Cough syrup gives clue to Tamoxifen drug dose

[Hopeful]

http://www.bbc.co.uk/news/health-11790275

Hopeful

[Rich66]

Blood samples were taken to see if the processing of the cough syrup drug matched that of the Tamoxifen.
The results showed that levels of the cough drug accurately predicted levels of the chemicals produced when tamoxifen is broken down


Ok...so...if checking labs, why not just check for the active part of Tamoxifen, endoxifen?