How are theta tab CI calculated?

Phoenix Certara Forums Modelling and Simulation
1

Hi,

I’ve written up the results of some modeling work and I have been asked to add how the parameter estimate CI are calculated. We expected it would be 1.96xSE, but that doesn’t appear to be the case. I cannot find the information anywhere in the help files. Someone suggested they are likely derived using the estimate covariance matrix assuming normality.

Can anyone confirm how the parameter estimate CI in the theta tab are calculated using Phoenix NLME?

Best regards,
Emma

2

can you share your theta sheet

tried on a random project on my end and it is all good:

Parameter Estimate Units Stderr CV% 2.5% CI 97.5% CI
tvKa 0.38615 0.02771 7.175942 0.33183 0.44047

see redo the computation in R you get exactly the same values

tvKa ← 0.3861499
seka ← 0.027709891
100*seka/tvKa

tvKa + 1.96seka
tvKa - 1.96seka
0.33183 0.4404698

3

I can’t share the whole theta sheet but can give an example parameter:

Parameter Estimate Units Stderr CV% 2.5% CI 97.5% CI
tvV 2767.255 ml 246.125 8.894195 2263.876 3270.633

If I use 1.96xSE then:

2.5% CI 97.5% CI

2284.849 3249.660

The model estimates the human PK parameters and allometrically scales those back to fit the observed animal PK data- could this be the cause?