Everything You Wanted to Know about Z Scores but were Afraid to Ask by Sharon Wallace MS RD CSP

I have been asked with increased frequency what a z score is and what is means when evaluating a growth curve. Z scores have been used as a research tool for many years to look at trends in growth in studies. However, now that many hospitals and physicians offices are moving towards an electronic health record, we are beginning to see them used more frequently as a nutrition evaluation tool to assess trends in growth as the scores are often provided as part of the plots on the computer generated growth curves in the chart.

What is a Z Score?

The score shows the standard deviation above or below the mean on the growth chart. If you have looked at a growth chart of a patient or your own child, you will recall it contains curve shaped lines with various percentiles on it. The middle line is the 50th%Ile and they extend out to the 97%ile percentile and 3rd%Ile. So, a z score of 0 is the equivalent of the 50th%ile or average of what you are measuring (weight, height, weight for height or BMI) for that age. A z score of +1 means your plots fall at the 15th%ile or 85th%Ile and a z score of +2 falls roughly at the 3rd or 97%Ile. z scores run positively (+1. +2.+3) or negatively (-1, -2. -3) and so on. A z score by itself would look like a bell curve (see figure 1).

What do they really mean? How do they help evaluate a pediatric patient?

The farther away a plot is from the median (again, a z score of 0=50th%Ile on the growth chart) in either direction, the higher likelihood that there is some sort of growth problem (obesity or undernutrition). Either of these issues can impact the decision making process regarding both nutrition and feeding goals. Obesity can exacerbate reflux symptoms and inhibit gross motor skills, which in turn can delay progression of feeding skills. Undernutrition can have significant impacts on all aspects of growth and development as well as place an infant or child at higher risk for lab abnormalities such as iron deficiency anemia. Degrees of pediatric malnutrition that are based on z scores are now developed by the ASPEN (American Society for Parenteral and Enteral Nutrition (ASPEN)) guidelines and are determined by z score trends (for example a five year old who’s BMI plots -1.5 for their z score would fall into a mild malnutrition category while a child who’s z score plots -3.2 would fall into a severe malnutrition category.

How do Registered Dietitians use these curves? What are the limitations?

Trends that the z score shows can track what impact adjustments in diet or tube feedings are having on their patient’s overall growth. The goal if a child has a negative z score is to adjust either the caloric density and/or volume of a formula or tube feeding and/or may affect what kind of foods are recommended in the diet. If the z score falls in the established ranges of malnutrition, oral feeding goals may need to be adjusted until the z score shifts in an upward direction (for example, that would not be the time to start cutting out a formula or tube feeding in efforts to improve appetite). If a z score is elevated (especially above +3), this may suggest obesity, and there may be more leverage regarding adjusting the food and/or formula intake and, if applicable, a tube feed schedule.

Certain limitations of the charts may include errors in weighing or measurement. Errors in measuring height are frequently made, especially if an infant or child has special health care needs or issues such as contractures which can make accurate measurement tricky. In addition, variations in weighing (naked vs fully clothed) especially in a young infant, can skew weights. Both of these inaccuracies will inadvertently affect a weight for height curve or BMI and in turn result in an inaccurate trend of z score.

Children who are stunted also may plot with a poor BMI curve and z score, but may not actually be malnourished. These scenarios are only a few of many factors that can make interpreting z scores challenging and because of this, decisions about how to utilize them should ideally be made with the guidance of a registered pediatric dietitian, who specializes in evaluating pediatric growth and nutrition issues.

* Sharon is a registered dietitian with a specialty in pediatrics. She works as one of the RD’s on the UNC pediatric feeding team.

Note: from Krisi. Because we treat our feeding team patients together as a team, I have been able to use the dietitian’s information including the z score to assist with getting insurance and medicaid approval for my feeding patients.