The analytic sensitivity of the drug monitoring method must be adequate. For some drugs, plasma levels are in the nanogram per milliliter range. Example: Tricyclic antidepressants, digoxin.
The specificity of the method must be known, because the drug’s metabolites or other drugs may interfere. Interference by metabolites, which may or may not be pharmacologically active, is of particular concern in immunologic assay methods using antibodies to the parent drug.
The precision of the method must be known to assess whether changes in levels are caused by method imprecision or by clinical changes.
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