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.
A urine sample (known as a urine toxicology screen) is typically used to test for marijuana exposure, and it can detect marijuana for days or weeks after use. Bloodwork can also be done to test for the amount of tetrahydrocannabinoid (THC) in the bloodstream. What to expect from urine and blood tests.