Solving the problem
The clinical estimate of digitalis toxicity is .50. It has already been determined in your laboratory that with a cutoff point of 2.0 ng/mL the digoxin assay is 90% sensitive and 80% specific for toxicity.
The PV+ is .82, and the PV- is .89. The essay reports 2.5 ng/mL; this is abnormal. At this level, the test offers even higher predictive values.
Therefore, the probability of digitalis toxicity has risen to more than .82; digoxin administration is stopped, and the patient should improve in several days, at which time he will be started on a smaller dose of the drug.