Low BMI | acute-gp

Low BMI & Eating Disorders

Acute medical risk, admission criteria and admission-avoidance options for adults (18+)

Scope of this page
This page supports decisions about acute medical assessment, admission and safe admission avoidance in adults with low weight or eating disorders. It does not cover referral criteria to eating disorder or dietetic services.

For routine and urgent referrals to the Adult Eating Disorder Service, see the RMS Eating disorders referral criteria

1. Indicators of acute medical risk: discuss with Acute GP

These are based on the RCPsych Medical Emergencies in Eating Disorders (MEED) guidance, CR233, which replaces MARSIPAN and Junior MARSIPAN. Any one of the following should prompt a discussion with Acute GP about acute medical assessment or admission.

Anthropometry

BMI <13 kg/m² (or MUAC <18 cm)
Weight loss >1 kg/week for 2 or more consecutive weeks

Cardiovascular and observations

Pulse <40 bpm
Any combination of: systolic BP <90 mmHg, recent syncope, postural BP drop >20 mmHg, postural pulse rise >30 bpm
QTc >450 ms (females) or >430 ms (males), or any other significant ECG abnormality or acute change
Temperature <35.5 °C
SUSS test score <2 (unable to sit up from lying flat or rise from a squat, or only by using arms)

Bloods

Na <130 mmol/L
K <3.0 mmol/L with symptoms or ECG changes, or K <2.5mmol/L
PO4 or Mg <0.5 mmol/L (requires IV replacement)
ALT >500 iu/L
Glucose <3 mmol/L and symptomatic, OR glucose <3 mmol/L with BMI <13 kg/m² (risk of coma and death)
AKI: consider lower thresholds, as a low baseline creatinine can mask significant renal impairment in malnutrition
Raised CRP and/or hypoalbuminaemia (possible intercurrent illness)

Other

Signs of intercurrent illness in a severely malnourished patient*
Any other acute deterioration or clinical concern

*Immune response is blunted in malnutrition (lower baseline WCC, neutrophils and temperature), so "normal" values may be falsely reassuring. Mortality rises significantly when intercurrent illness occurs in malnourished patients.

Patient declines admission?
If urgent medical need is identified and the patient declines admission, consider capacity (Mental Capacity Act) and whether the Mental Health Act applies. Discuss with psychiatric advice.

2. Electrolytes and bloods: admit, discuss or manage in the community

Interpret results alongside symptoms, ECG and trend. Always check Mg when K is low (low Mg causes refractory hypokalaemia), and do an ECG for any significant electrolyte abnormality.

Potassium

ADMIT / IV: K <2.5 mmol/L
DISCUSS (admission likely): K <3.0 mmol/L with symptoms or ECG changes
COMMUNITY: K 2.5–3.49 mmol/L, asymptomatic, normal ECG. Consider oral supplementation. Repeat bloods in 2–3 days. Consider a PPI if chronic purging.

Phosphate

ADMIT / IV: PO4 <0.5 mmol/L
DISCUSS (admission likely): PO4 <0.8 mmol/L and symptomatic, especially if high refeeding risk
COMMUNITY: PO4 0.5–0.79 mmol/L and asymptomatic. Consider oral supplementation. Repeat bloods in 2–3 days. Consider holding calcium supplements short term, as they bind phosphate. If not at high refeeding risk, replacement is not usually advised above 0.6 mmol/L; repeat the test instead.

Magnesium

ADMIT / IV: Mg <0.5 mmol/L
DISCUSS (admission likely): Mg <0.7 mmol/L and symptomatic
COMMUNITY: Mg 0.5–0.69 mmol/L and asymptomatic. Consider oral supplementation OR here. Repeat bloods in 2–3 days. Check ECG.

Sodium

ADMIT: Na <130 mmol/L
COMMUNITY (mild abnormality): support normal fluid intake (~35 ml/kg; ~2 L/day for most adults). Isolated low Na may reflect water-loading (urine SG <1.010). If there is laxative misuse, advise against it and explain the risks.

Adjusted calcium

DISCUSS (admission likely): Ca <1.9 mmol/L, OR <2.05 mmol/L and symptomatic
COMMUNITY: check Mg, PTH, vitamin D and ECG. Raised Ca usually reflects dehydration; support fluid intake.

