Clinical Chemistry 46: 1132-1135, 2000;
(Clinical Chemistry. 2000;46:1132-1135.)
© 2000 American Association for Clinical Chemistry, Inc.
Evidence-based Implementation of Free Phenytoin Therapeutic Drug Monitoring
Martha Burt1,
David C. Anderson1,
Julie Kloss1 and
Fred S. Applea,1
1
Departments of Laboratory Medicine and Pathology and Neurology, Hennepin County Medical Center, University of Minnesota School of Medicine, Minneapolis, MN 55415.
a Address correspondence to this author at: Hennepin County Medical Center, Clinical Laboratories 812, 701 Park Ave., Minneapolis, MN 55415. Fax 612-904-4229; e-mail fred.apple{at}co.hennepin.mn.us
 |
Abstract
|
|---|
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%.
Conclusions: 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.
|
Introduction
|
|---|
There is increasing interest in measuring free drug concentrations
for a variety of substances, based on the principle that the free drug
is the pharmacologically active component (1). This is
particularly true for phenytoin, which is highly bound to plasma
protein (~90%), producing a very small free fraction. The common
laboratory practice is to measure total phenytoin
concentrations, which assumes that phenytoin protein binding is
relatively constant, so that the concentration of the active unbound
drug can be predicted from the total drug plasma concentration
(2)(3). Despite the high percentage of protein
binding, there can be substantial interindividual variation in free
phenytoin concentrations, ranging from 9% to 25% (4). The
free drug fraction of phenytoin can be altered by various compounds and
clinical situations, including hypoalbuminemia (5), uremia
(6), pregnancy (7), critical illness
(8), HIV infection (9), and administration of
other drugs, including antiepileptic drugs
(4)(10)(11).
The potential for clinically significant discordance between free and
total phenytoin concentrations exists for the above reasons. Several
studies have addressed the correlation between total and free phenytoin
(12)(13), with earlier studies supporting a
strong correlation between total and free phenytoin
(4)(14) and more recent studies advocating free
drug measurements (1)(10)(13).
Kilpatrick et al. (13) examined the association between free
phenytoin and clinical status and found that in all patients the
unbound concentration of phenytoin reflected the clinical status
equally or better than the total phenytoin concentration.
Recognizing that free phenytoin concentrations are the most appropriate
information for clinical use, this report describes the process and
data we used to implement monitoring of free phenytoin only, replacing
monitoring of total phenytoin, at an urban medical center.
|
Materials and Methods
|
|---|
Over a 6-week period, both total and free phenytoin concentrations
were measured on every clinical phenytoin request sent to the
toxicology laboratory at Hennepin County Medical Center, a 400-bed
primary care, urban medical center. Total and free phenytoin
measurements are were performed on the Hitachi 911 using the Cedia
Phenytoin II assay (Boehringer Mannheim Systems). Abbott TDx
free phenytoin calibrators at 0.0, 0.5, 1.0, 2.0, 3.0, and 4.0 mg/L
were used.
Serum or plasma was obtained from whole blood collected in sodium EDTA,
sodium heparin, or lithium heparin tubes. Samples were separated into
total phenytoin and free phenytoin aliquots. Separation of free
phenytoin and protein-bound phenytoin was achieved by
filtering the sample through an anisotropic hydrophilic YMT
ultrafiltration membrane (Millipore Corporation), using centrifugation
at 1000g at 25 °C for 20 min as the driving force for the
ultrafiltration. The temperature of 25 °C was chosen for separating
the free fraction of phenytoin because at 25 °C, 10% exists as the
free fraction, compared with 15% at 37 °C, when dissociation
constants were studied in patient samples (15). Thus, our
medical center practice of using 10% as the reference for free
phenytoin agrees with the literature.
The therapeutic total and free phenytoin concentrations used in our
laboratory are 10–20 mg/L and 1–2 mg/L, respectively (16).
These ranges have been verified using in-house data (data not shown)
and are reported on the laboratory information system patient reports.
