Blood, serum or plasma are the matrices of choice for toxicological analyses if the objective is to assess the acute impact of a drug of abuse on a person. This is typically the case in roadside testing and post-accident workplace drug testing or when there is objective evidence of impairment (e.g. aberrant driving) or in case of accident. The detection of drugs (cocaine, cannabis etc) in the blood strongly suggest impairment.
Blood samples should not be used for a control of abstinence, since alcohol and drugs are rapidly metabolized. After 24 h the drug concentrations are most often very low and substances can not be detected any more.
The levels of alcohol or drugs in the blood generally correlate with the intoxicative effects (deviant behaviour, adverse effects) and impairment. In contrast the concentrations in urine or hair do not allow any conclusions on the behaviour of a person at the time of sample collection.