Glucose

ADMIT: <3 mmol/L and symptomatic, OR little or no nutritional reserve (e.g. BMI <13)
COMMUNITY (≥3 mmol/L or not for admission): eat every 2–4 hours, including starchy carbohydrate. Give fast-acting sugar if symptomatic, always followed by starchy carbohydrate to prevent rebound hypoglycaemia. Advise not to drive and to avoid long periods alone.

ALT

DISCUSS: ALT >500 iu/L or continuing to rise
COMMUNITY: a mild rise is common in malnutrition; manage with weight restoration.

ADMIT: raised CK with symptoms of rhabdomyolysis (reduced or dark urine, severe muscle pain, cramps or swelling)
DISCUSS: rising creatinine or AKI
COMMUNITY: asymptomatic (often exercise-related; can be up to ~20,000). Stop exercise and encourage hydration. Check U&E weekly if CK >300, and safety-net for rhabdomyolysis symptoms.

Creatinine / urea

DISCUSS: AKI, especially with intercurrent illness or purging
COMMUNITY: monitor trends, as a low baseline creatinine can mask AKI. Support normal fluid intake.

Refeeding syndrome: suspected or high risk
Low PO4 is often the first electrolyte to fall, frequently with low K and Mg. Treat low levels as above AND put high-risk refeeding precautions in place:

Once-daily A–Z multivitamin and mineral
Thiamine 50 mg QDS (or 100 mg BD)
Vitamin B co-strong 1–2 tablets TDS
Daily electrolyte monitoring

Seek CFT Adult Eating Disorder Service advice on refeeding risk and/or consider medical admission if daily monitoring cannot be delivered safely in the community.

3. ECG

Consider acute medical assessment if:

Heart rate <40 bpm
QTc >450 ms (females) or >430 ms (males)
Changes consistent with hypokalaemia. The earliest sign is reduced T-wave amplitude, and changes usually appear once K is 2.5–2.9 mmol/L.

The most common findings are bradycardia (energy conservation) and prolonged QTc (often linked to purging). Automated QTc can be inaccurate, so calculate it manually if raised.

Manual QTc

Rautaharju (recommended, as it has less rate-dependent bias): QTc = QT × (120 + HR) / 180
Bazett (simplest, but over-corrects at fast rates and under-corrects at slow rates): QTc = QT ÷ √RR

QT = small squares from the start of Q to the end of T × 0.04 s. RR = small squares between consecutive R waves × 0.04 s. The result is in seconds (e.g. 0.525 s = 525 ms).

4. Admission avoidance: options when criteria are not met

Oral electrolyte replacement, with repeat bloods in 2–3 days (see section 2)
Refeeding precautions with daily bloods, where risk is high and monitoring is achievable
Advice from the Adult Eating Disorder Service on concerning presentations and refeeding risk (this is not an acute or emergency service)
More frequent physical health monitoring in primary care (up to weekly) where results are abnormal, deterioration is rapid or physical-risk behaviours such as purging are frequent, in line with shared-care arrangements
Fluids: support normal intake (~35 ml/kg; ~2 L/day for most adults)
Hypoglycaemia risk: regular carbohydrate every 2–4 hours; not to drive (DVLA guidance here); avoid long periods alone
High CK: stop exercise, hydrate, and check U&E weekly if CK >300
Once-daily A–Z multivitamin and mineral for most low-weight or restrictive presentations

Safety-netting: seek urgent help (111 / 999 / ED) for

Sudden breathlessness, chest pain or palpitations
Fainting or collapse
Blood in vomit
Severe muscle cramps, weakness, numbness or tingling
Reduced or dark urine with muscle pain
Thoughts of self-harm or ending their life
Any other change in physical or mental health that concerns them or those around them

5. Contacts and resources

Acute GP (acute medical assessment and admission)
01872 226263

Adult Eating Disorder Service, CPFT (advice on clinical presentations; non-urgent)
01872 246884
cpn-tr.EatingDS@nhs.net

Referral criteria
https://rms.cornwall.nhs.uk/primary_care_clinical_referral_criteria/primary_care_clinical_referral_criteria/dietetics/eating_disorders

Guidance

RCPsych – Medical Emergencies in Eating Disorders (MEED), CR233 → https://www.rcpsych.ac.uk/improving-care/campaigning-for-better-mental-health-policy/college-reports/2022-college-reports/cr233
RCHT Clinical Guideline – Management of Low Body Weight Patients with a Primarily Psychological Cause → https://doclibrary-rcht.cornwall.nhs.uk/GET/d10297811