DWR Diagnostics

Therapeutic Drug Monitoring

Phenobarbitone

Sample into a plain serum tube. Separator gel can absorb the drug, so it is important to separate the serum of the gel as soon as possible. Alternatively use a plain tube without a gel separator. However samples are more likely to be haemolysed when a gel separator is not used and this can also affect the results. Moderately to markedly haemolysed samples should not be sent.

Potassium Bromide

Sample into a plain serum tube. Separator gel can absorb the drug, so it is important to separate the serum off the gel as soon as possible. Alternatively use a plain tube without a gel separator. However samples are more likely to be haemolysed when a gel separator is not used and this can also affect the results. Moderately to markedly haemolysed samples should not be sent.

Digoxin

Digoxin pharmacokinetics and dynamics may vary considerably from dog to dog. Time to ‘peak’ serum level is 2-4 hours after oral dosage with time to ‘peak clinical effect’ being 6-8 hours. ‘Trough’ serum levels usually occur after 8-12 hours. The elimination half-life is variably reported between 14 and 56 hours in dogs.

A standard starting dose is 3-4 micrograms/kg p.o. q12hrs for dogs ≤20kg and 0.22mg/m2 body surface area p.o. q12hrs for dogs >20kg. A dose of 250 microgram per dose should not be exceeded as an initial dose in any dog and this drug should be used with caution in breeds pre-disposed to ventricular ectopy such as Dobermann pinschers. Decreased doses or an increase in dosing intervals may be required in geriatric patients, obese animals or those with significant renal dysfunction.

Digoxin levels should be measured in serum without a gel separator 5-7 days after initiation or dose-adjustment. The timing of the sample depends on the requirement to either detect a minimum effective level (in which case a ‘trough’ sample at 8-12 hours post-pill would be sensible) or whether the clinical answer desired relates to potential for toxicity (in which case a ‘peak’ sample at 2-5 hours post-pill would be sensible).

Trough samples of 0.6 – 1.2ng/ml are usually sufficient to document adequate therapeutic digoxinaemia. However most cardiologists will base assessment of adequacy of ventricular rate control in patients with atrial fibrillation on history, clinical findings, an in-clinic ventricular response rate <150bpm, a 24 hour mean ventricular rate determined by Holter monitoring or an at-home ventricular response rate <110bpm.

If toxicity is suspected or risk of toxicity evaluated for then levels exceeding 2.4ng/ml should prompt dosage adjustment.

If you wish to discuss referral of patients with cardiac disease for further assessment please contact one of our cardiologists Jon Wray or  Ruth Willis on tel: 01638 572012

Test Protocols

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