Therapeutic Drug Levels and the Clotting Tests

📖 Terms in this lesson

Trough level: The lowest drug level in the blood, drawn just before the next dose, used to monitor aminoglycosides, and vancomycin unless pharmacy uses AUC-guided dosing.

Digoxin level: Therapeutic range about 0.5 to 2.0 ng/mL; toxicity brings nausea, bradycardia and yellow-green visual halos.

Lithium level: Therapeutic range 0.6 to 1.2 mEq/L; above about 1.5 brings coarse tremor, vomiting and ataxia.

Prothrombin time (PT): About 11 to 13.5 seconds, with an INR of 0.8 to 1.2 off anticoagulants; it lengthens with warfarin, liver disease and vitamin K deficiency, and on warfarin the INR, not the seconds, is what you report.

Fibrinogen: Normal about 200 to 400 mg/dL; it falls below 100 in disseminated intravascular coagulation because clotting consumes it, so a low fibrinogen with a rising D-dimer in a bleeding client points to DIC.

Phenytoin level: Therapeutic 10 to 20 mcg/mL; above 20 brings nystagmus, ataxia and slurred speech, and the drug also causes gingival hyperplasia with long use, so teach good oral care and never stop it abruptly.

Theophylline level: Therapeutic about 10 to 20 mcg/mL (newer targets 5 to 15); above 20 brings nausea, tachycardia and seizures, and caffeine, smoking changes and many antibiotics shift the level.

Carbamazepine level: Therapeutic 4 to 12 mcg/mL; above about 12 to 15 brings dizziness, diplopia and ataxia, and the drug can suppress the marrow, so report fever, sore throat or bruising and expect periodic CBCs.

Valproic acid level: Therapeutic 50 to 100 mcg/mL; above about 100 to 150 brings sedation, tremor and confusion, and the drug can injure the liver and lower platelets, so liver enzymes and a CBC are checked.

Vancomycin level: The trough goal was 10 to 20 mcg/mL (15 to 20 for serious MRSA infections); many pharmacies now dose to an AUC of 400 to 600 instead, and levels above that range threaten the kidneys and hearing.

Theophylline: An older bronchodilator with a narrow therapeutic range (about 10 to 20 mcg/mL); toxicity shows as nausea, tachycardia, restlessness and seizures, so check levels, limit caffeine, and know that smoking lowers the level while ciprofloxacin and heart failure raise it.

Phenytoin: A seizure drug with a narrow range (10 to 20 mcg/mL) whose toxicity shows as nystagmus, ataxia and slurred speech; push IV doses slowly in saline only with cardiac monitoring, expect gum overgrowth with long use, and hold tube feedings around oral doses.

Carbamazepine: A seizure, trigeminal neuralgia and bipolar drug (range 4 to 12 mcg/mL) with boxed warnings for aplastic anemia and Stevens-Johnson syndrome (worst in people of Asian ancestry with the HLA-B*1502 gene); report fever, sore throat, bruising or rash, watch sodium, and avoid grapefruit.

Valproate (valproic acid, divalproex): A seizure and bipolar drug (range 50 to 100 mcg/mL) with boxed warnings for liver failure, pancreatitis and birth defects; check liver enzymes, platelets and ammonia, report abdominal pain, jaundice or easy bruising, and expect weight gain and tremor.