Evaluation of the sensitivity of commercially available digoxin radioimmunoassay kits

We have evaluated the technical minimum detection limits of 12 different commercially available kits and the practical minimum detection limits of 11 kits used for the determination of the serum digoxin level by radioimmunoassay. The poor technical ability of many of the kits to distinguish between digoxin levels of 0.1 ng/ml intervals between 0 and 0.5 ng/ml and, additionally, the detection of a digoxin level up to 0.3 ng/ml with some of the kits in glycoside-free sera impose a practical working minimum detection limit of 0.5 ng/ml for the majority of these kits. We, therefore, recommend caution with interpreting or evaluating serum digoxin levels below 0.5 ng/ml.