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Serum levels of mood stabilizers: When to measure
Why does the time since last dose even matter?
There is a certain minimum amount of time that must elapse between the last dose of the medication and drawing a blood sample to measure the serum level. If the blood is drawn too early, will the level be higher or lower than the true level? Every time a person takes a medication, the serum levels gradually increase to a peak and then fall. However, because the person continues to take the medication before the levels fall all the way, there is always medication in the blood. This lower level is the “trough level” (as opposed to the peak level). The efficacy of medication is related to the trough level. The peak level is related to the likelihood of many side effects.
OK, so how long after the last dose?
Lithium: The prescribing information for sustained-release lithium carbonate (Eskalith CR) states that, “Blood samples for serum lithium determinations should be drawn … when lithium concentrations are relatively stable (i.e., 8 to 12 hours after the previous dose).”
Divalproex: While about 12 hours after the last dose was appropriate for divalproex (brand name Depakote), for the extended-release formulation (Depakote ER) we should ask the patient to wait for at least 15 hours after the last dose in order to reach the trough level (Reed and Dutta, 2006)
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Copyright 2015, Rajnish Mago, MD. All rights reserved. May not be reproduced in any form without express written permission.
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