TY - JOUR
T1 - Evidence-based implementation of free phenytoin therapeutic drug monitoring
AU - Burt, M.
AU - Anderson, D. C.
AU - Kloss, J.
AU - Apple, F. S.
PY - 2000
Y1 - 2000
N2 - Background: The majority of laboratories measure total phenytoin concentration for therapeutic drug monitoring. However, there are substantial interindividual variations in free phenytoin concentrations, the pharmacologically active component. Methods: We describe the process and data used to implement monitoring of free phenytoin only in an urban medical center. Over a 6-week period, total and free phenytoin concentrations were measured, clinical charts reviewed, and indications for alterations in the percentage of free phenytoin fraction were determined. Results: Of the 189 phenytoin requests from 139 patients, 136 data points were analyzed. Free phenytoin concentrations were 6.8-35.3%, with 50% outside the expected range of 8-12%. Clinical indications likely responsible for variations were hypoalbuminemia, drug interactions, uremia, pregnancy, and age. Overall, 30% of patients demonstrated a discrepancy between therapeutic, subtherapeutic, or supratherapeutic concentrations between free and total phenytoin concentrations. The largest discordance (53%) occurred in the patient group with free phenytoin <8% or >12%. Coclusions: This study supports previous clinical findings that monitoring total phenytoin is not as reliable as free phenytoin as a clinical indicator for therapeutic and nontherapeutic concentrations. Thus, we recommend that therapeutic monitoring should use free phenytoin concentrations only. (C) 2000 American Association for Clinical Chemistry.
AB - Background: The majority of laboratories measure total phenytoin concentration for therapeutic drug monitoring. However, there are substantial interindividual variations in free phenytoin concentrations, the pharmacologically active component. Methods: We describe the process and data used to implement monitoring of free phenytoin only in an urban medical center. Over a 6-week period, total and free phenytoin concentrations were measured, clinical charts reviewed, and indications for alterations in the percentage of free phenytoin fraction were determined. Results: Of the 189 phenytoin requests from 139 patients, 136 data points were analyzed. Free phenytoin concentrations were 6.8-35.3%, with 50% outside the expected range of 8-12%. Clinical indications likely responsible for variations were hypoalbuminemia, drug interactions, uremia, pregnancy, and age. Overall, 30% of patients demonstrated a discrepancy between therapeutic, subtherapeutic, or supratherapeutic concentrations between free and total phenytoin concentrations. The largest discordance (53%) occurred in the patient group with free phenytoin <8% or >12%. Coclusions: This study supports previous clinical findings that monitoring total phenytoin is not as reliable as free phenytoin as a clinical indicator for therapeutic and nontherapeutic concentrations. Thus, we recommend that therapeutic monitoring should use free phenytoin concentrations only. (C) 2000 American Association for Clinical Chemistry.
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U2 - 10.1093/clinchem/46.8.1132
DO - 10.1093/clinchem/46.8.1132
M3 - Article
C2 - 10926893
AN - SCOPUS:0033871187
SN - 0009-9147
VL - 46
SP - 1132
EP - 1135
JO - Clinical chemistry
JF - Clinical chemistry
IS - 8 I
ER -