Calculating half-life

[pharmpkParticipant]

I’m sure I should be able to find this in the manual but I’m having trouble finding the answer.

I have a set of data, one time column and three concentration data columns. The data fit well with an exponential function and DG provides Power as the slope. In my area of interest,,pharmacokinetics, the -slope is a rate constant and ln(2)/rate constant is the half-life. I’d like to turn the slope value into a half-life and use it in a text label on the graph. One value for half-life for each column of concentration data.

I’ll keep looking in the help but any help would be appreciated.

Thanks, David

PS Congratulations on the new version 5.0 whichI’m using.

[dgteamModerator]

There are two ways you could approach this.

 

First, if you add a token in a text command with a number from your fit, you can use that value in a function directly in the token.  Click the right side of the token to open the formatting options and click the top one, “Use function”.  Then you can add your equation, but note that we use the notation that the natural log is log(x). Base 10 is log10(x).

The other option is to extract the value from the command to an Expression variable.  Do the calculation in the Expression variable.  Then the output will be available to add as an token in a Text command.

Does that help?

 

[pharmpkParticipant]

OK, I have an answer. Maybe not the best but seems to work

I set the Fit function to arbitrary as a*exp(-b*x) with x = Time (column)

The fit gave me values of a and b (= kel1). I then calculated Half-life1 as log(2)/kel1. The same for the next columns using kel2 and kel3

 

 

[dgteamModerator]

We thought this would be a good example to add in to DataGraph.  If you go now to File > On-line examples you’ll see an “Exponential Fit” example that shows how to calculate the half-life using a variable or in a token.   This uses Exponential option directly and you don’t have to use the Arbitrary fit.  Hopefully this is helpful.

We also think having the half-life as an output from the command would make a lot of sense, as this is a common need.  So we are putting this on our To-do list 🙂

 

[pharmpkParticipant]

Thanks for adding this example. I finally worked out how to use the exponential fit and Extract Power as kel1 and thus half-life by using an expression.

I now have data which appears bi-exponential with

y = a*exp(-b*x) + c*exp(-d*t)

so I might be back to an arbitrary fit (using the logarithmic pop-up to the right of the equation field).

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