Malnutrition Universal Screening Tool (MUST)

Malnutrition affects virtually every organ system and is independently associated with adverse outcomes across all healthcare settings. Recognising these consequences helps justify the investment in screening and intervention.

• Immune function: Malnutrition impairs both innate and adaptive immunity, increasing susceptibility to healthcare-associated infections (pneumonia, urinary tract infections, surgical site infections). Protein-energy malnutrition particularly affects T-cell function and mucosal barrier integrity.
• Wound healing: Inadequate protein, zinc, vitamin C, and caloric intake significantly delays wound healing. Malnourished surgical patients have higher rates of wound dehiscence, anastomotic leak, and pressure ulcer development.
• Muscle function: Sarcopenia and muscle wasting reduce respiratory muscle strength (increasing pneumonia risk), impair mobility (increasing fall risk), and prolong rehabilitation. Loss of muscle mass accelerates with acute illness and bed rest.
• Psychological wellbeing: Malnutrition is associated with depression, apathy, fatigue, and impaired cognitive function. These effects reduce engagement with rehabilitation and treatment plans, creating a cycle of further nutritional decline.
• Healthcare utilisation: Malnourished inpatients have, on average, a hospital stay approximately 30% longer than well-nourished patients. The estimated annual cost of disease-related malnutrition in the UK exceeds £19.6 billion, largely driven by increased length of stay, readmissions, and community care needs.

Malnutrition in adults is multifactorial. The MUST score identifies risk, but a thorough nutritional assessment should explore the underlying cause to guide targeted intervention.

• Reduced intake: Anorexia from illness, medication side effects (nausea, altered taste), dysphagia, poor dentition, depression, social isolation, poverty, or restrictive diets.
• Increased requirements: Sepsis, trauma, burns, major surgery, cancer, chronic obstructive pulmonary disease, heart failure, or any hypermetabolic state.
• Impaired absorption: Coeliac disease, inflammatory bowel disease, short bowel syndrome, pancreatic exocrine insufficiency, or post-surgical malabsorption (e.g. gastrectomy, bariatric surgery).
• Increased losses: Chronic diarrhoea, fistulae, draining wounds, renal losses (nephrotic syndrome, dialysis), or excessive vomiting.
• Age-related factors: Physiological anorexia of ageing, sarcopenia, social isolation, cognitive decline affecting meal preparation, and institutional catering that does not meet individual needs.

In many patients, several of these factors coexist. For example, an elderly patient with COPD may have increased requirements, reduced appetite from breathlessness, poor intake from depression, and medication-induced taste changes — each contributing to cumulative nutritional risk.

Nutritional intervention should be proportionate to the degree of malnutrition risk and tailored to the patient’s underlying condition, preferences, and functional status. The general hierarchy of intervention is:

• Food first: Optimise oral intake with food fortification (adding energy-dense ingredients to meals), increasing meal frequency, providing snacks, and addressing barriers to intake (poor dentition, swallowing difficulties, timing of meals around treatments).
• Oral nutritional supplements (ONS): Prescribe energy- and protein-dense supplements when food intake alone is insufficient. Compliance is improved when ONS are given between meals rather than as meal replacements, and when a variety of flavours and textures are offered.
• Enteral nutrition (tube feeding): Indicated when the gut is functional but oral intake is inadequate despite food-first and ONS strategies. Routes include nasogastric, nasojejunal, PEG (percutaneous endoscopic gastrostomy), or jejunostomy tubes depending on anticipated duration and clinical context.
• Parenteral nutrition (PN): Reserved for patients with a non-functional or inaccessible GI tract (e.g. bowel obstruction, severe mucositis, short bowel syndrome). PN carries significant risks including line sepsis, metabolic complications, and liver dysfunction. It should be initiated and monitored by a specialist nutrition team.

Refeeding syndrome is a potentially fatal complication of nutritional repletion in severely malnourished patients. Patients at high risk (BMI < 16, negligible intake > 10 days, low baseline potassium/phosphate/magnesium) require careful electrolyte monitoring and gradual caloric introduction. NICE guideline CG32 provides detailed refeeding risk assessment criteria.

Mid-upper arm circumference (MUAC) is the recommended alternative when BMI cannot be calculated — typically because the patient is bedbound, has lower-limb amputations, or height cannot be reliably measured for other reasons.

• Measure at the midpoint between the acromion (shoulder tip) and the olecranon (elbow) on the non-dominant arm.
• Use a non-stretch tape measure with the arm relaxed at the side.
• MUAC < 23.5 cm: Suggests BMI is likely < 20 — assign BMI score of at least 1 (possibly 2 if MUAC is very low or clinical impression suggests BMI < 18.5).
• MUAC ≥ 23.5 cm: Suggests BMI is likely ≥ 20 — assign BMI score 0.

