TDM

Question 1

What is TDM (therapeutic drug monitoring)

Answer 1

Individualisation of drug dose to achieve the desired therapeutic effect

• The application of pharmacology, pharmacokinetics, genetics, pathology and clinical medicine to interpret and use measured drug concentrations
• Adjust the dose to treat the patient not to optimise the assay result

Question 2

Outine the process for TDM starting off with decision to treat the patient to assessment and changing therapy if required

Answer 2

1. Decision to treat
2. Drug and dose selection
3. Administer drug
4. Blood test (phlebotomist)
5. Analysis (laboratory)
6. Assessment* (clinical review, calculations, recommendations)

Samples for drug assay must be properly collected and accurately measured, and the results interpreted in an appropriate manner

7. Changes in therapy (if necessary)
8. Drug and dose selection again if necessary

Question 3

What are some reasons for doing in assay in TDM?

Answer 3

• Toxicity
• Non-compliance / Sub-therapeutic
• Drug interaction
• Disease state
• Monitoring after loading dose or starting therapy

Question 4

What information is required to interpret the assay results (request form)?

Answer 4

• Name, age, gender, weight
• Reason for assay
• Reason for drug
• Dose, time of administration
• Time of blood sample
• Duration of therapy

Other: drugs, clinical issues, biochemistry

Question 5

When to do the assay for TDM?

Answer 5

Do it eight hours after the dose is given (elimination stage)

Question 6

When to do the assay to determine steady-state concentration for TDM?

Answer 6

Usually 4-5 half-lives to get to steady state concentration

• Took 5-6 days to get to steady state in the image attached

Question 7

What are the reasons for measuring drugs in biological fluids? For TDM, pharmacokinetic studies and toxicology

Answer 7

TDM

• quick
• reasonable accuracy and precision
• cost
• several drugs

Pharmacokinetic studies

• high accuracy
• good precision and sensitivity
• one drug ± metabolites
• deriving parameters from data

Toxicology

• overdose
• sports medicine
• consideraiton (speed, sensitivity, selectivity)

Question 8

What are some collection problems for assays?

Answer 8

• Haemolysis
• In vitro metabolism (red blood cells in plasma sample)
• In vitro incompatibility
• Lithium heparin tubes (increased level of lithium)
• Gel tubes (may absorb drugs)

Question 9

What is the therapeutic range for digoxin?

Answer 9

• 0.5 - 1.8 mcg/L

Question 10

What are the standard phrases used in TDM? Explain why these terms are used

A) Aim for the therapeutic range

B) Assays are cheap and easy

C) Free drug concentration is best

D) TDM improves patient care

Answer 10

Aim for the therapeutic range

• Assay must be interpreted in conjunction with review of clinical factors

Assays are cheap and easy

• Require resources for data processing and interpretation
• Assay often wasted (lack of clear indication, inappropriate action)
• Can cause adverse event (invasive procedure)

free drug concentration is best (not done frequently)

• Assumes free (plasma) concentration is proportional to therapeutic effect
• Beware of active metabolites
• Expensive, time consuming

TDM improves patient care

• Decrease adverse effects and increase proportion of patients in therapeutic range