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Test Name

Digoxin (Free)

Alternate Name/ Synonym

Lanoxin (Free)

Test Code


Drug Class



Therapeutic Drug Monitoring - TDM


Abbott Architect ci4100

External Proficiency Testing


Turn Around Time

Daily (throughout the day)


Cardiac Glycoside

Clinical Significance

Refer to the Digoxin (Total) for a complete description of the clinical significance of Digoxin.

In patients receiving either Fab antibodies, such as Digibind, for treatment of digoxin toxicity or in patients with digoxin-like immunoreactive factors (DLIFs), such as neonates, patients with renal or liver disease, and women in the third trimester of pregnancy being treated with digoxin.

Digoxin-like factors are strongly bound by protein, and measurement of digoxin in the ultrafiltrate provides the free digoxin concentration and separates it from the cross-reactive DLIFs. In this way, the physician can evaluate the patient's digoxin status even in the presence of Digibind or DLIFs. By multiplying the conventional therapeutic range of 1.0 to 2.5 nmol/L by 0.8, one arrives at a therapeutic range for free digoxin of 0.8 to 2.0 nmol/L.

Finally, patients receiving Digibind will have very high plasma or serum concentrations of digoxin when measured by most methods. This is because the Fab antibody Digibind draws digoxin out of skeletal muscle and heart tissue. This Fab-bound digoxin is not pharmacologically active digoxin, and for this reason measurement of free digoxin in the ultrafiltrate is strongly recommended.

Specimen Type

Serum, Plasma (Heparin or Citrate)

Minimum Specimen Requirements

500 uL

Optimal Collection Time

Trough: 0-30 minutes prior to doseor 
Post: minimum 6 - 8 hrs after the dose


4°C (transport with a cool pack if possible)

Approval is not required

CPT Codes


Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Phone: 416-813-7200
Phone: 1-855-381-3212

Therapeutic Range

0.8 – 2.0 nmol/L

Critical Values

> 3.5 nmol/L

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