In the overdose setting, clinical parameters are more indicative of toxicity than serum concentrations. There are relatively few drugs for which quantitative measurement is beneficial. Drugs in which serum quantitation may be useful include:
- ethylene glycol
Serum concentrations may be useful to evaluate the need for hemoperfusion (lithium, methanol, ethylene glycol, phenobarb) and the need for antidotes (digibind, desferoxamine).
Drug concentrations need to be interpreted cautiously.
Similar concentrations may cause different clinical pictures for a number of reasons. For instance, patients who ingest carbamazepine therapeutically have a shortened metabolite half-life and also develop tolerance to the effects of the drug. A concentration of 18 mcg/ml may cause coma in a person who is not ingesting carbamazepine therapeutically. In a patient who is ingesting therapeutic carbamazepine, this serum concentration may cause only drowsiness.
Salicylate concentrations also need to be interpreted cautiously. Patients with relatively low concentrations following chronic ingestions may be critically ill. In overdose, salicylate metabolism changes from first order (increased metabolism with increased serum concentration) to zero order (constant amount metabolized per hour). Tissue stores become saturated and there is an increase of free salicylate which causes clinical toxicity.
There are many other examples, but for most drugs, serum concentrations are of no value in the overdose setting (including tricyclic antidepressants).
As always, call the poison center if you have any questions.
Donna Seger, M.D.
Medical Director, Middle Tennessee Poison Center