This test measures the concentration of zolpidem in blood serum as part of the Therapeutic Drug Monitoring (TDM) program. Zolpidem is a non-benzodiazepine hypnotic (a "Z-drug") that selectively acts on GABA-A receptors in the brain. Its levels are measured to assess treatment adherence, detect potential overdoses, and investigate the causes of inadequate efficacy or unusual side effects, particularly due to its rapid metabolism and significant inter-individual variability.