What the heck are percentiles, anyway?

The Pediatric Insider

© 2011 Roy Benaroch, MD

In pediatrics, our patients are growing targets. There’s no “best” weight or “correct” blood pressure—there’s averages and ranges that depend on things like a child’s age and sex. Since there’s no way we could possibly memorize all of the normals at every age, we rely on “percentiles.”

Talking in percentiles doesn’t always make sense to parents. I blame number grades in school, where the closer to 100% your child gets, the better the grade. “I scored a 97!” is great. Having a BMI (body mass index) percentile of 97%– that’s not so great.

A percentile is a way of comparing your child to kids of similar age and sex. If your son’s height percentile is 40%, that means he’d be number 40 in a line of boys of his exact age if they were lined up in height order. A percentile at or near 50% is about average, and anything between about 25-75 percentile is close enough to be considered average.

In most areas of health, average equals good. It’s the outliers, the ones with the highest blood pressure or the lowest blood counts, that we worry about.

Percentiles are especially useful when we look at growth and weight. Most children grow along about the same percentile range from age 2 through puberty—so if after two the percentile is changing much, something might be going wrong with growth (before two, there is a lot of percentile shifting as children move towards their expected growth pattern.)

A person’s overall “chubbiness” is usually expressed numerically as a BMI, or body mass index. In adults, a BMI of 25 is usually considered overweight; over 30 is obese. In kids, we rely on the BMI percentile—over 85% is overweight, over 95% is obese. From year to year, the BMI number will change, but the percentile should not vary very much.

Another thing about percentiles: in the middle of the pack, a very small change in a number will lead to an exaggerated change the percentile number that really isn’t very meaningful. For instance, a  9 year old boy who weighs 79 pounds is at the 50^th percentile. If he gains 3 pounds, that takes him to the 60^th percentile. But a change from 85 percentile to 95 percentile in the same boy would mean he’s gained 15 extra pounds. Percentile changes in the 25-75^th percentile range usually don’t mean there’s been a big change in absolute numbers, but percentile changes of only a few points way at the top of bottom of the percentile range can mean a big shift has occurred.

If you’re concerned about your child’s growth or weight, ask your pediatrician to review the growth chart and show you how the percentiles have trended over the years. For most kids, a nice stable percentile curve means that their overall health is good—even if the percentile isn’t right the middle. But a child who’s percentile is very far from average (especially those with BMIs higher than 85-95 percentile), may have significant health risks that ought to be addressed.

Early puberty in boys

The Pediatric Insider

© 2010 Roy Benaroch, MD

Allison posted, “I’ve got a friend whose (just turned) 8 year old son is showing signs of puberty ‘down there.’ I told her that was VERY early and that I’d get it checked out. Thoughts?”

Although it’s actually considered normal for girls to begin puberty at age 8, true pubertal changes in an eight year old boy are abnormal and ought to be evaluated.

But first: is he really experiencing puberty? The first sign of true puberty in a boy is enlarging testicles, followed by growth of the penis. These changes are driven by hormones produced in the pituitary glands and testicles. Although pubic hair, body odor, and acne often appear at about the same time, these changes are related to hormones from the adrenal glands, and are not really part of true puberty. A boy with only the appearance of hair, without testicle and penis growth, may not be entering puberty. He can be watched for signs of further development without further immediate workup. But a boy who’s truly entering puberty at age 8 needs a thorough evaluation.

Evaluating early puberty starts with careful measurement of height– puberty will coincide with an increased growth rate, so if a boy is suddenly becoming taller, that’s a definite sign of puberty. A careful physical exam by an experienced physician is essential. Sometimes, a bone age x-ray is useful, or blood tests of hormone status are needed. Some of these boys will also need an MRI scan of their pituitary glands, or other radiology exams.

Though early puberty in a girl is rarely caused by a serious underlying health condition, at least half of boys with true early puberty are going to have an underlying cause that needs to be found and addressed. Boys with true early puberty are much more concerning than girls. Allison ought to tell her friend to get her son to the pediatrician for an exam and further workup.