I think what s/he's referring to are tests for mutations in certain liver enzymes that are known to interfere with a drugs function. These are cheap and quick tests (these aren't sequencing based tests) and quite good at predicting non-responders

On the other hand it can take 4-6 weeks to see if someone responds to a drug, and the drug can cause adverse events in addition to not treating the disease

So in this case it is cheaper, faster and safer to take the test before prescribing