What is Normal? Blog from Dr. Farber

What is normal?

Part one:

In pediatrics, we often talk about a child, or a finding, being normal, but the term can be vague for many parents.  What exactly do we mean when we say your child is normal?

Sometimes, we use it to refer to something that is common.  Thus, it is normal for a two-year-old to have tantrums, or for a preschooler to have multiple viral infections in the winter.  Another meaning of the word, and the one I will discuss today, is in a numerical or statistical sense.

Here, we are not talking about an absolute number, but a range.  Thus, although many people think of a normal temperature as 98.6, in actuality it can range between 97 and 100.3 degrees, and we do not consider it a fever until it hits 100.4.

Statistically, the 50th percentile is average.  This means when we take a measurement, half of children will be at or above this value, and half at or below.  Importantly, this leads to the finding that being below average is, in most instances, normal.  I will use height as an example; an adult male who is 5 foot 8 is shorter than average, but no one would consider that abnormal. The 90th percentile in height means your child is taller than 90 out of 100 children their age, and the 10th percentile means they are taller than 10 out of 100 (or, alternatively, shorter than 90).  Arbitrarily, someone above or below the 5th percentile can be considered abnormal.

One conclusion to draw from this is that, by definition 10 percent of children are ‘abnormal’ on any given measure (5% above, and 5% below).  Intuitively, this does not sound correct, and in most cases, it is indeed not.  A professional basketball player who is 6 foot 6 is ‘abnormal,’ but if you try and find a cause with blood tests and the like, you will almost certainly not find anything, unless there is something else going on (for example, Marfan syndrome leads to extreme height and can have cardiac issues, so that playing basketball is not healthy for such a person, but there are other clues on the exam beside the height, which will direct us to look further).  Similarly, most children who are shorter than normal do not have genetic or other conditions, and a work-up would be unproductive

Growth curves are used as a guide, to tip us off to when something may be amiss.  Someone who is well below the fifth percentile in height, or has other associated features, is worth evaluating further.  The rate of growth is usually even more important; a child who has always been at the third percentile, and continues to grow along those lines, is usually not a concern. However, the child who used to be at the 75th percentile, and then 25th, and is now at the 5th is, even though he is taller than the child at the third percentile.  In this case, the growth rate, not the absolute height, is abnormal, and warrants investigation.  This is also why regular physical examinations are important, to help detect trends, and not just measurements at one point in time.

One very important point to make is that, for young children, normal predicts normal, nothing else.  Thus, a child who is a slow walker, but normal, will not necessarily be below average in athletic skills when older.  In the same manner, a taller than average child may wind up short (but normal) as an adult.  Better predictions, but still rather inaccurate, would be based on genetic information (tall parents usually produce tall adults.)

Another important aspect of growth is weight.  This is a valuable marker in the younger child/infant, but again we are usually most interested in the trend over time, rather than the absolute number (for example, has your newborn baby regained the birthweight by the two-week visit, and are they gaining around an ounce a day?).  Even more important than the weight is the Body Mass Index (BMI), which takes into account height as well.  Thus, going back to our tall basketball player, he might weigh 200 pounds, but that would not make him overweight, unlike a person the same weight but a foot shorter.  In older children, BMI is the key number I like to look at, in helping to decide how a child is doing here.

Unfortunately, as you are all aware, this country has been getting heavier, and this applies to children as well.  We are seeing an epidemic of diabetes in this country, and it is spreading to children.  We are often able to diagnose prediabetes, a precursor to diabetes, in children.  We can prevent diabetes in many, if not most cases, by establishing a good weight; thus, we recommend screening by blood work for older overweight children.

BMI is also an exception to the rule that above the 95th percentile is the cutoff for abnormal.  This is because of the rising incidence of excessive weight in this country.  Suppose you are at the 50th percentile for BMI, and suddenly everybody in the country gained 50 pounds overnight; you would still be at the 50th percentile when compared to everyone else, but nobody would argue that your current weight was healthy.  For BMI, therefore, we tend to use the 85th percentile for helping to decide who should be screened for diabetes (if you are at or above this, and have a family history of diabetes, I screen, and also if you are at or above the 95th percentile, regardless).

Next up:  abnormal lab tests