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What are Patient-Years?

Mary McMahon
Updated: Mar 03, 2024

The concept of patient-years is used in many clinical studies and statistical assessments of risk. Viewing things in these terms allows researchers to look at a population more generally, rather than trying to separate out and process data from each individual member of a group. This concept often show up in news articles about long-term studies, although this precise term may not be specified. To obtain the number, researchers add together all of the years that patients in a study were followed, and then divide those years by the event of interest.

For example, if ten patients participated in a study on heart attacks for 15 years (i.e., 150 patient-years (10 x 15)), and three of them had heart attacks, there would be one heart attack for every 50 patient-years in the study. While it is important to look at individual data in any study, looking at things in these terms can reveal trends.

In the heart attack example above, the researchers might choose to follow several different populations and compare them at the end of the study. If our group above was a control group, there might be several research groups with different heart attack rates, like one heart attack for every three patient-years, or one heart attack for every seven. By looking at the general average, the researchers might be able to draw some conclusions about the various means to prevent heart attacks that are being studied.

Many studies on new medications also view things in these terms. For example, if one death is experienced for every 1,000 patient-years of a study, this might be viewed as an acceptable risk, while a high death rate might be cause to reconsider the validity of a medication. Contraindications for medications are also sometimes processed in this light; if one group being studied experiences a high rate of complications while on the new medication, researchers might decide that the medication is contraindicated for similar people in the general population.

In addition to being used in discussions of clinical studies, patient-years also sometimes crop up in long term morbidity and mortality reports. For example, the organ donor waiting list is usually carefully tracked to see how many patients die each year waiting for organs, and calculations using this concept sometimes become important in determining who is entitled to new organs. For instance, if a population of specific individuals waiting for lungs on the list experiences 300 deaths for every patient-year of waiting while another group waiting for lungs experiences 25 deaths, patients in the more high-risk group are probably going to get lungs first.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a The Health Board researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By anon172474 — On May 04, 2011

What is this "300 deaths for every patient-year" supposed to mean?

If one patient was waiting for lung for one year, would he/she have died 300 times?

By anon67492 — On Feb 24, 2010

In the last para you mentioned 300 deaths in every patient-year but did not mention how many patient-years. explain.

By junyb66111 — On Jun 04, 2009

Can patient-years number be decimal or not?

When I analyze the data, year can be a decimal. If I sum the years of patients, it would be a decimal.


Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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