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Body Mass Index Study Guide
Clinical Skills Teaching & Learning Centre
Written by: Clinical Skills Lecturing Team
Reviewed by: Dr Jamie Fanning, Theme Lead for Clinical Examination & Procedural Skills
July 2020
1
Contents
Body Mass Index Study Guide ..................................................................................................... 1
Contents....................................................................................................................................... 2
Glossary ....................................................................................................................................... 3
Learning Objectives ..................................................................................................................... 4
Introduction .................................................................................................................................. 5
Preparation .................................................................................................................................. 6
Patient safety ............................................................................................................................ 6
Equipment .................................................................................................................................... 7
Procedure .................................................................................................................................... 8
Bibliography & Further Reading ................................................................................................. 11
Picture Credits ........................................................................................................................... 12
2
Glossary
BMI Body Mass Index
Comorbidities The presence of 1 or more additional conditions occurring in conjunction
with the primary condition.
NICE National Institute for Clinical Excellence
WHO World Health Organisation
kg kilogram
m metre
cm centimetre
3
Learning Objectives
• To understand the reasons for calculating Body Mass Index (BMI)
• To understand how to calculate BMI
4
Introduction
Obesity is a preventable condition that causes more deaths worldwide than malnutrition. In light
of this, NICE and WHO have recognised the importance of obesity screening to identify patients
who are overweight and at increased risk of ill health as a consequence, to allow for appropriate
interventions to occur.
BMI is such a screening tool that measures body fat based on weight and height and it can be
used as a tool to assess a patient’s risk for chronic disease, or if a patient has a sudden
dramatic change in weight (this is usually due to loss or retention of fluid). BMI should also be
considered in slower weight change which can occur in starvation or increasing body fat
storage.
There are limitations to this tool for adults as a BMI calculation does not take into account age,
ethnicity, gender or body composition. It also does not take into account the proportion of fat as
opposed to muscle.
Therefore the BMI is less accurate for certain groups;
• Body builders/ weight lifters
• Athletes
• Certain ethnic groups (there is research below, but there may be further factors rather
than just ethnicity)
• Elderly
• People with a physical disability
• People with eating disorders
• People with extreme obesity
• In critically ill patients it is often difficult to measure body height and weight
Patients with a higher BMI are more likely to develop several diseases, such as diabetes,
cardiovascular disease, stroke, osteoarthritis, hypertension or certain cancers, and patients with
pre-existing conditions are at higher risk.
Patients can have their waist circumference measured as well, as this is considered a good
measure of internal body fat deposits, patients with a larger waist circumference are at
increased risk of disease. NICE 2014, state that this may not be done routinely but should be
considered in anyone with a BMI less than 35kg/m 2.
This study guide is not discussing BMI measurements for children or pregnant women, although
there are ways of calculating these, please refer to the WHO 2018.
5
Preparation
Patient safety
• Introduce yourself
• Check the patient’s identity and allergies
• Explain what you want to do
• Gain informed consent
• Consider an appropriate chaperone
• Adequate exposure maintaining dignity
• Position the patient appropriately – consider moving and handling
• Wear Personal Protective Equipment as required.
• Wash your hands before and after you touch the patient (as per WHO guidelines)
On first meeting a patient introduce yourself, confirm that you have the correct patient, with the
name and date of birth, if available please check this with the name band and written
documentation and the NHS/ hospital number/ first line of address.
Ensure the procedure is explained to the patient in terms that they understand, in this instance
many patients will want to know the reason for the calculation. Gain informed consent and
ensure that you are supervised, a chaperone would not usually be required.
When positioning the patient consider moving and handling, some patients may not be mobile,
or may need additional support to move onto the scales. In some instances patients may be
immobile, consider whether hoist equipment has the facility for weight measurements,
occasionally certain beds also have the capacity to weigh patients.
PPE will not normally be required unless there is an infection risk. Maintain hand hygiene and
wipe down equipment before and after use.
6Equipment
You will require:
• Hand wash
• Equipment to measure height
• Scales to measure weight
• A calculator
• A disposable tape measure
Measuring Height:
In order to record a patient’s height you will need a device that is capable of measuring height
using a metric scale (metres). Across different clinical placements you will find a variety of
devices, some of which will be digital and others manual (free-standing or fixed to a wall) and an
example of a free-standing device is shown in figure 1.
Measuring Weight:
To measure the patient’s weight you will need a device that is capable of measuring weight
using a metric scale (kilograms). As demonstrated in figures 2 & 3, there are a variety of
devices which you will encounter in clinical practice, some of which will be digital (fig 2) and
others manual (fig 3).
Figure 3
Figure 1 Figure 2
7Procedure
Sequence for procedure:
1. Measure height (m)
2. Measure weight (kg)
3. Consider measuring waist circumference (cm)
4. Calculate body mass index (BMI).
5. Document and discuss with the patient.
1. Measure height:
The patient should be measured without footwear and should stand with their back to the
vertical scale (fig.4). The horizontal bar should be moved until it rests on the crown of the
patient’s head (fig.5) and the height should be recorded in metres (m).
Figure 5
Figure 4
2. Measure weight:
The patient’s weight should also be measured
without footwear on and heavy items of clothing
should be removed - they can wear light, indoor
clothing. Check that the scales are set to zero
before the patient gets on them and ensure the
patient is able to transfer onto and off the scales
safely. The weight should be measured in kilograms
(kg). Figure 6
83. Consider measuring waist circumference:
If you suspect the BMI is high you may measure the waist circumference. To do his measure
the waist circumference of the patient with the tape measure horizontal around the waist and
positioned above the hips. Ensure that the tape measure is neither compressing the skin or
loose and the circumference should be measured, in centimetres (cm) when the patient
breathes out.