Total imprecisions (CVs) for total phenytoin and free phenytoin at 24,
2.3, and 1.1 mg/L were 4.7%, 11%, and 22%, respectively. For
patients whose percentage of free phenytoin was >12% or <8%,
the medical record was reviewed, looking specifically at current
medical conditions, current and recent medications, blood urea
nitrogen, albumin, whether the patient was pregnant, and the patient’s
age.
|
Results
|
|---|
Over the 6-week period, 189 requests were received for phenytoin
therapeutic drug monitoring on 139 patients (84 males, 55 females; age
range, 7–77 years). Fifty-three patient data points were excluded from
the study, leaving 136 patient data points (72%) for analysis. Reasons
for data point exclusion included free phenytoin concentrations below
the detection limit (0.5 mg/L), total phenytoin concentration <3 mg/L,
and sample volume not sufficient for determination of both total and
free phenytoin concentrations. The unbound phenytoin fractions were
6.8–35.3% of the total phenytoin concentrations. Sixty-eight of the
patient data points (50%) had a free phenytoin fraction (FPF) of
8–12%, and 68 (50%) had a FPF outside of this range. Clinical
history was available for 62 of the 68 patient data points for those
patients whose FPF was >12% or <8%. Review of the medical records
showed that 46 (74%) had an identifiable explanation or reason for
this high or low FPF, leaving the remaining 16 (26%) with no
identifiable cause for the abnormal FPF. Table 1
shows the incidence of clinical situations known to affect the
FPF. Many patients had several reasons for increased or low FPF and
were included in multiple categories. Hypoalbuminemia and valproic acid
coadministration were the most common clinical situations associated
with an abnormal FPF. Uremia, critical illness, and age extremes were
also common clinical associations.
The clinical importance of the FPF became obvious when total
phenytoin concentrations were compared with free phenytoin
concentrations. For all patient data points, 56 (41%) had therapeutic
total phenytoin concentrations, 32 (24%) had total phenytoin
concentrations greater than therapeutic, and 48 (35%) had
subtherapeutic total phenytoin concentrations. For the 56 patient data
points with therapeutic total phenytoin concentrations, 5 had free
phenytoin concentrations >2 mg/L and 3 had free phenytoin
concentrations <1 mg/L. For the 32 patient data points with
supratherapeutic total phenytoin concentrations, there were no
subtherapeutic free phenytoin concentrations and 5 with therapeutic
free phenytoin concentrations. The majority of the subtherapeutic total
phenytoin patient data points had therapeutic free phenytoin
concentrations (28 of 48).
The last two columns in Table 2
divide the patient data points into those with an FPF between
8% and 12% (group 1) and those with a FPF <8% or >12%
(group 2). In group 1, 51% had both therapeutic total and free
phenytoin concentrations (35 of 68). Two of 37 patient data points had
subtherapeutic free phenytoin concentrations with therapeutic total
concentrations, and 3 of 19 patient data points had therapeutic free
concentrations with supratherapeutic total concentrations. Patient
data points in group 2 had fewer (19%) therapeutic total and free
concentrations (13 of 68) when compared with group 1. Of the 19 patient
data points with therapeutic total phenytoin concentrations, 5 had
supratherapeutic and 1 had subtherapeutic free concentrations. Group 2
demonstrated a relatively high number of patient data points (28 of 68;
41%) with therapeutic free phenytoin concentrations and subtherapeutic
total phenytoin concentrations. This finding was not observed in group
1.
When the concordance for all patient data points was compared, 95 of
136 (70%) of the free and total phenytoin concentrations were in
agreement; i.e., both the free and the total phenytoin concentrations
were supra-, sub-, or therapeutic. For those patient data points in
group 1, 93% were concordant, whereas those in group 2 had only 47%
concordant, leaving a majority of 53% discordant. These data were
distributed to the hospital staff along with notification of plans to
drop routine measurement of total phenytoin. The change was well
accepted. Only two total phenytoin concentrations have been requested
in the 5 months since the change was made.
|
Discussion
|
|---|
There is growing literature demonstrating that the relationship
between free and total phenytoin concentrations is more unpredictable
than previously thought. Kilpatrick et al. (13) reported a
range of 6.7–33.3% free fraction, and Peterson et al. (4)
reported a range of 9.7–24.7%. Our free fraction range of 6.8–35.3%
compares well to these previously published reports. However, 50% of
our patient data points fell outside the 8–12% range expected from
review of the literature.
