergency situations where acute forms of malnutrition are the predominant pattern, the weight for height index (W/H) is the most appropriate index to quantify levels of current acute malnutrition in the population with an assessment of oedema. Furthermore, weight for height does not require the determination of age which is often difficult in these situations. (Ref. 15,18, 20).
Calculation and expression of the indices
Indices can be calculated by using reference tables or by using appropriate computer software.
NORMAL DISTRIBUTION CURVE
For a given height, one can draw the distribution curve of the children according to their weight. This bell shaped curve is called Gauss's curve or the normal distribution. It has some specific characteristics. The curve is symmetrical around the mean weight, the mean weight being the sum of all weights divided by the number of observations. The mean weight is equal to the median) weight, the median weight being the weight which splits the sample in two parts of equal size according to weight. This curve can be defined by its mean weight and its standard deviation.
The standard deviation is the square root of the sum of the squares of the differences between each weight and the mean weight, divided by the number of observations minus one.
Standard deviation = sqrt(sum (observed weight - mean weight)2)/(n-1)
In fact, it is not exactly the case for a distribution according to weight for a given height. The distribution is slightly asymmetrical, because weight variations are greater in the upper part of the distribution. This is the reason why we will deal with the median rather than the mean, since it is a better indication of the distribution for weight for height.
Expression in percentage of the median
This mode of expression requires knowing the median weight of the children of the reference population of the same length/height. The value of the median weight can be found in reference tables for each height by 0.5 cm. Calculation is simple: the observed weight is divided by the median weight and multiplied by 100 in order to be expressed as a percentage of the median.
Weight / height index =Observed weight/Median weight x 100
For example, for a child of 80.5 cm weighing 9.6 kg, reference tables give a median weight of 10.9 Kg. The weight/height index expressed in percentage of median is:
9.6 / 10.9 x 100 = 88.1%
Expression in percentiles
In the reference population, for a given height/length, the weight of children aged between 6 to 59 months is normally distributed. The 50th percentile is the weight which divides the distribution into two equal parts, 50% above, 50% below. It coincides with the median weight. In a similar way one can define the 10th percentile as being the weight under which 10% of the children of the reference population lie (90% being above). In the survey sample, for a given height/length, one can express the weight of a child according to its position in the reference distribution. The various weights corresponding to the various percentiles are shown by the reference tables.
For example, for a child weighing 9.6 kg and measuring 80.5 cm, the tables show weight values corresponding to the 5th percentile, the 3rd percentile as well as the deciles in the reference population. By reading the table, the weight of the child can be expressed as corresponding to the 5th percentile.
For the whole sample, one can determine the number and thus the proportion of children situated below a given percentile. When one says that in the sample 12% of the children were found to have an index below the 3rd percentile, it means that where 3% of the children from the reference population are found below this weight, 12% are found in the sample.
Expression in Z-Scores
The expression in Z-Scores uses the standard deviation of the reference distribution for a given height/length as a unit. The weight/height index expressed in Z-Scores represents the difference between the observed weight and the median weight of the reference population expressed in standard deviation units:
Weight/height index = (Observed weight - Median weight) / Standard deviation
Reference tables give the standard deviation and the median weight for each given height/length. This allows us to calculate, for each child in the sample, the value of his index expressed in Z-Scores. For example, for a child 80.5 cm and 9.6 kg, reference tables show a median weight of 10.9 Kg and a standard deviation of 0.870 kg2. Hence, his index expressed in Z-Scores is:
WHICH MODE OF EXPRESSION SHOULD BE USED?
(9.6 - 11.0) / 0.870 = -1.61 Z-Scores
Expressions in percentiles and Z-Scores have a true statistical meaning, which percentage of the median does not have. A child is more malnourished if the weight/height index is 80% at 6 months than at 59 months. The expression in percentiles does not allow the identification of severely malnourished children since percentiles corresponding to severely malnourished children do not exist in the reference population. Expression in Z-Scores is recommended. However, if people in charge of the refugees or people going to use the information are used to another mode of expression, this should also be used in order to deliver meaningful information.
CALCULATION OF THE INDICES
Calculations do not need to be carried out in the field when collecting the measurements. The main aim of the survey is not to locate malnourished children (screening) but to gather information on the whole population. The major preoccupation is not individuals but rather the condition of the population. If a child is found to be obviously malnourished during the course of the survey, he has to be referred to an intensive nutrition unit in order to seek treatment, but this is not the objective of the survey.
If computer equipment is available, one of the existing anthropometric software packages maybe used for calculation and analysis of the results (see Ref. 21). Data is directly entered into these software packages and nutrition indices are calculated by the programme, thus avoiding mistakes in reading the tables. Computerization of the nutrition indices is more accurate than manual calculation and takes into account the sex of the children.
If computer equipment is not locally available, reference tables can be used (see Annex 13). They are derived from the NCHS reference curves and are valid for both sexes. The height/length is rounded to the nearest 0.5 cm, as shown in the next table.
Weight values corresponding to the different cut off values are read from the table, enabling us to classify a child as belonging to an interval of the percentage of the median. For example a child 80.5 cm tall and weighing 8.6 kg will qualify for the interval of 75% to 80% of the median. Similar tables exist for Z-Score classification (see Annex 13).
We will only focus on the weight/height index in this part, since it is the most appropriate for assessing acute malnutrition and thus in meeting the objectives of a nutrition survey in emergency situations.