What do clinical trial results mean?

There are lots of ways for researchers to measure the results of clinical trials. Here are some of the terms they use. We have grouped them together into information about:

What are end points?
How long people live (survival)
How well treatment works (response)
Side effects
Other end points

What are end points?
Researchers use different factors to see how well a treatment works. They call these the trial end points. Many trials have more than one end point. They are called primary and secondary end points. 

The primary end point is the thing that the research team are most interested in finding out. It is the question that the trial is designed to answer. It may be how long people live after different treatments. Or how long it is before the cancer comes back. 

Secondary end points are other things that the researchers are also interested in, but they are not the main aim of the trial. It may be to find out what happens to the drug in the body, or what the side effects are.

Below are some of the terms researchers use when they are looking at trial results.

How long people live (survival)

Overall survival (OS) How long people live after treatment
Disease free survival (DFS) The time between treatment aimed at curing cancer, and signs that it has come back
Progression free survival (PFS) The time between treatment aimed at shrinking or controlling cancer, and signs that it has started to grow again
Event free survival (EFS) The time between treatment and having a specific ‘event’ such as a symptom, type of pain or a fracture, for example
Median overall survival The time from treatment to the point at which half of the patients are living


You may also hear the terms 5 year survival or 10 year survival. This means how many people are alive at least 5 years after treatment. It does not mean the number of people who live for 5 years or 10 years and then die. 

How well treatment works (response)
Many researchers use something called the Response Evaluation Criteria in Solid Tumours (RECIST) to analyse how well trial treatments work. They measure the area of cancer on a scan before and after treatment, to see how much it’s changed.

Complete response There are no signs of cancer on scans or tests
Partial response The cancer has shrunk by at least one third (30%) and there are no signs the cancer has grown anywhere else in the body
Stable disease The cancer has stayed the same size, it hasn’t got better or worse
Disease progression The cancer has grown by at least a fifth (20%) or there are new areas of cancer
Overall response rate (ORR) The total number of people whose cancer has either gone away (a complete response) or shrunk (a partial response)


There are other terms which you may hear. If you are in remission, it means that there are no signs of cancer on scans or tests. 

A recurrence means the cancer has started to grow again after it had gone away. And a relapse means the cancer has started to grow again after it had got smaller or stopped growing.

Side effects
A side effect is a medical problem which doctors think is probably caused by the treatment in the trial.  An adverse event (AE) is similar, but it may or may not be caused by the treatment in the trial. It could be a coincidence.

A serious adverse event (SAE) is a medical problem that causes you to be seriously ill. You may be admitted to hospital or have long term medical problems.

Other end points
Pharmacokinetics (PK) means how the drug gets to different parts of the body, and how the body breaks down and gets rid of the drug.

Pharmacodynamics (PD) means finding out how a drug works and the effect it has on the body.

Quality of Life (QoL) is also important. Researchers measure any positive or negative effects that treatment has on people and their day to day lives.

Related Information

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Clinical trial results

How clinical trial results are used

Find a clinical trial


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