“When faced with a difficult question, we often answer an easier one instead, usually without noticing the substitution.” -Daniel Kahneman
As physical therapists we have many tools to track patient progress; outcome surveys, range of motion measurements, etc. However, which of these measurements meaningfully reflect patient outcomes?
This might seem obvious. But we often forget to ask these big picture questions in the clinic. We also forget such questions when reading research or choosing con-ed courses.
To pick the right measurements, we need to understand “clinical endpoints“. This term comes from clinical medicine trials. A clinical endpoint is “an event or outcome that can be measured objectively to determine whether the intervention being studied is beneficial”.1
There are three types of clinical endpoints2:
1. Direct clinical endpoints
These directly relate to patient outcomes. For example, consider a soccer player with knee pain. A direct clinical end point would be their ability to play soccer with an acceptable level of pain. We could measure that with the Victorian Institute of Sport Assessment–Patella (VISA-P). An improvement in the VISA-P likely means an improved ability to play soccer with an acceptable level of pain.
Of course, direct clinical endpoints are not the only meaningful measurements. But they most directly relate to patient outcomes.
2. Surrogate endpoints
These predict patient outcomes. We use these if we can’t directly measure an outcome. For example, we don’t know whether an elderly patient will fall – until they fall. But a measure like the modified 30-second Sit to Stand (m30STS) predicts fall risk.3 If we improve the patient’s m30STS score, they should be at a lower risk of falling.
A surrogate endpoint should be validated. Meaning that data supports its ability to predict an outcome.2
However, some surrogate endpoints are not validated. Such endpoints theoretically relate to an outcome. But data doesn’t show that connection. For example, consider the elderly patient at risk of falling. Low calf muscle strength might correlate with falls. However, there are many other factors related to fall risk, so we can’t predict fall risk with calf muscle strength alone.
These correlate with biological activity in a patient. They are least relevant to patient outcomes. For example, a PT might use a tool shown to “increase blood flow” in a patient with low back pain. “Increased blood flow” is a biomarker. This biomarker might correlate with tissue healing. And tissue healing might decrease low back pain. But this is a weak connection. The increased blood flow is an interesting effect, but may not be relevant to improving low back pain.
As we see, clinical endpoints are not all equally relevant. Understanding them is crucial to interpreting research findings, picking con-ed courses, and tracking patient progress. The goal is to not forget the goal: getting patients back to activities they love.
- NCI Dictionary of Cancer Terms. (n.d.). Retrieved November 13, 2020, from https://www.cancer.gov/publications/dictionaries/cancer-terms/def/endpoint
- Sullivan, E. (n.d.). Clinical Trial Endpoints [Powerpoint slides]. Retrieved November 13, 2020 from https://www.fda.gov/media/84987/download
- Applebaum, E. V., Breton, D., Feng, Z. W., Ta, A. T., Walsh, K., Chassé, K., & Robbins, S. M. (2017). Modified 30-second Sit to Stand test predicts falls in a cohort of institutionalized older veterans. PloS one, 12(5), e0176946. https://doi.org/10.1371/journal.pone.0176946