To the Editor: The recent review of digoxin by Campbell and MacDonald1 pointed out that a serum digoxin level of over 1.0 ng/mL is associated with excess mortality. Indeed, higher blood levels (≥ 1.2 ng/mL) are associated with higher crude rates for all-cause hospitalisation, and for increased hospitalisation for worsening heart failure or suspected digoxin toxicity.2

The post hoc analysis of the DIG trial2 suggests that the optimal range is 0.5–0.8 ng/mL.

I recently surveyed 31 private pathology laboratories across Australia to determine their recommendations about the therapeutic range of serum levels of digoxin. In summary, their recommendations ranged from a lower limit between 0.5 ng/mL and 1.0 ng/mL, and an upper limit between 1.6 ng/mL and 2.1 ng/mL. Twenty-six of the 31 suggested that values below 0.8 ng/mL were subtherapeutic. It is likely that many doctors will heed such advice and inappropriately increase the dose of digoxin in patients being treated for heart failure.

It is possible that adverse effects will flow from current laboratory industry recommendations, and these should be revised.