Therapeutic drug monitoring: introduction and rationale MCQs With Answer

Therapeutic drug monitoring (TDM) is an essential clinical pharmacokinetics tool for optimizing drug therapy by measuring drug concentrations and interpreting them against pharmacologic targets. This blog offers focused MCQs tailored for M.Pharm students, emphasizing the rationale behind TDM, pharmacokinetic and pharmacodynamic principles, indications, sampling strategies, interpretation challenges, and real-world applications. Questions explore steady-state concepts, therapeutic index, free versus total drug levels, assay limitations, and special populations (renal/hepatic impairment, pediatrics, pregnancy). These items are designed to deepen understanding, promote critical thinking for dose individualization, and prepare students for both exams and clinical practice in therapeutic drug monitoring.

Q1. What is the primary clinical rationale for performing therapeutic drug monitoring (TDM)?

• To measure drug metabolites for pharmacogenetic studies
• To optimize individual patient’s drug exposure to improve efficacy and reduce toxicity
• To replace clinical assessment in therapeutic decision-making
• To routinely monitor all prescribed medications

Correct Answer: To optimize individual patient’s drug exposure to improve efficacy and reduce toxicity

Q2. Which pharmacokinetic parameter is most directly estimated by trough concentration used in TDM?

• Volume of distribution (Vd)
• Clearance (CL)
• Absorption rate constant (Ka)
• Bioavailability (F)

Correct Answer: Clearance (CL)

Q3. For a drug with a narrow therapeutic index, which statement best supports the use of TDM?

• Therapeutic effect is predictable from dose alone
• Small concentration changes can lead to ineffective therapy or toxicity
• Wide interpatient variability is negligible
• Therapeutic range is extremely broad

Correct Answer: Small concentration changes can lead to ineffective therapy or toxicity

Q4. Which drug property least favors the need for routine TDM?

• Low interpatient pharmacokinetic variability
• Narrow therapeutic index
• Poor correlation between dose and effect
• Long-term therapy with potential accumulation

Correct Answer: Low interpatient pharmacokinetic variability

Q5. When is the ideal time to collect a trough sample for most once-daily oral drugs in TDM?

• At peak concentration, 1–2 hours after dosing
• Immediately before the next scheduled dose
• Any time during the dosing interval
• 12 hours after dosing regardless of interval

Correct Answer: Immediately before the next scheduled dose

Q6. Which of the following drugs is commonly monitored using TDM due to narrow therapeutic range and interindividual variability?

• Amoxicillin
• Gentamicin
• Ibuprofen
• Ranitidine

Correct Answer: Gentamicin

Q7. In patients with hypoalbuminemia, which concentration measurement is more informative for highly protein-bound drugs?

• Total plasma concentration
• Urinary concentration
• Free (unbound) plasma concentration
• Red blood cell concentration

Correct Answer: Free (unbound) plasma concentration

Q8. Which statement best describes steady-state in the context of TDM?

• It is achieved immediately after the first dose
• Average concentration over dosing interval equals dose divided by clearance at steady-state
• Peak concentrations are independent of dosing interval at steady-state
• Steady-state is irrelevant for drugs with linear kinetics

Correct Answer: Average concentration over dosing interval equals dose divided by clearance at steady-state

Q9. Bayesian forecasting in TDM is used primarily to:

• Assay drug metabolites directly from urine
• Combine prior population pharmacokinetic data with individual concentrations to individualize dosing
• Estimate the bioavailability from a single concentration
• Replace the need for clinical monitoring of efficacy

Correct Answer: Combine prior population pharmacokinetic data with individual concentrations to individualize dosing

Q10. Which factor is least likely to cause variability in measured drug concentrations between patients?

• Genetic polymorphisms affecting metabolizing enzymes
• Drug–drug interactions altering clearance
• Consistent adherence to dosing schedule
• Renal or hepatic impairment

Correct Answer: Consistent adherence to dosing schedule

Q11. The therapeutic range for a drug is best defined as:

• The single concentration that guarantees therapeutic success
• The plasma concentration range associated with desired effect and acceptable toxicity risk
• The concentration below which side effects always occur
• The concentration measured only at peak

Correct Answer: The plasma concentration range associated with desired effect and acceptable toxicity risk

Q12. Which analytical assay characteristic is most critical for reliable TDM results?

• High limit of detection only
• High specificity and accuracy at clinically relevant concentrations
• Use of colorimetric reagents irrespective of interference
• Assay speed over accuracy

Correct Answer: High specificity and accuracy at clinically relevant concentrations

Q13. In a patient on phenytoin, which non-linear pharmacokinetic property complicates TDM interpretation?

• Zero-order (saturable) metabolism at therapeutic concentrations
• Renal excretion as unchanged drug only
• Constant fraction eliminated per unit time (first-order)
• Complete lack of protein binding

Correct Answer: Zero-order (saturable) metabolism at therapeutic concentrations

Q14. Which clinical scenario is a recommended indication for initiating TDM?

• Monitoring a drug with wide therapeutic index and predictable kinetics
• Suspected nonadherence or unexpected toxicity with a drug having variable kinetics
• Routine measurement of analgesics after a single dose
• When drug assays are unavailable

Correct Answer: Suspected nonadherence or unexpected toxicity with a drug having variable kinetics

Q15. For aminoglycosides, which TDM strategy best correlates with efficacy and reduced toxicity?

• Monitoring peak concentrations to predict nephrotoxicity
• Monitoring trough concentrations to minimize toxicity and ensure clearance between doses
• Measuring only total weekly exposure without timing
• Using single random concentrations

Correct Answer: Monitoring trough concentrations to minimize toxicity and ensure clearance between doses

Q16. When interpreting a single TDM concentration, which additional information is most important to collect?

• Patient’s favorite food
• Exact time of last dose, sample time, dosing history, and renal/hepatic function
• Weather at time of sampling
• Manufacturer of the drug only

Correct Answer: Exact time of last dose, sample time, dosing history, and renal/hepatic function

Q17. Why is free drug concentration sometimes preferred over total concentration for TDM of highly protein-bound drugs?

• Free drug does not correlate with pharmacologic effect
• Only free drug is pharmacologically active and available for clearance
• Total concentration is always lower than free concentration
• Protein-bound drug crosses membranes easily

Correct Answer: Only free drug is pharmacologically active and available for clearance

Q18. Which limitation is commonly encountered with immunoassays used in TDM?

• They always require mass spectrometry confirmation
• Potential cross-reactivity with metabolites or structurally similar compounds
• They are incapable of high throughput
• They provide inaccurate results only at very high concentrations

Correct Answer: Potential cross-reactivity with metabolites or structurally similar compounds

Q19. Therapeutic drug monitoring is most cost-effective when:

• Applied universally to all medications regardless of need
• Targeted to drugs with narrow therapeutic index, significant PK variability, and clear concentration–effect relationships
• Used only for investigational drugs in trials
• Performed without clinical interpretation

Correct Answer: Targeted to drugs with narrow therapeutic index, significant PK variability, and clear concentration–effect relationships

Q20. Which practice improves the clinical utility of TDM results?

• Reporting concentrations without reference ranges or interpretation
• Integrating concentration data with clinical context, PK principles, and dosing recommendations
• Delaying sample collection arbitrarily
• Using only population means to dose every patient

Correct Answer: Integrating concentration data with clinical context, PK principles, and dosing recommendations

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