Blood levels of drugs are used to monitor attainment of therapeutic drug levels, compliance with therapeutic regimens, and potential excess dosing. They are also used in situations when accidental or deliberate drug overdose is suspected. In therapeutic situations, serial samples may be drawn to determine peak (highest) and trough (lowest) blood levels of drugs. Samples for peak drug levels are generally drawn within 30 to 60 minutes of drug administration. Trough levels are drawn immediately before the next dose of the drug is to be given. It is necessary to know as exactly as possible the time the drug was administered or ingested for accurate interpretation of test results.

Many potential toxins are present in the household and in industrial settings. Data regarding circulating levels of toxic substances may be used to diagnose either acute or chronic poisoning with metals or common commercial substances.

• Therapeutic and toxic levels of various drugs are shown in Table 5-30. Toxic doses and effects of industrial and household toxins are listed in Table 5-31. For child values, refer to agency laboratory information.

• Determination of therapeutic levels of prescribed drugs, especially those with narrow therapeutic ranges or serious toxic effects, or both
• Evaluation of the degree of compliance with the therapeutic regimen
• Known or suspected drug overdose
• Known or suspected exposure to environmental toxins
• Evaluation of chronic exposure to industrial products known to be toxic

Nursing Care Before the Procedure

Client preparation is the same as that for any test involving the collection of a peripheral blood sample (see Appendix I).

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported to the laboratory immediately. For drug levels, the name of the drug, dosage, and time administered or ingested should be noted on the laboratory request form.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any test involving the collection of a peripheral blood sample.

• It may be necessary to withhold subsequent doses of drugs administered for therapeutic reasons until test results are available, but this practice should be confirmed with the person prescribing the medication.
• Abnormal values: Note and report both therapeutic and toxic values for the drug test performed. Notify the physician at once if any value is at a critical level. Prepare for immediate interventions to prevent cardiac arrest or other manifestations of toxicity, such as ECG monitoring, oxygen, or intubation and ventilation. Administer ordered antidote or other medications.
• Long-term drug therapy: Support client and instruct in long-term medication regimen, which symptoms to note and report, and when to discontinue the medication.
• Medicolegal aspects: Collection, delivery, possession, and transportation of the specimen should be witnessed by a legally responsible person, and the possession of it must remain unbroken from the time of collection to the completion of any legal court action (chain of evidence). Seal specimen to prevent tampering and label "Medicolegal Case." In some cases a number is used instead of a name for identification. Toxicology tests to determine abuse, suicide, intentional overdose, or suspected murder or attempted murder require these considerations.