Therapeutic Drug Monitoring

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Therapeutic drug monitoring can provide information useful in tailoring drug therapy to the individual patient. A clinical need for TDM may arise from a lack of therapeutic response, suspected drug toxicity, or the desire to confirm a therapeutic approach.

Diagnostic Services can arrange for analyses of serum or plasma concentrations of digoxin, gentamicin, phenobarbital, potassium bromide, procainamide, quinidine, theophylline and other therapeutic drugs along with interpretation of results by the clinical pharmacologist. Advance notification is required if test results are urgently needed.

Laboratory Requisition Forms

A Therapeutic Drug Monitoring Requisition Form is available from Diagnostic Services. When submitting samples for analysis, all information requested on the form should be provided. This is particularly true when requesting a consultation.

Assay Schedule

TDM test samples should arrive at Diagnostic Services before 10 AM on the day the assay is to be run.

1. Digixon Referred – Daily
2. Gentamicin Referred – Daily
3. Phenobarbital Daily
4. Potassium Bromide…………Tuesday and Friday
5. Procainamide Referred
6. Keppra (Levitiracetam) …… Referred
7. Zonisamide Referred

Other therapeutic drugs can be measured by special request with Diagnostic Services.

* General Collection – Red-top vacutainer, spin down, and put in glass tube.

** Note: Do not use serum separator tubes for therapeutic drug monitoring. The silicone gel can bind drugs and cause artificial decreases in concentrations.

Digoxin

Therapeutic Range

Serum or plasma concentrations, established for steady state trough samples: Canine. 1.2 – 3.8 nmol/L

• Feline……..………… 1.2 – 2.6 nmol/L
• Equine……………… 0.6 – 2.6 nmol/L

Specimen Collection and Handling

1. Collect the sample immediately prior to the next dose if dosing BID. (If dosing SID, especially in cats, collect the sample 10 – 12 hours post digoxin administration).
2. Collect blood by venipuncture into a plain, red-topped tube, separate serum from cells. The required volume of serum is 0.5 ml.
3. Store and transport in glass containers only (cardiac glycosides absorbs to plastic).
4. Transport samples to the laboratory on ice; if a delay of greater than 24 hours is anticipated between sample collection and arrival at Diagnostic Services, the sample should be shipped frozen.

Gentamicin

Measurement of both peak and trough serum or plasma concentrations of gentamicin provides better assessment of drug efficacy and nephrotoxic potential.

Therapeutic Range

Serum or plasma concentrations:

Toxic Concentrations

Serum or plasma concentrations:

Specimen Collection and Handling

1. Collect blood sample 1 hour after IM or SQ dosing for peak determination or just before the next dose for trough determination.
2. Collect blood by venipuncture into plain, red-topped or heparinized tube. Separate serum or plasma from the cells and place into a plastic tube. The required volume of serum is 0.5 ml.
3. Store and transport in plastic containers only (gentamicin absorbs to glass).
4. Transport samples to the laboratory on ice; if a delay of greater than 24 hours is anticipated between sample collection and arrival at Diagnostic Services, the sample should be shipped frozen.

Phenobarbitol

The sampling time in relation to drug administration depends upon the therapeutic considerations. If seizures are not being controlled in the patient, trough sampling is best. If toxicity is suspected, peak sampling (4 to 6 hours after drug administration) is best. Alternatively, peak and trough sampling will allow for estimation of serum or plasma half-life and will facilitate the calculation of an adjusted dose of phenobarbital for the patient. For best assessment of steady state serum or plasma concentrations, at least two weeks should have elapsed since the last adjustment in the dosage regimen.

Therapeutic Range

Serum or plasma concentrations:

• Canine…………..……. 54 – 190 μmol/L
• Feline…………..…….. 65 – 130 μmol/L

Specimen Collection and Handling

1. Select the desired dosage regimen and administer the drug until steady state is reached (approximately 16 days in canines).
2. Collect blood by venipuncture into plain, red-topped tube; separate serum from cells. The required volume of serum is 0.5 ml.
3. Store and transport in glass or plastic tube
4. Transport samples to the laboratory on ice; if a delay of greater than 24 hours is anticipated between sample collection and arrival at Diagnostic Services, the sample should be shipped frozen.

Potassium Bromide

Therapeutic Range

Serum or plasma concentrations:

• Monotherapy……………… 12.5 – 37.5 mmol/L
• With Phenobarbital… 12.5 – 31.3 mmol/L

Sample Collection and Handling

1. Collect a sample at any time during the day after the patient reaches steady state (~ 3 months).
2. Collect blood by venipuncture into a plain, red-topped tube. Separate serum from the cells ( at least 1 ml serum is required).
3. Store and transport in glass or plastic tube.
4. Transport samples to the laboratory on ice; if a delay of greater than 24 hours is anticipated between sample collection and arrival at Diagnostic Services, the sample should be shipped frozen.
5. Avoid hemolysis and/ or lipemia as it interferes with the assay

Procainamide

Therapeutic Range

Serum or plasma concentrations:

• Canine……………………… 85 – 125 μmol/L

Toxic Concentrations

Serum or plasma concentrations:

• Canine….………………….. > 125 μmol/L

Sample Collection and Handling

1. Collect the sample immediately prior to the next dose for trough serum concentration.
2. Collect blood by venipuncture into plain, red-topped tube. Separate serum from cells. The required volume of serum is 0.5 ml.
3. Transport samples to the laboratory on ice; if a delay of greater than 24 hours is anticipated between sample collection and arrival at Diagnostic Services, the sample should be shipped frozen.

Keppra® – Levetiracetam

Minimum Sample Required: 0.5ml Serum or Plasma (heparinized). Do not use serum separator tubes. Fasting recommended but not required. No ice needed.

Steady-state” never truly occurs for levetiracetam in dogs (and may not in cats) because the drug does not accumulate with each dose. Because minimal accumulation occurs, concentrations can be measured within days to 1 week of initiating a new dosing regimen.

Recommend a trough sample: Just prior to next dose (8hrs post on 8hr dosing). (Dr. McConkey, Pharmacologist, AVC).

If peak sampling is also wanted collect the peak sample: 2-4hrs post dose. Diagnostic Services will forward a submission form from Auburn University to include with your submission.

Zonisamide

Minimum Sample Required: 0.5ml Serum or Plasma (no serum separator tubes). Fasting recommended but not required. No ice needed.

Recommend trough sample: Just prior to next dose (12hrs post on 12hr dosing). (Dr. McConkey, Pharmacologist, AVC). Testing 14 days after starting treatment or following a dosage change is suggested. Testing annually after the first test is sufficient, unless there is a problem.

If peak sampling is also wanted collect the peak sample: 3 hrs post dose. Diagnostic Services will forward a submission form from Auburn University to include with your submission.