A small specimen can support a quantitative result if the laboratory has validated the method for that volume and specimen type. Franco and colleagues developed a liquid chromatography–tandem mass spectrometry method using 50 microliters of serum or saliva. It measured CBD and its active metabolite 7-hydroxy-CBD as separate analytes.
The authors reported limits of quantification of 2.5 ng/mL for CBD and 5 ng/mL for 7-hydroxy-CBD. They assessed accuracy, precision and stability and demonstrated the method using samples from patients treated with CBD for epilepsy. A validated method can establish what a result means within its stated analytical limits.
Measuring the same compounds in two specimens does not establish that their concentrations are interchangeable. Comparing a saliva result with a serum reference would require evidence linking the measurements under the relevant collection conditions. For an individual report, the first details to check are the specimen, named analyte and applicable reporting limit.