As a concept, FTT assumes that the growth pattern seen is a failure that should be identified and reversed. In population‐based cases, however, there is also the risk that otherwise flourishing children will be mislabelled, resulting in unnecessary and potentially harmful parental anxiety. It could well be argued that, as undernutrition is relatively uncommon in affluent countries such as Denmark, most children below a single growth threshold are simply growing normally at the extreme end of the distribution. However, undernutrition still does occur and its identification in population screening, by necessity, relies heavily on anthropometric measures.
The concurrence among clinically used criteria for growth delay and undernutrition proved to be poor, with no single measure reliably identifying children with significant undernutrition. Criteria based on a low weight or height centile have the limitation of including normally growing, constitutionally small children, yet missing larger children not falling below the cut‐off. By contrast, definitions relying on a decrease on an ordinary centile chart tend to overidentify initially large and miss very small infants. Accordingly, low length for age and downward crossing of centiles, without adjustment for regression to the mean, proved to have low sensitivity and positive predictive value and, thus, on both theoretical and practical grounds seems invalid. The concordance between low weight for length and conditional weight gain proved to be surprisingly low and a low weight for length seemed to be associated more with relatively tall stature than with low weight. Although the Waterlow criterion may have a high positive predictive value in populations where wasting is common, in this affluent population it predominantly seems to identify relatively tall, slim children at low risk of being nutritionally compromised. This was shown to be an intrinsic limitation of the method's lack of adjustment for age,27
but the same tendency was also seen with BMI, although to a lesser extent.
This is the first population‐based study to examine this question. The study population was a large representative birth cohort with prospectively recorded data, a high participation rate and no substantial differences between participants and non‐participants.
Using a routine screening system still involves the risk of selection bias, with over‐representation of data on at‐risk children—for example, children of low birth weight more often had measurements from all four visits. Condensing data into two age groups reduced this tendency and subsequently no differences were found between children with full versus partial growth data, making it likely that the prevalence rates found were not inflated.
Misclassification due to inadequate growth references, which has been a problem in other studies,28
was dealt with by using the cohort as its own reference.25
Inevitably, there will be some measurement errors, as data were not collected under careful research conditions, but this is the case in any real‐life screening situation, which this study aims to assess. Combining deceleration in weight gain with low weight for length seems a theoretically valid definition of significant undernutrition, but criteria depending on multiple measurements are also prone to multiple sources of error, particularly if they use length, which is especially difficult to measure. Overestimation of length in proportion to weight would increase the probability of being identified by criteria using both of these measures. The relatively high gestational age of children identified by the Waterlow criterion and BMI suggests a true difference in length, but the association found between low BMI and higher length does question the validity of our measure of significant undernutrition in infancy.
Two previous studies have compared criteria for FTT or undernutrition in affluent societies. Wright et al21
compared the criteria of Gomez and Waterlow in referred children with weight for age less than the 5th centile or deceleration of weight crossing two major centiles. Likewise, Raynor and Rudolf22
investigated five criteria for undernutrition in children referred to an FTT clinic with weight below the 3rd centile or deceleration in weight crossing two centile channels. Both studies found large differences regarding prevalence, but concurrence was not investigated. Raynor and Rudolf also investigated the cross‐sectional association of the criteria with developmental delay and eating difficulties. They found no definition to be more predictive than another, and suggested weight as the most feasible measure of FTT. However, both study populations had already been referred because of low weight or poor weight gain and were thus highly selected, so results cannot easily be applied to the general paediatric population. Ideally, we would test the performance of screening criteria in an unselected population at high risk of undernutrition. In developed countries, however, the only possible high‐risk groups would usually have underlying somatic morbidity, covering less than a quarter of children with FTT, and thus rendering the findings non‐generalisable and unsuitable for screening purposes.
However, only testing criteria against subsequent outcomes can truly determine whether a growth pattern is pathological. Bairagi et al29
found weight for age to be the best discriminator of the 1‐year mortality in a Bangladeshi paediatric population, whereas weight velocity seemed to be a good indicator of short‐term mortality. Mortality is a rare outcome in more affluent societies, but poor conditional weight gain was recently found to be associated with sudden infant death syndrome.30
Alternatively, cognitive delay and behavioural difficulties are possible outcomes of FTT, and, in a recent meta‐analysis, Corbett and Drewett3
found that poor early weight gain, measured in a range of ways, was associated with minor cognitive deficits in later childhood. Slow conditional weight gain is commonly followed by catch‐up weight gain,31
more rapidly in children receiving intervention,32
suggesting that this is not a constitutional pattern. No studies have yet considered outcomes for low BMI in infancy.