Final answer:
GC-MS is the method of choice for confirmation testing following a positive opiate drug screening result.
Step-by-step explanation:
The method of choice for confirmation testing following a positive opiate drug screening result is **GC-MS** (gas chromatography-mass spectrometry). GC-MS offers high specificity and sensitivity, allowing for the identification and quantification of drugs and metabolites in biological samples.
It is commonly used in pharmacokinetic evaluation, as well as in vitro and in vivo studies. Other methods mentioned, such as AAS (atomic absorption spectroscopy), HPLC (high performance liquid chromatography), and immunoassay, may be used for preliminary screening but may not provide the same level of accuracy and specificity as GC-MS.