# Thread: I need help figuring out type of analysis for duration of therapy and adverse outcome

1. ## I need help figuring out type of analysis for duration of therapy and adverse outcome

Hi everyone,

I have a prospective cohort of 90 subjects who were previously on drug X (duration of drug X = "varx"), now on drug Y and followed for 24 weeks. Continuous dependent laboratory variables were checked at week 0 and at week 24.

1) What type of analysis can I run to evaluate how "varx" affected variables at week 0?

I'll be running a paired analysis (likely nonparametric) to evaluate changes from week 0 to week 24 while on drug Y. However,

2) What kind of analysis can I run to evaluate how "varx" correlate with changes while on drug Y?

Should I create a categorical variable for duration of therapy on drug X based off quartiles? Linear regression analysis? I'm not sure what to do.

2. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

Can you better describe the study design? In particular being on drug x. So they were on x for varying time periods or not and when was outcome collected for x and y? Example data would help!

3. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

Originally Posted by hlsmith
Can you better describe the study design? In particular being on drug x. So they were on x for varying time periods or not and when was outcome collected for x and y? Example data would help!
Prospective cohort followed for 24 weeks; labs performed at week 0 and week 24. The cohort was switched form drug X (with variable duration) to drug Y at week 0. Repeat labs done at week 24.

Example Data:
Subj | Time on drug X | Week 0 GFR | Week 24 GFR
1 44 months 113.5 113.5
2 80 121.1 103.8
3 0 76.24 80.2
4 32 155.1 152.1
5 60 96.2 97.4

4. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

For question 1, I thought using linear regression analysis with the dependent variable being the baseline laboratory values and independent variable being the duration on drug X would suffice.

For question 2, I though using linear regression analysis with the dependent variable being the delta values between the two time periods would suffice.

Any thoughts?

5. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

But you don't seem to have a baseline variable for before X was initiated, do you?

6. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

Originally Posted by hlsmith
But you don't seem to have a baseline variable for before X was initiated, do you?
No, no variables from while on drug X. Merely a time variable of duration previously on X.

7. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

What can you do with it then, since subject could have GFR = 110 then on X 20 months then had GFR = 110. So its effect is completely unknown. So you want to predict effect of Y while controlling for baseline value of GFR and prior weeks of X. That seems more reasonable then try to model varx!

This disconnect is where you are confusing me.

8. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

Originally Posted by hlsmith
What can you do with it then, since subject could have GFR = 110 then on X 20 months then had GFR = 110. So its effect is completely unknown. So you want to predict effect of Y while controlling for baseline value of GFR and prior weeks of X. That seems more reasonable then try to model varx!

This disconnect is where you are confusing me.
The question is whether longer duration of X affects week 0 variables or possibly has an effect on how drug Y affects at week 24.

The first part seems simple with linear regression. The second question does seem a bit unreasonable...I was thinking of recalculating a new value as a delta value from week 24 and week 0 and using linear regression again. Does this seem somewhat reasonable?

9. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

You keep top-rock'in around my issue, you need to freeze and take it to the floor to provide more substantive info. How can you predict the effect of X's course on GFR if you only have GFR at time "0" which appears to be after the application of X. You need to have the value of X before the initiation of X. Unless you know that, but didn't listed it up above.

11. ## Re: I need help figuring out type of analysis for duration of therapy and adverse out

Originally Posted by hlsmith

You keep top-rock'in around my issue, you need to freeze and take it to the floor to provide more substantive info. How can you predict the effect of X's course on GFR if you only have GFR at time "0" which appears to be after the application of X. You need to have the value of X before the initiation of X. Unless you know that, but didn't listed it up above.
Frozen!
Thanks, I see what you mean vs having a categorical variable (stating previously on or not) and running some descriptive analysis.

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