To convert digoxin to nanomoles per liter, multiply by 1.281.
eSupplement. Digoxin survey
Hauptman PJ, McCann P, Romero JMR, Mayo M. Reference Laboratory Values for Digoxin Following Publication of Digitalis
Investigation Group (DIG) Trial Data. JAMA Intern Med. 2013;173(16):1552-1554. doi:10.1001/jamainternmed.2013.7756
The translation of new findings into clinical practice is an ongoing challenge for physicians and health systems. The definition of a reference range for serum digoxin concentration (SDC) in patients with heart failure provides an example in which published data have not been incorporated into laboratory practice, which as a result may have an adverse impact on clinical care.
Specifically, in a post hoc analysis from the Digitalis Investigation Group (DIG) heart failure trial, higher mean SDCs were associated with increased mortality; the optimal therapeutic range for clinical benefit among men with a left ventricular ejection fraction of less than 45% was 0.5 to 0.8 ng/mL.1 A second analysis indicated that SDCs of 1.2 ng/mL or higher may be harmful in women.2 (To convert digoxin to nanomoles per liter, multiply by 1.281.) In light of these studies, we sought to determine the current practice of reporting SDCs in hospital-based chemical laboratory analyses.
A brief written survey (with telephone follow-up) (eSupplement) was sent to chemistry laboratory directors at hospitals listed in the top 50 for cardiovascular medicine reported by US News and World Report3 and an additional 50 from the top 100 hospitals rated by Thomson-Reuters (now Truven Health Analytics)4 in 2012. The study was approved by the Saint Louis University institutional review board, St Louis, Missouri.
A total of 60 surveys were completed and returned for analysis (a 60% response rate). Respondents were 27 laboratory directors or assistant directors, 21 supervisors, 11 technicians, and 1 laboratory medicine fellow. Five different commercial assays were used; in the year prior to the survey, 5 laboratories changed their commercial assay citing upgrades in equipment or laboratory processes. No laboratory reported a change in the SDC reference range.
Most respondents defined a therapeutic reference range as 0.8 to 2.0 ng/mL (Figure 1); 56 of 60 report SDCs of 2.0 ng/mL or greater as being within the normal range.
A total of 41 laboratories reported the mean SDC evaluated over a period of up to 1 year, most commonly over the prior month (18 of 41). Nearly half (19 of 41) reported mean concentrations of 1.0 ng/mL or greater (Figure 2). A subset (33 of 41) reported on the proportion of SDCs higher than various thresholds; a significant number reported levels of 1.5 ng/mL or higher (Figure 2). When asked if SDC correlated with clinical efficacy, most respondents answered “don’t know” or “no” (76%); of the sites that answered in the affirmative (24%), only 1 site used a reference range with an upper limit lower than 1.0 ng/mL, whereas 8 listed a range up to 2 ng/mL, 1 each listed 1.5 ng/mL and 1.0 ng/mL, and 2 respondents did not provide a range.
In an early but influential study that helped establish the therapeutic range for digoxin, Smith et al5 reported digoxin toxicity based on electrocardiographic (ECG) manifestations. In the 0.80 to 2.4 ng/mL range, patients without evidence of ECG changes were considered to have nontoxic levels. Subsequently, data from the DIG trial suggested that SDCs lower than 0.9 ng/mL are associated with therapeutic benefit and values higher than 1.2 ng/mL may be harmful.1,2 This observation has been incorporated into practice guidelines.6
Our survey demonstrates that most laboratories closely adhere to the recommendations of the original study by Smith et al.5 Although our survey is not comprehensive, we selected a sample of hospitals ranked highest in 2 independent surveys of cardiovascular care since other hospitals likely use similar assays and ranges. In addition, the self-reported SDCs were generally higher than 1.0 ng/mL. This may partially reflect a greater propensity on the part of clinicians to check SDCs when there is a clinical suspicion of toxicity. However, although digitalis toxicity can become manifest at therapeutic serum levels and toxicity remains a clinical diagnosis, clinicians may be inappropriately reassured that their current dosing is providing a therapeutic benefit at a safe level given the published reference range.7
Of note, we obtained only a cross-sectional overview of SDCs detected in hospital-based chemical laboratory analyses and have no data on whether the levels were appropriately timed relative to dose. Furthermore, most data correlating concentrations to outcomes were generated from the DIG trial in heart failure, which excluded patients with atrial fibrillation at baseline; however, the 2 conditions often coexist. There is also no empirical evidence to suggest a therapeutic benefit of digoxin in atrial fibrillation at concentrations beyond those established for heart failure.
We previously showed that despite a secular decline in the use of digoxin, admissions for toxicity have not declined in parallel.8 As such, the persistence of a broad reference range for digoxin, a drug with a well-documented narrow therapeutic window, is a cause for concern. Based on our survey findings and a clear evidence base, we recommend the adoption of a redefined reference range for digoxin by chemical laboratory analyses with an upper limit no greater than 0.9 ng/mL and a change in laboratory reporting processes.9
Corresponding Author: Paul J. Hauptman, MD, Department of
Medicine, Saint Louis University Hospital, 3635 Vista Ave, St Louis, MO 63110 (firstname.lastname@example.org).
Published Online: June 24, 2013. doi:10.1001/jamainternmed.2013.7756.
Author Contributions: All the authors had full
access to all the data in the study and take responsibility for the integrity of the data and the
accuracy of the data analysis.
Study concept and design: Hauptman.
Acquisition of data: All authors.
Analysis and interpretation of data: Hauptman and McCann.
Drafting of the manuscript: Hauptman, McCann, and Ramirez Romero.
Critical revision of the manuscript for important intellectual content:
Hauptman, McCann, and Mayo.
Statistical analysis: Hauptman, McCann, and Ramirez Romero.
Administrative, technical, and material support: Hauptman, McCann, and Mayo.
Study supervision: Hauptman.
Conflict of Interest Disclosures: Dr Hauptman
received an honorarium for his participation in a CME event titled “Digoxin Toxicity:
Awareness, Recognition, and Treatment,” sponsored by BTG International Inc (released August
21, 2012, and available at http://theheart.medscape.org/viewarticle/767832 [sign-up required]).
Correction: This article was corrected online on June 26, 2013, to add a text
citation for the eSupplement.