Interpretation

The choice of charts for growth monitoring is bewil-dering—size charts, growth charts, and parental adjustment, each with many different cutoffs. In the industrialized world, the aim is to detect growth disorders as early as possible, and the key question is ''Which form of growth monitoring is most effective at detecting disease?''

A common view is that growth monitoring requires measurements on two or more occasions, whereas growth screening involves a single measurement, and monitoring is therefore better than screening. Indeed, this is a fundamental tenet of growth monitoring as practiced in the developing world. Yet there is no direct evidence either way.

Figure 3 The British 1990 boys infant weight chart with thrive lines superimposed. The thrive lines represent downwards centile crossing corresponding to the 5th velocity centile, i.e., moderate weight faltering. (© Child Growth Foundation.)

A Dutch study by van Buuren and colleagues for the first time addressed this question, treating height monitoring as a diagnostic test with associated sensitivity and specificity. The study aimed to detect Turner's syndrome in girls using a regional growth survey as the corresponding normal population. Three measures of size and growth were used: height SD score, height SD score adjusted for parental height, and height velocity, each with a range of cutoffs. The study found that height alone was not very effective at identifying cases of Turner's syndrome, whereas height adjusted for parental height was very effective. Height velocity was useful for ruling out Turner's syndrome, but it was poor at ruling it in. Overall, the study emphasized the value of height adjusted for parental height and tended to discount the value of repeated measurements.

Similar studies need to be done for other outcomes, but until they are performed, the choice of charts and cutoffs needs to be based on simpler criteria. The British height reference uses the 0.4th centile to screen for short stature (Figure 1), which screens in 0.4% of the population and corresponds to a specificity of approximately 99.6%. It is very important for the false-positive rate to be as low as 0.4% to avoid overwhelming growth clinics with referred patients. In the United Kingdom, height velocity is viewed as too noisy, because of the two measurement errors involved, to justify its routine use.

Of course, these conclusions apply to growth monitoring in the industrialized world. They should not be extended uncritically to the developing world, where the purpose of growth monitoring is different—to reduce malnutrition. Here, underweight is judged by the Road to Health chart and is the universal indicator of malnutrition. Weight can fluctuate rapidly due to disease, and encouraging mothers to weigh their infants regularly is a logical way of encouraging the child to grow along the 'road to health.' This philosophy is just one component of UNICEF's GOBI program, which combines growth monitoring, oral rehydration, breast feeding, and immunization.

When height is also available, weight-for-age can be separated into height-for-age and weight-for-height, where low values are known as stunting and wasting. Stunting reflects long-term malnutrition and wasting short-term malnutrition. The implications of the two conditions are different, the latter indicating a need for medical intervention, possibly urgent, and the former is a proxy for more deeply seated socioeconomic problems that are less amenable to intervention. In practice, however, height is rarely measured in infancy and the main focus is on detecting underweight.

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