# How to calculate the slope per day using 3 measurements?

I have a dataset of 134 lung cancer patients who had an inflammatory biomarker measured at 3 time points before during and after therapy - we're trying to investigate if the biomarker is predictive of their survival risk. Thing is, the time between the three measurements is different for each patient so we want to calculate the slope per day to account for the unequal time between the measurements - does that make sense and if so how to do so?

• Just to be clear, are these arthritis patients receiving anti TNF-$\alpha$ treatment who subsequently developed lung cancer? Or is the treatment an anticancer therapy with inflammation as a serious side effect? – AdamO Mar 2 at 20:30