Several clinical conditions and medications are known to cause
alterations in FPF. The majority of the patient data points in this
study (74%) had an identifiable clinical diagnosis or concurrent drug
therapy to account for an FPF outside the 8–12% range. This leaves
26% of the patient data points for which the free, pharmacologically
active fraction cannot be accurately predicted based on the total
phenytoin concentration. Coadministration of valproic acid is a
well-documented cause of altered phenytoin protein binding, but
the FPF is not predictable (17). Valproic acid
coadministration was commonly associated with altered phenytoin binding
in this study. In our patients, hypoalbuminemia accounted for the
largest clinical diagnosis associated with an abnormal FPF. Even when
albumin was within the reference range, there was an inverse
relationship between the plasma albumin concentration and the unbound
phenytoin fraction (13), such that those patients with
borderline albumin concentrations might not have predictable FPF.
One of the most striking findings of this study was the 30% overall
rate of discordance between the total and free phenytoin concentrations
(47% for group 2). Put another way, in 30% of patients on phenytoin
therapy, the total phenytoin concentration was not accurately
portraying the pharmacologically active component, free phenytoin
concentration. This has serious implications for the management of
epilepsy with phenytoin and for therapeutic drug monitoring of
phenytoin. Phenytoin has a narrow therapeutic range, with potentially
significant adverse events associated with under- or overmedicating a
particular patient (18). Additionally, our data show that
even in patients without known causes of altered FPF, the free fraction
cannot be predicted reliably based on the total phenytoin
concentration.
On the basis of the evidence presented in this study, our clinical
laboratory replaced total phenytoin measurements with free phenytoin
measurements after this 6-week period of parallel free and total
phenytoin concentration measurements. In the 5 months subsequent to
this, only two total phenytoin measurements have been performed, one
because of technician error and one in response to an unusual clinical
situation. We postulate that in a given patient, the FPF fluctuates in
comparison with the total phenytoin over time because of changes in the
clinical status of the patient. Because we and the practitioners taking
care of these patients cannot predict how a change in management or
clinical status will affect the free and total phenytoin
concentrations, we recommend measuring only free phenytoin in all
patients. This has been widely accepted at our medical center, without
reports of any adverse outcomes. In contrast, for example, before
implementation of our free phenytoin only policy, four of our patients
who had therapeutic total concentrations were found to have toxicity
with free phenytoin concentrations >3.0 mg/L. Additionally, to
identify those patients with normal vs abnormal binding fractions, both
total and free phenytoin concentration measurements would have to be
done on every new patient and at every point when the clinical status
of the patient changed, to verify if the FPF had changed. In
comparison, measuring only free phenytoin on all patients is much
simpler, less costly than performing both the total and free
measurements, and is a true measurement of the pharmacologically active
drug. Therefore, we recommend free phenytoin concentration
determinations for all therapeutic drug monitoring laboratories, with
the elimination of total phenytoin determinations for routine practice.
Finally, we point out that the parameter "functional sensitivity",
defined as an imprecision (CV) of 20%, has been proposed to serve as a
clinically relevant estimate of the lowest reporting limit for an assay
(19). This parameter accounts for the imprecision
associated with interassay variables, such as different calibration and
reagent lots, and biological variables. A potential limitation of our
findings could be attributed to our low-end CV of 22% at 1.1 mg/L,
which falls above the 20% functional sensitivity required. Our patient
groupings of data could possibly have been influenced, thus affecting
our conclusions.
|
References
|
|---|
-
Soldin S. Free drug measurements: when and why? An overview. Arch Pathol Lab Med 1999;123:822-823.
[ISI][Medline]
[Order article via Infotrieve]
-
Kutt H, Winters W, Kokenge R, McDowell F. Diphenylhydantoin metabolism, blood levels and toxicity. Arch Neurol 1964;11:642-648.
