BEST-MSU Trial: Mobile Stroke Units vs Standard EMS in Acute Stroke
In patients with acute stroke in the United States, does prehospital mobile stroke unit care (with on-board CT, vascular imaging, and thrombolysis capability) improve excellent 90-day functional outcome (mRS 0-1) in tPA-eligible patients compared with standard EMS transport?
Grotta JC, et al. (NEJM 2021) · 1515 patients
Population
Included
- Acute stroke symptoms within 24 hours
- Located within the MSU service area of Houston, TX
- Alert or minimally drowsy on initial evaluation
- Age 18 or older
Enrollment criteria (identical for MSU and EMS groups, assessed on scene)
- Examination features consistent with acute stroke that produced any degree of disability (no formal cutoff according to the National Institutes of Health Stroke Scale)
- Stroke onset within the previous 4.5 hours (time the patient was last known to be well)
- No obvious guideline contraindications to the use of tissue plasminogen activator (t-PA)
- Considered enrolled if screening criteria for t-PA treatment were met on MSU or EMS arrival at the scene, whether or not the patient ultimately became eligible for the primary outcome analysis
Excluded
- Comatose or unresponsive at presentation
- Traumatic etiology
- Location outside MSU service area at time of activation
- No qualifying acute stroke on assessment
- Stroke-mimicking condition
- Cerebral hemorrhage detected on noncontrast CT (rendered the patient ineligible for t-PA)
- Acute stroke with established guideline contraindications to t-PA
Source: Grotta JC et al., N Engl J Med 2021;385(11):971–981· Retrieved 2026-06-09
Primary Outcome
Design note: BEST-MSU used an alternating-week controlled design, not a parallel-arm RCT. MSU and standard EMS care alternated weekly at each site. Results are compelling but should be interpreted in the context of this quasi-experimental framework.
mRS 0-1 at 90 Days (tPA-Eligible Patients)
Study Arms
- Agent
- Mobile stroke unit (MSU) management with prehospital t-PA
- Route
- Prehospital, ambulance-based
- Co-interventions
- Transport to the destination stroke center on the basis of local EMS triage criteria; in-hospital care including imaging and reperfusion decisions after arrival
On-scene MSU steps: establishment of intravenous access, determination of the NIHSS, noncontrast CT of the head, blood-pressure control, and, if criteria were met, t-PA initial bolus and start of infusion. At three sites, CT angiography could be performed if large-vessel occlusion was suspected. MSU staffed by one or two paramedics, a CT technologist, and a critical care nurse, with a vascular neurology specialist supervising on board or remotely by telemedicine. t-PA used was Activase (recombinant; supplied by Genentech). The publication and these pages do not state a per-kilogram t-PA dose.
- Agent
- Standard EMS transport
- Route
- Prehospital, ambulance-based
- Co-interventions
- In-hospital stroke evaluation and reperfusion treatment (t-PA and endovascular therapy as indicated) after arrival at the emergency department
On EMS weeks, a study nurse (but not the MSU) met the patient and EMS at the destination emergency department without delaying intake, and the hospital-based stroke team managed care including imaging and t-PA decisions. Allocation to MSU or EMS was by alternating-week cluster, not individual randomization.
Safety
Death at 90 days
8.9%
11.9%
Mortality at 90 days was numerically lower with MSU care.
Trial Design
Type
- Prospective, multicenter, alternating-week controlled trial
- Observational comparison of MSU vs standard EMS
- Primary analysis in patients adjudicated eligible for thrombolysis
Timeline
United States; August 2014 to August 2020
N
1515
Enrollment
1,515 patients at 5 MSU sites in the United States. Alternating-week controlled trial. August 2014 to August 2020. Published NEJM 2021.
ClinicalTrials.gov
NCT02190500Bedside Pearl
BEST-MSU showed that prehospital MSU care saved 36 minutes and improved outcomes in tPA-eligible patients (NNT 13). The mechanism is faster treatment, not better treatment. For hospitals without MSU access, the equivalent message is: every minute saved on door-to-needle time translates to measurable benefit.
See also