Evaluating the Appropriateness of Digoxin Level Monitoring

Abstract

Background Digoxin level determinations can be useful clinically in patients receiving digoxin therapy but are sometimes misused.

Methods Explicit appropriateness criteria were adapted from previously published criteria and revised using local expert opinion. They were then used to evaluate the appropriateness of random samples of inpatient and outpatient serum digoxin levels. Overall agreement between reviewers regarding appropriateness was good (κ=0.65). Patients in the study included 162 inpatients in whom 224 digoxin levels were measured and 117 outpatients in whom 130 digoxin levels were measured during a 6-month period. The main outcome measure was the proportion of digoxin levels with an appropriate indication.

Results Among inpatient levels, only 16% (95% confidence intervals [CI], 11%-20%) were appropriate. Of the 189 digoxin levels considered inappropriate, only 26 (14%) had a result of 2.3 nmol/L or more (≥1.8 ng/mL). None of these levels resulted in an important change in therapy, and no patient had a toxic reaction to the therapy. Among inappropriate levels, daily routine monitoring accounted for 78%. Of the 130 outpatient levels, 52% (95% CI, 44%-61%) were appropriate. Of 62 inappropriate levels, only 4 (6%) had a result of 2.3 nmol/L or more (≥1.8 ng/mL). One result led to a change in therapy, but none of the patients were believed to experience a toxic reaction. Among the inappropriate levels, 87% of patients underwent early routine monitoring before a steady state was achieved.

Conclusions A high proportion of digoxin levels were inappropriate, particularly among inpatients. In both groups, the primary reason tests were judged inappropriate was early routine monitoring. Few inappropriate tests resulted in important data. Interventions to improve the use of digoxin levels could potentially save substantial resources without missing important clinical results.

MANY FACTORS contribute to the rising costs of health care. These rising costs have led to attempts to rethink the ways services are provided and identify services that could be omitted, while maintaining high-quality care. While diagnostic tests are relatively inexpensive individually, they are costly in the aggregate and contribute significantly to the rising expenses of the health care system.1-3 At our institution, digoxin level monitoring accounts for 21% of all therapeutic drug concentration measurements.

Digoxin level determinations can be clinically useful in patients receiving digoxin therapy for a variety of reasons, including evaluating compliance and verifying toxicity of the therapy.4-6 While many studies have evaluated the use of digoxin therapeutic drug–monitoring services and sampling for drug monitoring of digoxin, few have evaluated the proportion of digoxin level determinations with appropriate indications.7-10

Since we wanted to measure the appropriateness of serum digoxin levels in our institution, with the aim of developing interventions to improve it, we undertook a study with the following goals: (1) to develop explicit criteria that define appropriate indications for measuring digoxin levels in both inpatients and outpatients; (2) to use these criteria to evaluate the proportion of digoxin determinations with an inappropriate indication; (3) to describe the indications for ordering determinations of drug levels; and (4) to determine how often levels with an inappropriate indication generated clinically important results.