How PPE Projections Are Calculated

Translating patient projections into PPE demand projections

We found it helpful to distinguish between two types of PPE consumption: contact-based consumption and staffing based consumption.

  • Contact-based consumption refers to PPE usage in which the number of items utilized is a direct function of the number of contacts staff members have with COVID-19 patients.

  • Staff-based consumption refers to PPE usage in which the number of items consumed only depends on the number of staff in a shift and not on the number of their contacts with patients.

Modeling contact-based consumption

Depending on the PPE item and the scenario, an item might be used only once (this practice is typical of the “standard” scenario) or might be reused multiple times (especially in scenarios of PPE shortage, such as “contingency” or “crisis”) before being discarded.

PPE use of an item on a particular day=(# of patients that day)(# of daily contacts per patient)# of patient contacts before discarding item{PPE\ use\ of\ an\ item\ on\ a\ particular\ day} = \frac{(\#\ of\ patients\ that\ day)(\#\ of\ daily\ contacts\ per\ patient)}{\#\ of\ patient\ contacts\ before\ discarding\ item}

This formula does not require that the PPE be used for every patient contact. For example, if gloves were only used every other contact on average, then the denominator would be two patient contacts per pair of gloves because, on average, the patient would be contacted twice before a single pair of gloves is discarded.

The total PPE demand for the hospital for a given item on a given day can be found by summing the demand across all portions of the hospital.

Modeling staff-based consumption

In the case of staff-based consumption, rather than specifying that the clinician discard PPE after a given number of patient contacts, we instead specify that the equipment be (re)used for a certain number of shifts. Staff-based calculations must be done separately for the multiple clinical roles (nurse, resident, RT), since staffing ratios vary across roles.

Average # new PPE item per day=(# of patients that day)(# of shifts per day)(# of patients per provider)(# of shifts item is used){Average\ \#\ new\ PPE\ item\ per\ day} = \frac{(\#\ of\ patients\ that\ day)(\#\ of\ shifts\ per\ day)}{(\#\ of\ patients\ per\ provider)(\#\ of\ shifts\ item\ is\ used)}

This calculation assumes that clinicians will be re-using the same mask from previous shifts. The total PPE demand for the hospital can be found by summing the demand across all clinical units and clinical roles.

Scenario-dependent consumption

The consumption of some PPE items might be best modeled as contact-based in the standard scenario but as staff-based in cases of shortages (i.e., the contingency or the crisis scenario). For example, in our surveys, N95 masks in typical times are consumed based on the number of patient contacts. However, in light of supply shortages, many hospitals have switched utilization policies to a contingency mode limiting each staff member to one mask per shift, thus corresponding to a staff-based consumption.