Glossary

Risk

The risk for a disease or adverse event represents the probability for the event to occur among a defined susceptible (or exposed) population. For example, the risk of developing a skin rash in the first week of treatment in patients receiving an antimicrobial. When risk is presented as a proportion (per 100 patients), we assume that all patients were exposed for the same length of time. In the examples above, all subjects were observed for 1 week. However, it is expected that the longer a person is exposed, the higher is the probability for the event to be observed.³

Incidence rate (or incidence density rate or person-time incidence rate)

Incidence rates can be used to account for different lengths of times exposed when measuring risk.  It is a statistic that represents a summary measure of an event occurrence per population, taking into account the length of time that each member of the population was susceptible or exposed (person-time at risk). In technical terms, incidence rate of an event in a population is the ratio of new events in a specified time period (numerator) divided by the number of patients at risk during that period (denominator). For example, 1 case per 1,000 patient-years exposed, expresses 1 new case observed among 1,000 patients during one year of exposure.³

Post-marketing

Refers to the time after a drug or biological product has been approved by FDA.⁴

Carry-over PML

PML that develops a few months after stopping one disease modifying therapy (DMT) and starting a different DMT. In these cases, PML could have developed without causing symptoms while the patient was still on the previous DMT, or shortly after stopping the previous DMT.⁵