CBD Finger-Prick Testing: Five Patients Cannot Establish Universal Matrix Equivalence
A small CBD microsampling study found no significant concentration difference among finger-prick blood, venous blood and plasma measurements. That finding supports feasibility under its conditions, but five patients cannot establish universal equivalence among these specimens.
A useful clinical sampling problem
Researchers developed an LC–MS/MS assay using 30-microliter volumetric absorptive microsampling tips. The clinical comparison involved five people with Dravet syndrome receiving purified prescription CBD; their ages ranged from 6 to 26 years.
Capillary blood was collected on the tips, venous blood was also applied to tips, and plasma was prepared separately. The study therefore compared both collection sources and different specimen preparations. It was not a trial of CBD’s seizure benefit or a test of driving impairment.
A null difference is weaker than demonstrated equivalence
CBD results did not differ significantly between capillary samples and the venous or plasma comparisons, with reported P values of 0.69. The analysis used significance testing, rather than a prespecified equivalence margin demonstrating that differences were acceptably small.
With only five patients, failure to detect a difference leaves substantial uncertainty about uncommon or patient-specific discrepancies. Multiple tips prepared from one person improve analytical replication; they do not increase the number of independent patients.
Hematocrit was examined over a narrow range
The methods specify hematocrit values of 45% to 47% in the donors and patients used for validation. Although volumetric tips address sample-volume problems associated with conventional dried spots, these experiments do not establish identical performance across all hematocrit values.
Extraction recovered approximately one-quarter of CBD and THC, yet the calibrated method met the reported precision and accuracy criteria. Recovery, reproducibility and agreement between specimen types are different validation questions.
What follows for a monitoring result
The assay quantified CBD and THC down to 1 microgram per liter under its validated conditions, and fortified tips met the study’s stability criteria for up to four weeks at the tested temperatures. Those findings concern an analytical workflow, not a universal clinical target concentration.
For a particular result, retain the tip type, sampled volume, hematocrit, storage history and matrix-specific calibration. A convenient finger-prick method may reduce collection burden while still requiring larger patient comparisons before interchangeable interpretation is justified.
Related: Interpreting THC concentration and impairment.
Primary research
Cannabidiol Determination on Peripheral Capillary Blood Using a Microsampling Method and Ultra-High-Performance Liquid Chromatography Tandem Mass Spectrometry with On-Line Sample Preparation. DOI: 10.3390/molecules25163608.
