A digoxin test is used to monitor the concentration of the drug in the blood. The dose of digoxin prescribed may be adjusted depending on the level measured. A healthcare practitioner may order one or more digoxin tests when a person begins treatment to determine if the initial dosage is within therapeutic range and then order it at regular intervals to ensure that the therapeutic level is maintained. A digoxin test may also be used to determine if someone's symptoms are due to an insufficient amount of the drug or to digoxin toxicity.

A healthcare practitioner will order the test to measure digoxin at the beginning of drug therapy to ensure correct dosage. Digoxin takes approximately one to two weeks to reach a steady level in the blood and in the target organ, the heart. A test done at that time will reflect more accurately whether a person is receiving the right amount of digoxin.

Once the dosage level is determined, routine monitoring of digoxin levels, at a frequency determined by the healthcare practitioner, will verify correct dosage.

A digoxin test may be ordered when it is suspected that levels are too low in someone who is taking the medication and has symptoms of heart failure, such as:

• Fatigue
• Shortness of breath
• Swelling in the hands and feet (edema)

The test may be ordered when toxicity is suspected and the affected person has signs and symptoms such as:

• Dizziness
• Blurred vision or seeing yellow or green halos
• Vomiting
• Diarrhea
• Irregular heartbeat
• Difficulty breathing
• Loss of appetite

Changes in health status can affect levels of digoxin and its ability to control symptoms. Digoxin tests may be done, and the dose adjusted if necessary, when someone experiences a physiologic change that may affect blood levels and effectiveness of digoxin, for example, kidney or thyroid problems, cancer, or stomach or intestinal illness.

For congestive heart failure, the ideal range of levels of digoxin in the blood, known as the therapeutic range, may be between 0.5 and 0.8 ng/mL. Each person's response to medications is different. Many factors, including kidney function and concurrent medications, may be involved. If someone's symptoms do not improve or if the person is experiencing side effects, then the healthcare provider may need to adjust the digoxin dose up or down according to that person's needs.

When prescribed digoxin, you should discuss with your healthcare provider and pharmacist all other prescription and over-the-counter medications, dietary supplements, and herbal remedies you are taking. These can affect the level of digoxin in the blood and its effectiveness. Be sure to notify your healthcare provider about any changes in use of these products while you are taking digoxin.

Prescription drugs that can interact with digoxin include: quinidine, flecainide, verapamil, amiodarone, amiodarone, azole antifungals (such as itraconazole, ketoconazole), cyclosporine, lapatinib, macrolide antibiotics (such as clarithromycin, erythromycin), propafenone, ranolazine, rifampin, and ciprofloxacin. Herbal remedies such as St. John's wort, oleander, and lily of the valley may affect levels of digoxin in the blood. Eating licorice may also affect blood levels of the drug.

Digoxin is primarily cleared from the system by the kidneys. When someone has kidney problems, the person's healthcare provider may want to monitor kidney function and blood potassium levels since kidney dysfunction and low levels of potassium can result in symptoms of digoxin toxicity.

Digoxin toxicity can be aggravated by potassium and magnesium levels, so a healthcare provider may monitor electrolytes and other ions like magnesium as well.

In cases where toxic levels of digoxin are found, a healthcare practitioner may administer a specialized antidote (digoxin immune FAB) to reverse the effects of the digoxin.