This table lists the classification of risk for patients who are overweight or obese:
Low High Very High
waist circumference waist circumference waist circumference
Men 102cm
Women 88cm
NICE 2014.
4. Calculating the Body Mass index:
Body mass can be calculated by dividing the patient’s weight in kilograms by the square of
the patient’s height in metres:
Weight = BMI
Height2
For example, the BMI of a patient who is 1.9m tall and weighs 82kg:
__82__ = 22.7 BMI
1.9 x 1.9
This table lists the BMI classification:
BMI Classification
40 Obesity class 3
WHO 2020
95. Document and discuss with the patient:
Once you have calculated the BMI and determined their weight classification, you need to
discuss this with the patient in a sensitive manner and document the results and details of
your discussions within their medical notes.
During the discussion it would be important to explore the patient’s level of risk and specific
modifiable risk factors to help inform them of ways in which they could reduce their own risk.
To facilitate these conversations, NICE have produced a scoring system which incorporates
waist circumference and BMI and highlights the level of intervention required for different risk
categories. Please remember that the BMI may be less accurate for certain groups, and
clinical judgement may be required:
Table classifying level of intervention from NICE 2014:
BMI Waist Circumference Comorbidities
Classification present
Low High Very High
Overweight 1 2 2 3
Obesity I 2 2 2 3
Obesity II 3 3 3 4
Obesity III 4 4 4 4
1 General advice on healthy weight and lifestyle
2 Diet and physical activity
3 Diet and physical activity: consider drugs
4 Diet and physical activity: consider drugs: consider surgery
Be aware that intervention should be higher for patients with comorbidities.
10Bibliography & Further Reading
Field, A. E., Aneja, P., Austin, S. B., Shrier, L. A., De Moor, C., & Gordon‐Larsen, P. (2007).
Race and gender differences in the association of dieting and gains in BMI among young
adults. Obesity, 15(2), 456-464.
Heymsfield, S., Peterson, C., Thomas, D., Heo, M., & Schuna Jr, J. (2016). Why are there
race/ethnic differences in adult body mass index–adiposity relationships? A quantitative critical
review. Obesity Reviews, 17(3), 262-275.
Kitahara, C. M., Flint, A. J., de Gonzalez, A. B., Bernstein, L., Brotzman, M., MacInnis, R. J., . . .
Singh, P. N. (2014). Association between class III obesity (BMI of 40–59 kg/m2) and mortality: A
pooled analysis of 20 prospective studies. PLoS Medicine, 11(7), e1001673.
Logue, J., Walker, J. J., Leese, G., Lindsay, R., McKnight, J., Morris, A., . . . Scottish Diabetes
Research Network Epidemiology Group. (2013). Association between BMI measured within a
year after diagnosis of type 2 diabetes and mortality. Diabetes Care, 36(4), 887-893.
doi:10.2337/dc12-0944 [doi]
NHS; BMI healthy weight calculator; BMI healthy weight calculator : NHS [Accessed 01/7/20]
NICE (2014, updated 2018) Obesity: identification, assessment and management. Clinical
guideline [CG189] Obesity: identification, assessment and management [Accessed 01/7/20]
Paeratakul, S., White, M. A., Williamson, D. A., Ryan, D. H., & Bray, G. A. (2002). Sex,
race/ethnicity, socioeconomic status, and BMI in relation to self‐perception of
overweight. Obesity Research, 10(5), 345-350.
Taylor Jr, H. A., Coady, S. A., Levy, D., Walker, E. R., Vasan, R. S., Liu, J., . . . Fox, C. (2010).
Relationships of BMI to cardiovascular risk factors differ by ethnicity. Obesity, 18(8), 1638-1645.
Wang, J., Thornton, J. C., Russell, M., Burastero, S., Heymsfield, S., & Pierson Jr, R. N. (1994).
Asians have lower body mass index (BMI) but higher percent body fat than do whites:
Comparisons of anthropometric measurements. The American Journal of Clinical
Nutrition, 60(1), 23-28.
WHO; WHO BMI [Accessed 01/7/20]
WHO (2009); WHO guidelines on hand hygiene in Health Care; WHO hand hygiene and
resources [Accessed 01/07/2020]
Zilanawala, A., Davis-Kean, P., Nazroo, J., Sacker, A., Simonton, S., & Kelly, Y. (2015).
Race/ethnic disparities in early childhood BMI, obesity and overweight in the United Kingdom
and united states. International Journal of Obesity, 39(3), 520
11Picture Credits
1. Figure 1: Height measurement: Clinical Skills Teaching and Learning Centre, University of
Liverpool.
2. Figure 2: SECA scales for standing patients: Permission for Images kindly granted by
SECA, seca, sales@seca.co.uk
3. Figure 3: SECA chair scales: Permission for Images kindly granted by SECA, seca,
sales@seca.co.uk
4. Figure 4: Height measurement: Clinical Skills Teaching and Learning Centre, University of
Liverpool.
5. Figure 5: Height measurement: Clinical Skills Teaching and Learning Centre, University of
Liverpool.
6. Figure 6: SECA chair scales: Permission for Images kindly granted by SECA, seca,
sales@seca.co.uk
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