Digoxin

Chemical Pathology

Notes

Digoxin is a cardiac glycoside used for controlling ventricular response in persistent and permanent atrial fibrillation and atrial flutter, but also for treatment of heart failure, being reserved for patients with worsening or severe heart failure due to LV systolic dysfunction who remain symptomatic despite other medication.

Digoxin levels are not routinely monitored but can be helpful when:

• poor compliance is suspected
• response to treatment is poor
• renal function is fluctuating
• drugs that interact with digoxin are co-prescribed
• confirmation of clinical toxicity is required (symptoms include confusion, anorexia, nausea, vomiting, disturbance of colour vision, arrhythmia but note that clinical presentation can be very variable and not always preceded by nausea and anorexia. For healthcare professionals, please access TOXBASE in cases of suspected overdose).

Sample requirements

For adults, 5 ml of blood taken into a narrow gold top tube (or rust top for the Acute Unit) (see below for collection time requirements)

• Renal impairment and hypokalaemia are two of the most important factors affecting digoxin dosage hence sodium, potassium, magnesium and creatinine will also be assayed when digoxin levels are requested (see notes below)

If patient on high dose biotin therapy (>5mg/day) collect sample at least 8 hours after the last dose.

Digifab interferes with digoxin immunoassays so there is no clinical benefit in checking digoxin levels after administration.

For children, blood collected into a 3.5mL rust top tube

Sampling times

• Samples should be collected at least 6 hours after the last dose.
• A steady state should have been achieved before sampling. When no loading dose has been given this may take 7 days in patients with normal renal function and 14 days for elderly patients.
• In patients with advanced kidney disease or on haemodialysis, samples for digoxin should be taken 12 to 24 hours after the prior dose. Samples collected before these times may be falsely elevated.

Storage/transport

Do not store. Send immediately to the laboratory at ambient temperature.

Required information

Relevant clinical details including time post dose and other drug history.

Turnaround times

The assays are run throughout the day and night.

The in-lab turnaround time is normally less than 24 hours.

The test can be ordered as an urgent request.

Reference ranges and therapeutic guidelines

Interpretation of results should be done in the context of the overall clinical status of the patient.

Digoxin levels are reported in µg/L. To convert from µg/L to nmol/L multiply by 1.28.

• Approximately 90% adult patients with proven digoxin toxicity have serum digoxin levels greater than 2.0 µg/L if the sample was collected at least 6 hours after the last dose.
• Target range in heart failure is 0.5 to1.0 µg/L. Target range in AF is 0.5 - 2.0 ug/L.

Digoxin levels should not be interpreted without considering the factors listed below and never if sample collected too early:

• Patients may exhibit signs of toxicity above 2.0 µg/L but the incidence of toxicity increases markedly above 3.0 µg/L
• For children older than 12 months adult guidelines can usually be followed. For younger children the trend for increased risk of toxicity at increased plasma digoxin concentrations appears to hold but the threshold for toxicity may be higher, especially in children less than 3 months old.
• Hypokalaemia significantly increases sensitivity to digoxin and digoxin levels less than 2.0 µg/L may be toxic if the patient is hypokalaemic e.g. a reduction of serum potassium from 3.5 to 3.0 mmol/L can increase tissue sensitivity by 50%.
• Hypercalcaemia and hypomagnesaemia can increase tissue sensitivity to digoxin.
• Hyperkalaemia, hypocalcaemia and hypermagnesaemia decrease tissue sensitivity to digoxin.
• Lower digoxin doses may be required in hypothyroidism and increased doses in hyperthyroidism.
• Always consider potential interaction from other medications. Significant drug interactions can occur with many drugs, some requiring adjustment of digoxin dose. Please refer to latest BNF or contact Medicines Information service (Tel GRH 0300 422 6108 or CGH 0300 422 3030).
• Falsely elevated digoxin levels may be seen in certain patients who have digoxin-like substances in their serum (patients with renal or hepatic dysfunction, pregnant women and neonates).

Further information

BNF

Lab Tests Online

Page last updated 15/05/2017