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.
Urine testing is the most common method for detecting marijuana / cannabis consumption. It can be detected up to 5 days after consumption in infrequent users while it can be detected up to 15 days after consumption in heavy users or those with a high percentage of body fat.