Therapeutic Drug Monitoring Flashcards
What is Therapeutic Drug Monitoring (TDM)?
TDM is the measurement of specific drug levels in a patient’s bloodstream to ensure optimal therapeutic effects and minimize toxicity.
What is the purpose of TDM?
(1) To determine the most effective dosage, while preventing toxicity.
(2) To identify noncompliant patients (patients who, for whatever reason, either cannot or will not comply with drug dosages as prescribed by the physician).
What factors influence blood drug levels and interpretation of plasma drug concentrations (pdc) during TDM ?
(1) Characteristics of the drug
(2) Characteristics of the patient
(3) The laboratory methods used to test for the drug
(4) Timing of sampling
What are drug characteristics that may influence drug levels during TDM?
(1) dosage of the drug
(2) drug interactions
(3) pharmacokinetics of the drug: route of administration, delivery rate, absorption rate, bioavailability, plasma protein binding, excretion rate
What are patient characteristics that may influence drug levels during TDM?
(1) age and weight of the patient
(2) pregnancy
(3) other medications the patient is taking
(4) other diseases that the patient has e.g. renal, liver, cardiac, thyroid diseases
(5) the patient’s compliance to the drug treatment regimen
(6) habits: diet, smoking, drinking
(7) electrolyte balance: digoxin vs K+ and Ca2+
Why is TDM not required for most drugs?
Their clinical effect is easily measurable. Therefore, their dosage can generally be varied according to the clinical response that a patient gets from the drug.
What are some indications for TDM?
(1) Drugs with low therapeutic index e.g. lithium, phenytoin, valproic acid, and phenobarbitone, where a concentration range is established for which most users experience the desired outcome without serious adverse effects.
(2) Poorly defined clinical end point e.g. epilepsy.
(3) When there is potential patient noncompliance, to find out whether a patient is taking a prescribed drug, particularly a patient suffering from psychiatric illness.
(4) Therapeutic failure to a particular drug without any apparent reason.
(5) Drugs with saturable metabolism e.g. phenytoin.
(6) Wide variation in the metabolism of drugs e.g. slow or fast metabolizers.
(7) Dose adjustment in major organ failure.
(8) Prevention of adverse drug effects; for example, nephrotoxicity of aminoglycoside antibiotics is difficult to distinguish clinically from that caused by a severe deneralized infection.
(9) Diagnosing under-treatment
(10) Monitoring and detecting drug interactions.
(11) Guiding withdrawal of therapy.
What is therapeutic index?
This is the ratio of the median toxic dose (TD50) to the median effective dose (ED50) of a drug for some therapeutically relevant effect.
The TI is a measure of a drug’s safety. A larger value indicates a wide margin between doses that are effective and doses that are toxic.
What is therarapeutic window (range)?
This is the range between minimum toxic dose and the minimum therapeutic dose.
Why is it that measurement of drug concentrations in urine is not suitable for TDM?
(1) Varying urine volumes result in varying concentration of the drug in urine.
(2) Drugs that are metabolized in the liver and are largely eliminated in the bile through the intestine are not accurately measured in the urine.
Name three cardiac drugs that require TDM.
digoxin, digitoxin, amiodarone, lidocaine, quinidine, procainamide, N-acetyl-procainamide
Name three antibiotics that require TDM.
aminoglycosides (gentamicin, tobramycin, amikacin), vancomycin, chloramphenicol
Name three antiepileptics that require TDM.
phenobarbital, phenytoin, valproic acid, carbamazepine, ethosuximide, lamotrigine, levetiracetam, topiramate, zonisamide, felbamate
Name two bronchodilators that require TDM.
theophylline, caffeine
Name three immunosuppressants that require TDM.
cyclosporine, tacrolimus, sirolimus, mycophenolate, mofetil, azathioprine
