Several factors are taken into account to check whether someone is malnourished or at a high risk of malnutrition.

In adults, these include:

• body mass index (BMI) – a measure that can be used to determine if you're a healthy weight for your height
• whether you've unintentionally lost weight in recent months
• whether an illness means you're unable to feed yourself or absorb nutrients from your diet

You would normally be considered malnourished if you have a BMI of less than 18.5 or you've unintentionally lost more than 5% of your body weight during the last three to six months. However, in some cases, doctors may be concerned about malnutrition if you have a BMI between 18.5 and 20.

You may be considered at a high risk of malnutrition if:

• an illness means you have eaten nothing for the last five days, or you're likely to eat nothing for the next five days
• you don't absorb nutrients from food well – for example, you have a condition, such as Crohn’s disease, that causes the digestive system to become inflamed
• there's an underlying reason why your body is likely to use up nutrients at a higher rate or have an increased need for nutrients
• you have difficulty eating and drinking

A vitamin or mineral deficiency can usually be diagnosed with a blood test.

Diagnosing malnutrition in children

Diagnosing malnutrition in children involves taking a measurement of their weight and height and comparing it against the expected average height and weight for a child of that age.

Some children will be below average because they're naturally smaller, but a significant drop below the expected level could indicate a risk of malnutrition.

Blood tests can also be used to measure protein levels in the blood. Low levels of protein may suggest that a child is malnourished.