Pathology Handbook

Digoxin

Synonyms

Lanoxin

Clinical Indications

Digoxin inhibits the Na+,K+ ATP (sodium pump) and its main properties are the ability to increase the force and velocity of myocardial contraction. Digoxin is  useful in the control of SVT, especially for slowing the ventricular rate in AF and in associated CCF.

There is wide ranging clinical response  for a given digoxin concentration and routine monitoring in patients on long- term therapy has little relevance unless renal function is changing or interacting drugs (e.g. quinidine or verapamil) are added or withdrawn. Other indications are:

  • On initiating therapy, if poor response, or to guide dose adjustment

  • To confirm toxicity

  • In deciding if continued therapy is necessary - if levels consistently below 1.0 nmol/L, patients are unlikely to suffer adverse effects if drug withdrawn.  

Drug Kinetics

After oral administration there are 2 distinct phases - a distribution phase during which the drug equilibrates between central (liver, kidney) and peripheral (muscle, myocardium) tissues and an elimination phase. The digoxin receptor sites are located in the peripheral compartment, and the high degree of tissue binding means that equilibrium drug concentrations in cardiac tissue are 15-30 times those in serum. Unless adequate time (at least 6 hours after last dose) for equilibration is allowed, serum concentrations will be unrepresentative of tissue concentrations. 

Digoxin elimination is predominately renal and half-life is prolonged in patients with impaired renal function.

Request Form 

Request on ICE.  Please state time of last dose on request form.

Availability

On request during routine hours. Laboratory must be contacted regarding urgent requests.

Specific Criteria

None

Patient Preparation

Sample must be taken at least 6 hours after last dose. If collected earlier, results cannot be interpreted.

Turnaround Time

Daily

Specimen

Serum; SST sample

Volume

2 ml

Container

Vacutainer gold top

Causes for Rejection

Unlabelled or incorrectly labelled sample; collected less than 6 hours after last dose of digoxin

Target Range

Samples collected 6 hrs post-dose:

Cardiac failure:

0.94 - 1.48 µg/L

IAtrial fibrillation

1.48 - 2.03 µg/L

Toxic

>2.03 µg/L

Conversion to SI units: 

µg/L x 1.28 = nmol/L

Half-life 

36 - 48 hours

Toxicity

Symptoms include nausea, vomiting, visual disturbances (especially colour vision) bradycardia, sweating, convulsions and coma.