The international reference population
The designation of a child as having impaired growth implies some means of comparison with a "reference" child of the same age and sex. Thus, in practical terms, anthropometric values need to be compared across individuals or populations in relation to an acceptable set of reference values. This need has made the choice of a growth reference population an important issue that has received considerable attention in the last decades (5).
The database uses as a basis for comparison across countries the National Center for Health Statistics (NCHS) growth reference, the so-called NCHS/WHO international reference population. The international reference growth curves were formulated in the 1970s by combining growth data from two distinct data sets, which were originally planned to serve as a reference for the USA. The reference for ages 0 to 23 months is based on a group of children in the Ohio Fels Research Institute Longitudinal Study which was conducted from 1929 to 1975. The height curves for this part of the reference are based on recumbent length measurements. The reference from 2 to 18 years of age is based on data of three cross-sectional USA representative surveys conducted between 1960 and 1975. The height curves for this part of the reference are based on standing height measurements. All samples consisted of healthy well-nourished US children. A detailed account of the historical background of the NCHS/WHO growth charts can be found elsewhere (5, 6).
The World Health Organization adopted the reference curves of the NCHS for international use in the late 1970s (7) based on the then growing evidence that the growth patterns of well-fed, healthy preschool children from diverse ethnic backgrounds are very similar (8). Differences of genetic origin are evident for some comparisons; however, these variations are relatively minor compared with the large worldwide variation in growth related to health and nutrition (9).
The adoption by WHO of the NCHS-based growth curves resulted in their wide international dissemination. Throughout the 1980s, several microcomputer-based software versions of the NCHS/WHO international growth reference were developed and supported by CDC and WHO (6). These software-based references have contributed to the wide acceptance of the concept of the international growth reference because they simplified the handling of anthropometric data from surveys, surveillance, and clinical studies.
Although the NCHS/WHO international growth curves have served many useful purposes throughout these years, because of a number of serious drawbacks, the suitability of these curves for international purposes has recently been challenged (5, 10). Work supported by WHO has demonstrated that the current international reference is sufficiently flawed as to interfere with the sound health and nutritional management of infants and young children. These flaws arise from both technical and biological considerations. In particular, the current reference may lead to the early introduction of complementary foods in exclusively breast-fed infants, which often has adverse consequences for the health and nutritional well-being of infants (11, 12). As a result, an international effort is currently underway to develop a new international growth reference (13). Until the new reference is developed, the NCHS/WHO growth reference curves will remain the reference values recommended for international use.
General issues that need to be considered when using international reference values are discussed elsewhere (10). One essential consideration is the appropriate use of the reference data. The way in which a reference is interpreted and the clinical and public health decisions that will be based upon it are often more important than the choice of reference. The reference should be used as a general guide for screening and monitoring and not as a fixed standard that can be applied in a rigid fashion to individuals from different ethnic, socioeconomic, and nutritional and health backgrounds. For clinical or individual-based application, reference values should be used as a screening tool to detect individuals at greater risk of health or nutritional disorders; and they should not be viewed as a self-sufficient diagnostic tool. For population-based application, the reference values should be used for comparison and monitoring purposes. In a given population, a high prevalence of anthropometric deficit will be indicative of significant health and nutritional problems, however, it is not only those individuals below the cut-off point who are at risk; the entire population is at risk, and the cut-off point should be used only to facilitate the application of the indicator.