How Obesity Is Diagnosed

Diagnosing obesity involves much more than simply stepping onto a scale. In fact, a thorough evaluation of a person's weight status is a complex procedure that involves taking into consideration many factors and the use of various tools and diagnostic tests, including body mass index (BMI), waist circumference measurement, physical exams, and lab tests to check for comorbidities.

Self-Checks/At-Home Testing

In the last 40 years, obesity has become a major worldwide health issue. “Obesity is a serious concern because it is associated with poorer mental health outcomes, reduced quality of life, and the leading causes of death in the U.S. and worldwide,” says the Centers for Disease Control and Prevention (CDC).

The most successful strategy for obesity treatment involves early diagnosis and intervention. Studies have shown that getting a formal diagnosis of obesity is more likely to result in weight loss (compared to those who are never diagnosed). 

Body Mass Index (BMI)

The most commonly used scale that healthcare providers use to diagnose obesity is called the body mass index. BMI takes into consideration the body’s overall fat content, expressed by using a person’s weight (in kilograms) divided by the square of one’s height (in meters).

A normal BMI ranges from 18.5 to 24.9 (kilograms of weight per squared meter of height).

BMI can be evaluated as part of a self-test for obesity screening by accessing the CDC's online tool for measuring BMI for adults, or the online tool for measuring BMI for children and teens.

Waist Circumference Measurement

Fat stored in the abdominal area is called visceral fat, which may further increase the risk of diseases such as heart disease and diabetes. Among people with a BMI in the range of 25–34.9, a waist measurement of over 35 inches in women and over 40 inches in men is associated with an increased risk for disease. 

It can be useful to keep an eye on this measurement since changes in waist circumference are an independent predictor of risk, regardless of overall weight, if you're within the normal to overweight range on the BMI scale.

The waist measurement should be considered at least annually. A person can perform a waist measurement as a self-test to screen for a high risk of obesity. However, other measurements (such as BMI) need to be taken into account as well since waist circumference thresholds are not reliable for patients with a BMI greater than 35. Also, waist circumference may not be a reliable indicator of abdominal fat for all ethnicities, genders, and age groups.

Other Diagnostic Measures

Other diagnostic measures used to identify or evaluate obesity include:

• A physical exam evaluating height, weight, and vital signs, as well as a general head-to-toe assessment
• A health history
• A history of weight loss efforts, exercise, and eating habits
• A family history review (to evaluate the possibility of inherited factors)

Labs and Tests

It's important to understand the significance of seeking professional help when it comes to diagnosing childhood, adolescent, or adult obesity.

Diagnostic testing for obesity and overweight conditions may involve some lab tests to evaluate the extent of how the condition has impacted a person’s overall health and to check for signs of underlying disease. The lab tests your healthcare provider will order depend on many factors, such as your risk factors for obesity-related diseases and current symptoms.

Lab tests may include:

• Cholesterol levels: Low "good" cholesterol (HDL) and high "bad" cholesterol (LDL) levels, which are commonly associated with obesity
• Fasting blood sugar and hemoglobin A1C (HbA1C) to check for signs of prediabetes or diabetes
• A thyroid test to observe for signs of thyroid disease, commonly linked with obesity
• Liver function tests to screen for the potential of fatty liver disease, which often accompanies obesity

Other tests may be ordered by your healthcare provider to evaluate the overall impact that obesity has on the body. One such test is an electrocardiogram (ECG or EKG), used to look for signs of heart disease.

Diagnosing Childhood Obesity

To diagnose childhood obesity, a healthcare provider will use a growth chart to evaluate how a child’s weight and height compare to other kids of the same age and sex. For example, a child who is considered in the 90th percentile weighs more and has a higher BMI than 90% of other children of that same age and sex.

