Weight alone is not the sole factor in diagnosing failure to thrive. As the definitions in this blog post indicate, it is “growth parameters” that are recorded, not just the weight. It is the weight to height ratios, body mass index, and z-scores that should be evaluated when considering a diagnosis of failure to thrive. Weight to height is considered to be normal if the percentage is between 5-85%. BMI values are normal between 18.5 and 25. Both of these are relatively well-known or understood even in non-medical communities. Z-scores are a calculation of the child’s standard deviations from the norm, with normal being zero, and are generally used when the child falls into the extremes (<3% or >97%) of the weight to height percentiles as they are a more precise calculation of the child’s growth and weight gain. Therefore, z-scores are not as well known and are not used as often, except in severe cases of failure to thrive.
There are also certain conditions, such as Down syndrome, that have their own growth charts because these children do not plot correctly on the standard growth charts. In the example of Down syndrome, they are considered normal between -2 and -3 standard deviations, or a z-score of between -2 and -3. Clearly, this would have an affect on the assessment of failure to thrive.