MUAC is a pragmatic surrogate, not a precise measure. It correlates with BMI at a population level but has limitations in patients with peripheral oedema, significant muscle wasting with preserved fat, or upper-limb abnormalities. Document the reason for using MUAC rather than measured height/weight.

Refeeding syndrome occurs when nutritional repletion — particularly carbohydrate — drives a shift from fat to carbohydrate metabolism, causing rapid intracellular uptake of phosphate, potassium, and magnesium. The resulting electrolyte disturbances can cause cardiac arrhythmias, respiratory failure, rhabdomyolysis, seizures, and death.

NICE identifies patients at risk if they have one or more of:

• BMI < 16 kg/m²
• Unintentional weight loss > 15% in the past 3–6 months
• Little or no nutritional intake for more than 10 days
• Low levels of potassium, phosphate, or magnesium before feeding starts

Or two or more of: BMI < 18.5, unintentional weight loss > 10% in 3–6 months, little/no intake > 5 days, history of alcohol misuse or drugs (insulin, chemotherapy, antacids, diuretics).

Management involves: checking electrolytes before feeding, starting at 10 kcal/kg/day and increasing slowly over 4–7 days, supplementing thiamine and B-complex vitamins before and during the first 10 days, monitoring electrolytes daily for the first week, and correcting any deficiencies promptly. A high MUST score should always trigger assessment for refeeding risk.

The most common and consequential failing of nutritional screening is completing the MUST score but failing to implement the corresponding care plan. Audit data consistently show that MUST completion rates have improved significantly since the tool’s adoption, but the proportion of patients who receive an appropriate nutritional intervention after a high score remains disappointingly low in many settings.

A MUST score only has value if it triggers action — documenting the score, initiating food-first strategies, referring to dietetics where indicated, and re-screening at the recommended frequency. A screening programme without a care pathway is a wasted exercise.

Weight estimation introduces significant error. Studies show that clinical estimates of body weight can deviate by ±10% or more from actual measured weight, particularly at the extremes of body size. When a patient cannot be weighed (bedbound, critically ill, bariatric patient without appropriate scales), clinicians often estimate weight without documenting the method used.

This matters because an inaccurate weight affects both the BMI score and the weight-loss calculation. If estimation is necessary, document it clearly (“estimated weight” or “weight from previous admission on [date]”), and use MUAC as a complementary measure. When possible, weigh the patient as soon as clinically feasible and update the MUST score accordingly.

It is a persistent misconception that obese patients cannot be malnourished. In reality, obesity and malnutrition frequently coexist. Sarcopenic obesity — characterised by low muscle mass with preserved or increased fat mass — is increasingly recognised, particularly in older adults and cancer patients. Micronutrient deficiencies (iron, vitamin D, B12, folate, zinc) are also common in obese individuals, especially those with poor dietary quality.

An obese patient who has lost 15% of body weight over 6 months may still have a BMI > 30 — the BMI score will be 0, but the weight-loss score should be 2, correctly identifying them as high risk. Clinicians must calculate the weight-loss percentage and not be falsely reassured by an apparently adequate BMI. MUST is designed to capture this scenario, but only if the weight-loss component is completed accurately.

Measured weight in patients with significant fluid retention (ascites, peripheral oedema, anasarca) does not reflect true nutritional status. A patient with decompensated liver disease may weigh 85 kg but have lost substantial lean body mass masked by 15 kg of ascitic and oedematous fluid. Similarly, aggressive diuresis can produce rapid “weight loss” that is fluid removal, not nutritional decline.

When fluid status is a significant confounder, use clinical judgement to estimate dry weight, rely on MUAC as a surrogate for nutritional status, and document the reasoning. Re-assess after fluid balance is optimised. Failing to account for fluid overload can either mask malnutrition (falsely reassuring BMI) or overestimate weight loss (aggressive diuresis misinterpreted as nutritional decline).

MUST is designed for serial screening, not a one-off assessment. Nutritional status is dynamic — a patient who scores 0 on admission may deteriorate significantly during a hospital stay, particularly if they develop complications, undergo surgery, or have prolonged periods of reduced oral intake. Equally, a patient who scores high initially may improve with intervention and appropriate re-screening documents this improvement.

BAPEN recommends weekly rescreening for inpatients, monthly for care home residents, and at least annually (or when clinical concern arises) for community patients. Failing to re-screen means missed deterioration, delayed intervention, and inability to evaluate whether nutritional care plans are working.