Therapeutic Drug Monitoring

Question 1

When to conduct TDM
- Step by step process

Answer 1

1. Can the drug be measured in the desired biological matrix
2. Is there a good relationship between drug concentration (PK) and drug response (PD)
3. Does the drug have a narrow therapeutic range
4. Is the drugs pharmacological response easily accessible
5. Are PK variabilities unpredictable (wide-inter individual variation)
6. Is the duration of drug therapy sufficient to warrant drug monitoring

Question 2

Example of drugs where TDM is useful

Answer 2

Vancomycin
Gentamycin

Question 3

Extended Interval Dosing
- Benefits

Answer 3

• Less toxic than conventional dosing
• Takes advantage of concentration killing (optimizes Cmax/MIC)
• Cost savings

However, has contraindications

Question 4

Conventional Dosing
- Benefits

Answer 4

• Used in patients with contraindications to extended interval dosing
• Synergy Dosing
  (Gram+ Vancomycin)
  (Aerobic Gram- Beta-lactams)

Question 5

Contraindications to Extended Interval Dosing

Answer 5

1. Patients with renal dysfunction
2. Patients on dialysis
3. Aminoglycosides being used for synergy
4. Abnormal clearance (Burn patients > 20% surface area)
5. Endocarditis
6. Surgical Prophylaxis

Question 6

Conventional Dosing
- Graph

Answer 6

More doses throughout the day
- Has peaks and troughs

Question 7

Extended Interval Dosing
- Graph

Answer 7

One high dose (based on weight) that is given every 24 hours

Question 8

Calculating One Compartment Intermittent IV Infusion Problems
- Steps

Answer 8

1. Draw a picture
2. Calculate k using patient’s individualized data
3. Determine half life
4. Determine Vd
5. Determine dosing interval
   - What is the new Cmax and Cmin, how many half lives does it take to go from Cmax to Cmin
6. Calculate maintenance dose using Cpeak
7. Round dose up or down, recalculate estimated Cpeak
8. Estimate trough concentration

Question 9

How to calculate Estimated Cpk

Answer 9

Desired Cpk * (Actual Dose / Calculated Dose)

Question 10

How to calculated Estimated Ctrough

Answer 10

Cpk * e^-KT

Question 11

Calculating T

Answer 11

How long after infusion was peak concentration drawn

Question 12

Calculating t

Answer 12

How long is the infusion

Question 13

Calculating tau

Answer 13

What is the dosing interval

Question 14

Draw Graph
Calculate k
Calculate half life
Calculate Vd (T, t, tau)
Determine how many half lives
Calculate New Dose (Use new infusions and new tau)
Calculate Estimated Cpk
Calculate Estimated Cthrough