The ionic composition of interstitial and intravascular fluids is the same. Proteins, however, are kept with the vascular compartment, at least to a degree, by the vascular wall, which is a semi-permeable membrane. The concentration of proteins is much higher in the plasma sector than in the interstitial sector.
Detection of marijuana using swab drug test can vary from person to person taking into account the different conditions like general health, different fluid intake, metabolism, etc.
Saliva drug testing. Saliva (oral) testing, also called a mouth swab test, may be used if an employer or other tester is interested in knowing about recent drug use, but is not ideal to survey long term use of drugs. Most saliva drug tests can detect most usage within a few hours up to 3 days.