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.
Hair follicle drug tests can detect marijuana use starting around 7-10 days after first use. To do this, test labs use mass spectroscopy to detect the presence of a non-psychoactive byproduct of THC, a metabolite called THC-COOH.