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Unit-hour utilization

Unit-hour utilization can measure ambulance transports per available hour or the share of time units are busy. Learn the difference.

Unit-hour utilization, or UHU, compares ambulance activity with the staffed ambulance time available to do the work. The name is used for different calculations. One counts transports per unit-hour; another measures the share of unit time spent on assignments. A UHU number is meaningful only when its formula and units are stated.

An El Dorado County advanced life support agreement appendix defines its ratio as transports divided by unit-hours produced. By contrast, Alamance County's EMS performance page describes UHU as the percentage of time ambulances are between dispatch and return to service. Those are distinct local measures, not interchangeable versions of the same percentage. El Dorado County agreement appendix, 2018 board attachment; Alamance County EMS measures

The denominator: unit-hours

One fully staffed, equipped ambulance provided for one hour contributes one unit-hour. Two such ambulances provided for twelve hours each contribute twenty-four unit-hours. This counts ambulance capacity over time, not individual employee hours. A two-person crew does not turn one ambulance hour into two unit-hours.

The denominator must follow the service's rules. Scheduled hours, hours actually staffed, and hours available after removing out-of-service periods can differ. A published ratio should identify which it uses. Otherwise a staffing shortfall could disappear into a denominator that still assumes every scheduled ambulance operated.

Example

Hypothetical: Two ambulances each operate for twelve hours, producing twenty-four unit-hours. Together they complete six patient transports. Under a transport-based formula, UHU is 6 ÷ 24 = 0.25 transports per unit-hour.

During those shifts, the ambulances spend a combined nine hours between dispatch and return to service. Under a busy-time formula, utilization is 9 ÷ 24 = 37.5%.

Both calculations describe the same hypothetical shifts. The first is an output rate; the second is a time share. Calling 0.25 “25% busy” would be incorrect. Six brief transports and six long transports produce the same transport-based ratio but can occupy very different amounts of time.

A response with no transport may still consume significant capacity. It adds no transport to the first calculation, while its occupied time can contribute to the second if the local rules include it.

Common confusion

UHU does not directly measure how hard an individual clinician worked. It aggregates vehicle activity, and different duties may or may not be represented. A period outside an assignment is not proof that a crew was resting; a transport count is not a complete inventory of clinical or operational work.

A systemwide average can also hide uneven demand. The same daily utilization can arise from calls spread across a day or several calls concentrated in one hour. The latter pattern may leave fewer resources ready for a new emergency even when the daily ratio is identical.

No universal safe or ideal threshold follows from the definition. The meaning of a local target depends on coverage, response expectations, trip lengths, staffing arrangements, and the chosen formula.

Using UHU in a comparison

Report the numerator, denominator, time period, and whether the measure covers one ambulance, a station, or the whole service. Check how hospital turnaround, canceled responses, nontransport calls, and out-of-service time are handled.

Pair utilization with concurrent-call patterns, resource availability, and response intervals. That combination can reveal whether a service is frequently committed when new calls arrive. A ratio alone cannot determine how many additional ambulances are needed or establish that staffing is adequate.

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