The measurement of carbamazepine (CBZ) in samples from the emergency r
oom (ER) raises several issues for drug monitoring. First, the ER freq
uently requires rapid turnaround time for clinical samples; this need
may be more conveniently met by automated immunoassays than by high-pe
rformance liquid chromatography (HPLC), the other major analytical tec
hnique for measurement of carbamazepine. On the other hand, immunoassa
ys often do not completely measure the pharmacologically active carbam
azepine epoxide metabolite and therefore may not indicate the full ext
ent of serum anticonvulsant activity. Second, patients may be admitted
to the ER specifically because of seizure activity, which may be an i
ndication of under- or overmedication with carbamazepine and which, if
due in large part to high levels of the epoxide metabolite, may not b
e fully assessed by immunoassay. We examined the results of carbamazep
ine determination in 102 consecutive samples sent from an ER. Each sam
ple was analyzed by a fluorescence polarization immunoassay (FPIA) and
by HPLC. There were good correlations between the FPIA and the HPLC f
or the parent drug and for the sum of the parent drug plus metabolite
(carbamazepine-10,11-epoxide, CBZ-E) with these regression equations:
FPIA = 1.13 (HPLC CBZ) + 0.09 (r2 = 0.93) and FPIA = 0.93 (HPLC CBZ CBZ-E) - 0.55 (r2 = 0.89), respectively. There were weak correlations
between the FPIA and the epoxide and between the parent drug and the e
poxide. Based on the FPIA and HPLC results, we classified each value r
elative to the therapeutic range, i.e., supratherapeutic, subtherapeut
ic, or therapeutic. The FPIA result agreed with the HPLC result in 10
of 11 supratherapeutic levels. 14 of 17 subtherapeutic levels, and 69
of 74 therapeutic levels. Where there was a difference in classificati
on, the results tended to fall around the cutoff point for one or the
other method. To address the possibility of elevated epoxide levels as
a cause of seizure-producing toxicity, we studied 45 patients with re
cent seizure activity. Of them, 12 had modest elevation of the epoxide
, and in six of these cases the sum of the parent drug plus epoxide wa
s elevated above the therapeutic range for HPLC. In five cases, the FP
IA indicated a supratherapeutic value. We did not observe a patient wi
th an elevated epoxide level where the FPIA gave a seriously misleadin
g value. We conclude that the FPIA provides an adequate measurement of
carbamazepine level in the majority of patients from the ER.