Because growth patterns and body frames can differ drastically from one child to the next, pediatricians take several factors into consideration when diagnosing a child’s weight status. These include:

• Growth charts
• Family history of obesity
• Eating habits
• Activity level
• Psychosocial history (includes sleep pattern, mood disorders such as depression, social interactions, and factors such as being bullied)
• Other health conditions

Lab tests that may be ordered when a child is suspected of being overweight include:

• A cholesterol test
• A blood sugar test
• Blood tests to check for hormone imbalances
• Blood tests to check for obesity-linked conditions

Diagnosing Adolescent Obesity

For diagnosing obesity in adolescents, the BMI scale is used in conjunction with comparing adolescents with other teens of the same age and sex. Adolescents in the 95^th percentile or more (for age and sex) or those with a BMI of 30 or more are considered obese.

Teenagers in this category are given a complete medical examination, which includes:

• A medical history
• A physical exam
• Lab tests
• X-rays

According to Stanford Children's Health, adolescents with a BMI between the 85th and 95th percentile or those with a BMI equal to 30 are automatically put into a risk category in which they will receive a second screening in five areas. These include:

• A family history
• A blood pressure screening
• A total cholesterol lab test to check for LDL, HDL, and triglycerides
• An annual BMI evaluation (looking for large jumps in BMI from year to year)
• An assessment of personal concerns about weight (including a psychological screening and an evaluation of self-perception and emotional response to being overweight)

Morbid Obesity Diagnosis

The BMI scale is the primary method for differentiating between obesity and morbid obesity. According to the World Health Organization (WHO), obesity is defined as having a BMI equal to or greater than 30. 

A person is considered morbidly obese when body weight is 100 pounds over ideal levels for his or her height, with a BMI of 40 or more. Morbid obesity is also diagnosed with a BMI of 35 or more for a person who has an obesity-related illness such as diabetes or high blood pressure.

Differential Diagnosis

There are many factors to consider as part of a thorough obesity diagnosis other than just a person’s weight status. Knowing what to expect and how to ensure an accurate weight evaluation may be the difference between a correct diagnosis—leading to early intervention—and a misdiagnosis of a person’s weight status.

Accuracy of the BMI Rating Scale

BMI is not always a completely accurate measurement when it comes to obesity diagnosis. Some individuals, namely athletes who have a large percentage of muscle mass, can throw off the accuracy of the scale. This is because athletes have very high body mass, but very little body fat content.

According to the American Medical Association's AMA Journal of Ethics, “A BMI equal to or greater than 30 has a sensitivity of 50% in detecting excess adiposity, meaning that half of those with a high body fat percent will not be called obese. Furthermore, because BMI calculations use total weight in the denominator, some lean subjects with preserved muscle mass may be labeled overweight.”

In addition, BMI measurements do not consider overall fat distribution, meaning that those with slightly overweight or normal weight, who have, for example, high levels of abdominal fat (visceral fat) may not be considered at risk according to BMI criteria.

Tests That May Improve the Accuracy of an Obesity Diagnosis

Body fat can be measured using a variety of methods. In addition to BMI, these measurements can help to improve the accuracy of an obesity diagnosis:

• Waist circumference: Helps to identify adipose (fat) tissue called visceral fat, which surrounds the body’s organs
• Ultrasound: Measures the thickness of the body’s fat tissue
• Skinfold measurements: A pinch test using a device called a bioelectrical impedance tool to estimate the amount of total body fat

The bioelectrical impedance tool pinches the skin at various locations of the body, pulling the skin away from the underlying muscle tissue, to measure the width of the fat tissue.

Several skinfolds are usually measured, including:

• Biceps 
• Triceps 
• Subscapular (under the shoulder blade)
• Suprailiac (above the hip bone)
• Pectoral (the mid-chest area)
• Midaxilla (midline of the side of the torso)
• Abdomen 
• Quadriceps (the upper thigh)

A skinfold test and other diagnostic tools are often used in conjunction with the BMI scale to more accurately identify the signs and symptoms of obesity.