An underweight person is one who has a BMI of?

Appeared in: NHM UP Staff Nurse - 2016

Explanation

The World Health Organization (WHO) defines an adult with a Body Mass Index (BMI) of less than 18.5 kg/m² as underweight.
The formula for BMI is weight in kilograms divided by the square of height in meters (kg/m²).
Among the given options, <18 kg/m² is the closest value to the standard cutoff of <18.5 kg/m² and is therefore the best answer.

Option A: A BMI of less than 15 kg/m² is classified as very severely underweight, not just underweight. It represents a more extreme condition.
Option B: A BMI of less than 20 kg/m² falls within the normal weight range (18.5–24.9 kg/m²). A person with a BMI between 18.5 and 19.9 is considered to have a normal weight.
Option D: A BMI of less than 12 kg/m² indicates an extremely severe and life-threatening level of underweight, far below the standard threshold for simply being 'underweight'.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Body Mass Index (BMI) Classification as background academic context rather than a clinical decision trigger.
Nurses frequently use BMI as a quick screening tool to identify patients at risk for health problems related to their weight, both underweight and overweight.
Being underweight can be as detrimental to health as being obese, increasing risks for infections, poor surgical outcomes, and osteoporosis. Nurses must assess for underlying causes like malnutrition, eating disorders, or chronic diseases.
What if the patient is an older adult? For individuals over 70, some guidelines suggest a slightly higher 'normal' BMI range (e.g., 22-27 kg/m²), and a BMI below 22 kg/m² might be considered underweight, as it's associated with higher mortality and frailty in this population.

How to Approach the Question

First, identify the key term in the question, which is 'underweight' and its relation to 'BMI'.
This is a factual recall question. Access your knowledge of standard clinical values.
Recall the WHO classification for BMI. The cutoff for underweight is <18.5 kg/m².
Compare this standard value to the given options.
Select the option that is closest to the official definition. In this case, <18 kg/m² is the best fit for <18.5 kg/m².

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 2 p. 521-523

E6 Parks TextBook of Preventive & Social Medicine part 1 — Subpart C (pp 404-604 of 604) p. 69-71

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