Besides blood, which sample type is listed as a potential data source for concentration-time profiling?

• A. Spinal fluid
• B. Hair
• C. Tears
• D. Saliva

Answer: (A)

Concentration-time profiling relies on measuring drug levels in a biologic matrix that truly reflects exposure over time, and for CNS-active drugs the spinal fluid provides a direct window into CNS exposure. Spinal fluid is in close contact with the brain and spinal interstitial fluid, so its concentrations more accurately reflect CNS drug levels and distribution across the blood–brain barrier. This makes it the best added data source when CNS pharmacokinetics are of interest, despite being more invasive to obtain.

Hair, on the other hand, records cumulative exposure over long periods but does not track rapid concentration changes after dosing, so it isn’t suitable for shaping a time–concentration profile. Saliva and tears can sometimes mirror free drug levels, but they usually show greater variability and lower concentrations, limiting their reliability for detailed PK profiling.