Obesity: Tests and Diagnosis

What tests are used to diagnose obesity?

The most important test for obesity is the calculation of body mass index or BMI, using weight and height. To calculate BMI we take the child’s weight in kilograms and divide it by their height in meters, squared. The BMI is usually a number between 10 and 40 in most children. In adults, cutoff values have been established to define “normal” (BMI between 18 and 25), “overweight” (25 to 30), “obese” (30 to 40) and “morbidly obese” (more than 40). In children, however, there are no cutoffs because the ranges change throughout childhood.

Typically BMI falls in the first few years of life until about age 4, and then rises until reaching the adult cutoffs at age 18. Therefore, someone with a BMI of 24 at age 10 would be considered “obese” though the same BMI for an adult would be within the normal range. As a result, we use percentiles to define the range of normal BMI in children. A BMI between the 5th and the 85th percentile for a given age is defined as “normal.” A BMI between the 85th and 95th percentile is then considered “overweight” and above the 95th is considered “obese”. Again, parents who consider their children as “thick” or “chunky” are often surprised when their BMI shows that they are above the 95th percentile and medically “obese.”

There are some children, specifically those with a large amount of muscle mass, where the BMI may overestimate the amount of fat on their bodies, so the level must be taken into context. If an obesity problem is identified,  your doctor may recommend screening your child for related conditions. This would typically include blood tests to check for elevated cholesterol or triglyceride levels, fatty liver disease, and type 2 diabetes. In addition, your health care provider may perform additional screening for thyroid disease or kidney disease.

Sometimes, a doctor may suggest further testing if your child shows concerning signs or symptoms at the time of their visit. For example, if a child has had high blood pressure over a series of visits, further screening for cardiovascular or kidney disease may be considered with tests such as an echocardiogram or a kidney ultrasound. Other studies to look for rarer complications of obesity are sleep studies to check for sleep apnea and an MRI for double vision or neurological problems.