Use the MRC-ICU score for adult ICU patients with a current medication chart and ICU support data available at a defined time point, most commonly around 24 hours after ICU admission. The score helps describe medication regimen complexity in critically ill adults and can support critical care pharmacist triage, workload review and serial monitoring rather than acting as a stand-alone severity-of-illness or disposition tool.

Evidence and validation

The tool was derived in a single-centre medical ICU retrospective cohort of 130 adults using 24-hour medication data to build a 39-item weighted list of high-risk medicines, continuous infusions and organ support devices. Higher MRC-ICU scores correlated with total medication count, longer ICU length of stay, higher ICU mortality and greater illness severity, with little association with age or sex.

Study	Setting	Key findings for MRC-ICU
Multicentre validation (AJHP 2020)	230 adults in medical and surgical ICUs at two US hospitals; scores calculated at 24 hours, 48 hours and ICU discharge.	Confirmed convergent validity with number of orders and medication orders and divergent validity from age, weight and sex across centres and ICU types.
Pharmacist interventions and DDIs (JACCP 2020)	100 adults in a mixed medical–surgical ICU.	MRC-ICU had a stronger relationship with critical care pharmacist interventions and potential drug–drug interactions than APACHE II and remained significant after adjustment for age, sex and admission type.
Machine-learning validation (Ann Pharmacother 2021)	130 adults from the original medical ICU cohort.	Including the 24-hour MRC-ICU score alongside APACHE III in machine-learning models improved inpatient mortality prediction; MRC-ICU was among the most important predictors.
Dynamic prospective evaluation (Sci Rep 2026)	202 adults in a tertiary medical ICU with daily MRC-ICU calculations.	Median 24-hour score was 8; higher scores were associated with more clinical pharmacist interventions, more drug-related problems, higher APACHE II, SOFA and mNUTRIC scores and higher ICU mortality, with good discrimination for mortality.

Limitations and practical use

Interpret absolute scores relative to your own unit: published adult ICU cohorts report median 24-hour values in the high single digits to low teens, and higher scores consistently track with more pharmacist interventions, drug-related problems and illness severity rather than a single universal cut-off.
The tool has been derived and most extensively validated in adult medical and mixed ICUs with ICU length of stay of at least 24 hours; it has not been validated in paediatric units or non-ICU wards.
MRC-ICU describes regimen complexity and pharmacist workload and should complement, not replace, physiological severity scores when estimating mortality risk or escalation needs.

The original derivation excluded ICU stays less than 24 hours and hospice or active transfer orders at 24 hours.

Medication Regimen Complexity-ICU (MRC-ICU) Score