Final answer:
The primary bodily fluid collected for a drug screen is urine due to its non-invasive collection method and the no-preparation-required aspect for analysis. While urine is most common, blood plasma can also be used in certain cases, with appropriate sample preparation such as anticoagulation and centrifugation.
Step-by-step explanation:
For a drug screen, the bodily fluid most commonly collected from a patient is urine. Drug screening through urine is a non-invasive and widely used method that can detect the presence of drugs and their metabolites. Urine samples typically require no preparation before analysis, which makes them convenient for both the patient and the medical staff. In a clinical setting, urine not only provides data on drug use but also on the excretion rates of various ions like sodium, potassium, and chloride. However, for certain analyses such as calcium and phosphate, a 24-hour urine collection might be required due to the variability of these ions' output over a day.
In certain cases, other bodily fluids such as blood plasma might be used for drug screening. Blood samples are commonly collected into tubes with anticoagulants such as lithium heparin or K2EDTA and are then centrifuged to obtain plasma. The plasma can then be quenched with methanol before analysis to preserve the sample. Although blood tests can provide comprehensive information about drug levels and physiological conditions, urine tests remain the standard primary method for drug screening due to their simplicity and effectiveness.