-
Lund L. Anticonvulsant effect of diphenylhydantoin relative to plasma levels. Arch Neurol 1974;31:289-294.
[ISI][Medline]
[Order article via Infotrieve]
-
Peterson GM, McLean S, Aldous S, von Witt RJ, Millingen KS. Plasma protein binding of phenytoin in 100 epileptic patients. Br J Clin Pharmacol 1982;14:298-300.
[ISI][Medline]
[Order article via Infotrieve]
-
Fedler C, Stewart MJ. Plasma total phenytoin: a possibly misleading test in developing countries. Ther Drug Monit 1999;21:155-160.
[ISI][Medline]
[Order article via Infotrieve]
-
Reidenberg MM, Odar-Cederlof I, von Bahr C, Borga O, Sjoqvist F. Protein binding of diphenylhydantoin and desmethylimipramine in plasma from patients with poor renal function. N Engl J Med 1971;285:264-267.
-
Perucca E, Richens A, Ruprah M. Serum protein binding of phenytoin in pregnant women. Br J Clin Pharmacol 1981;11:409-410.
-
Boucher BA, Hanes SD. Pharmacokinetic alterations after severe head injury. Clinical relevance. Clin Pharmacokinet 1998;35:209-221.
[ISI][Medline]
[Order article via Infotrieve]
-
Dasgupta A, McLemore JL. Elevated free phenytoin and free valproic acid concentrations in sera of patients infected with human immunodeficiency virus. Ther Drug Monit 1998;20:63-67.
[ISI][Medline]
[Order article via Infotrieve]
-
Lenn NJ, Robertson M. Clinical utility of unbound antiepileptic drug blood levels in the management of epilepsy. Neurology 1992;42:988-990.
[Abstract/Free Full Text]
-
Kwong TC. Free drug measurements: methodology and clinical significance. Clin Chim Acta 1985;151:193-216.
[ISI][Medline]
[Order article via Infotrieve]
-
Ohshima T, Hasegawa T, Johno I, Kitazawa S. Variations in protein binding of drugs in plasma and serum. Clin Chem 1989;35:1722-1725.
[Abstract/Free Full Text]
-
Kilpatrick CJ, Wanwimolruk S, Wing LMH. Plasma concentrations of unbound phenytoin in the management of epilepsy. Br J Clin Pharmacol 1984;17:539-546.
[ISI][Medline]
[Order article via Infotrieve]
-
Barth N, Alvan A, Borga O, Sjoqvist. Two-fold interindividual variation in plasma protein binding of phenytoin in patients with epilepsy. Clin Pharmacokinet 1976;1:444-452.
[ISI][Medline]
[Order article via Infotrieve]
-
Ratnaraj N, Goldberg VD, Hjelm M. Temperature effects on the estimation of free levels of phenytoin, carbamazepine, and phenobarbitone. Ther Drug Monit 1990;12:465-472.
[ISI][Medline]
[Order article via Infotrieve]
-
Winkler S, Luer MS. Antiepileptic drug review: part I. Surg Neurol 1998;49:449-452.
[ISI][Medline]
[Order article via Infotrieve]
-
Anderson GD. A mechanistic approach to antiepileptic drug mechanisms. Ann Pharmacother 1998;32:554-563.
[Abstract]
-
Smith DB, ed. Antiepileptic drug selection in adults in
epilepsy: current approaches to diagnosis and treatment. New York:
Raven Press, 1990:111–39..
-
Spencer CA, Takeuchi M, Kazarosyan M. Current status and performance goals for serum thyrotropin (TSH) assays. Clin Chem 1996;42:140-145.
[Abstract/Free Full Text]
The following articles in journals at HighWire Press have cited this article:
|
|
|
|
 
R. De Smet, J. Van Kaer, B. Van Vlem, A. De Cubber, P. Brunet, N. Lameire, and R. Vanholder
Toxicity of Free p-Cresol: A Prospective and Cross-Sectional Analysis
Clin. Chem.,
March 1, 2003;
49(3):
470 - 478.
[Abstract]
[Full Text]
[PDF]
|
|
|
Top
Abstract
Introduction
