**What are Patient-Years?**

A participant at one of our recent conferences asked a good question—“What are patient-years?”

“Person-years” is a statistic for expressing incidence rates—it is the summing of the results of events divided by time. In many studies, the length of exposure to the treatment is different for different subjects, and the patient-year statistic is one way of dealing with this issue.

The calculation of events per patient-year(s) is the number of incident cases divided by the amount of person-time at risk. The calculation can be accomplished by adding the number of patients in the group and multiplying that number times the years that patients are in a study in order to calculate the patient-years (denominator). Then divide the number of events (numerator) by the denominator.

- Example: 100 patients are followed for 2 years. In this case, there are 200 patient-years of follow-up.
- If there were 8 myocardial infarctions in the group, the rate would be 8 MIs per 200 patient years or 4 MIs per 100 patient-years.

The rate can be expressed in various ways, e.g., per 100, 1,000, 100,000, or 1 million patient-years. In some cases, authors report the average follow-up period as the mean and others use the median, which may result in some variation in results between studies.

Another example: Assume we have a study reporting one event at 1 year and one event at 4 years, but no events at year 2 and 3. This same information can be expressed as 2 events/10 (1+2+3+4=10) years or an event rate of 0.2 per person-year.

An important issue is that frequently the timeframe for observation in studies reporting patient-years does not match the timeframe stated in the study. Brian Alper of Dynamed explains it this way: “If I observed a million people for 5 minutes each and nobody died, any conclusion about mortality over 1 year would be meaningless. This problem occurs whether or not we translate our outcome into a patient-years measure. The key in critical appraisal is to catch the discrepancy between timeframe of observation and timeframe of conclusion and not let the use of ‘patient-years’ mistranslate between the two or represent an inappropriate extrapolation.”[1]

**References**

1. Personal communication 9/3/13 with Brian S. Alper, MD, MSPH, FAAFP, Editor-in-Chief, DynaMed, Medical Director, EBSCO Information Services.