Download material present was very low in con

sensitivity.
In addition, to ensure the
precision of the measurement,
we
found it necessary to pass the incoming
blood through a miniature
heat exchanger to equilibrate
thermally
the
analyte and calibration solutions. The
Fetal-Tek 200 method.
One institution reported to us that
they were getting falsely positive PG
data on genetic screening samples
from amniotic fluid of 12-week fetuses.
We saw one of these plates and agreed
with their apparent findings.
Other institutions
have sent us
plates that they believed had PG on
them when the bands didn’t line up
with the marker
or the amount
of
material
ChemFET sensors are coupled to the
instrumentation
system through a specially-designed electrical isolation system, which conforms to the required
safety standards (5). The waste fluid
line from the ChemFET
cell passes
through a downstream reference electrode cell, which incorporates a cabmel-saturated
KCI, single-junction,
porous-plug reference electrode (Russell pH Ltd., Auchtermuchty,
Fife,
Scotland), and then, by way of a penstaltic pump, to waste.
present was very low in con-
centration.
Still other institutions collected clinical data and decided they could determine the difference between PG and
artifactual material.
In putting any new test on line in an
institution,
Helena strongly suggests
that the procedure should be evaluated
The two calibration
solutions were
chosen to bracket the clinical range of
potassium concentration. A “baseline”
solution (per liter: 1 mmol of K and
150 mmol of Na ) and the incoming
blood are alternately
sampled by the
ChemFET cell, after pre-analysis auto-
by either comparing
it with another
method or by collecting adequate clinical data tojustif’
its use. This practice
assures individual
laboratory
compe-
tence.
All this re-emphasizes the necessity
for using all available informationincluding patient history, clinical findings, and the judicious use of laboratory tests-in efforts to determine fetus
status.
Eric Petersen
Helena Labs.
Beaumont,
TX
77704
Fig. 1. Encapsulated K ChemFET on TO-5
type header with flow-through cap
so as to divert
computer
one of two calibration
cell,
flow-through
either
matic calibration with the second solution (per liter: 7 mmol of K4 and 150
mmol of Na4). With a standard calibration solution (per liter: 4.5 mmol of
blood or
solutions to the
thus
allowing
on-
system
and
line calibration.
The
Ej.d44
instrumentation
integrated-circuit
chips are designed for simultaneous, four-channel
application (e.., continuous analysis
for K, W, Ca , and Na), although
we have so far restricted
On-Line Measurement of Potassium
In Blood by Chemical-Sensitive
Field-Effect Transistors: A
Preliminary Report
To the Editor:
Several chemical-sensitive
field-effect transistors
(ChemFETs)
have been
described (1) sincethe first report of
Bergveld (2) in 1970, but, as far as we
are aware, these devices have not yet
been entirely
successfully
applied to
clinical laboratory
measurements.
We recently developed a system for
direct analysis of blood from an in-
the clinicallaboratory application to on-line analysis for K
only, using valinomycinadded polyvinyl
chloride as the electroactive
gate material
(3). Separate
experiments
in solutions
containing
KC1 in the range 0.1-100 mmoIfL gave
a nernstian
response of 59 mV per
decade.
The ChemFET devices are operated
in the constant-current mode, with the
drain current set at the athermal level
(4) so as to minimize
inherent thermal
K and 150mmobofNa)wetestedthe
system with aqueous samples to determine replicability
and speed of response. Assay of 25 samples sequentially gave a mean computed
concentration of 4.516 mmolIL (SD 0.040
mmol/L). The 100% response time of
the ChemFET
flow-through
cell was
2 s.
Under microcomputer control, blood
was drawn into the sensor-head for an
8-s period at 32-s intervals with an
LKB Vanoperpex
12000 peristaltic
pump. The system flow rate is approximately 25 VU/s, giving an effective
exsanguination rate of 6.25 MU/s, although this can be decreased by reprogramming the system.
Figure 2 shows a chart record of the
60
S
A
dwelling cannula by using ChemFETs.
The devices used were the Ed44
I.C. chip (2.52 x 2.52 mm) developed at
Newcastle,
comprising
four enhance-
ment-mode, dual-dielectric
and four insulated-gate
ChemFETs
field-effect
transistors (IGFETs), all transistors
having 206 x 12 m channels and a
composite Si02/Si3N4 gate insulator.
The devices were suitably encapsulated so as to form a miniature,
flowthrough
cell assembly
(Figure 1),
which is incorporated into a remote
sensor-head (76 x 49 x 16 mm), together with a pair of miniature,
12-V
solenoid valves (type LFAA1200118H;
Lee Products
Ltd., Harrow, Middlesex,
U.K.). These valves were controlled by
a Hewlett-Packard
Model 9815 micro-
Fig. 2. On-line ChemFET analysis of K in human blood
A, Calibration with 7 mmolof K and 150 mmolofNa per liter;B, baselinesolutionof 1 mmolofK and 150
mmol of Na#{176}
per liter; C, instant
ofconnectionof subject;D. onset of clot formation
inintravenous
cannula.
The slight baselinedrift presumablyis due to the heparinization
CLINICAL CHEMISTRY, Vol. 29, No. 2, 1983
405
first successful application
of the system (on one of the authors, A.S.). We
found that heparinization of the systern (sodium heparin, 5000 USP units!
SD,Janata J, Johnson CC. Potassium ion-sensitive field effect transistor. Anal
C/tern 47, 2238-2243 (1975).
4. Leistiko 0. The selectivity
and tempera-
mined by a NADH-coupled continuousflow endpoint reaction.
Using a Technicon SMA II analyzer,
mU) before use was essential to prevent clot formation; even so, more adequate anti-coagulation
procedures
ture
ing that a metronidazole
must be
developed
for long-term
use,
3. Moss
characteristics
effect transistors.
(1978).
5. British
of ion
Phys
Standard
sensitive
Scr
18, 445-450
Specification
(1979). Specification
forSafetyof Medical
Electronic Equipment. British Standards
Institution, London, W1A 2BS, U.K.
analysis
Depts. of Phys.
lAnaesthesia
by
ChemFET,
a separate
The most important
and unique advantage of determination
by use of
ChemFET
devices in this manner is
that precise, continual,
on-line
monitoring is readily
achieved. The small
size and solid-state
nature of ChemFETs permit ex vivo measurements
by
sensors connected with minimal
deadspace directly
to the subject, a technique
that
provides fresh, reliable
physiological
data without the numerous hazards and difficulties
associated
with in vivo measurement.
We are continuing
this work, taking
steps to provide adequate heparinization of the system and to extend this
on-line analysis
to the simultaneous
measurement
of K4, H,
Ca24, and
Na (each of which has already been
separately measured in this way). Use
of ChemFETs in clinical chemistry laboratory
feasible
instrumentation
now
in a very few years.
seems
We acknowledge partial SERC support.
We are grateful to Dr. John Robertson and
the staff of the SERC Edinburgh Microfabrication Facility for processing the E1.d44
I.C.s, and particularly Alan Gundlach for
his expert advice.
A. Sibbald
NE1
AST activity
will show
SMA II computer-controlled
analyzer
does not provide for an AST blank
channel in its configuration.
Consequently, we programmed our system,
using the “Configuration”
(CF) com-
R. F. Carter
Dept. of Anaesthesia
Freeman Hospital
Newcastle
upon Tyne
mand,
UK
with
for alanine
the similar
configuration
aminotransferase
(ALT),
which does include
an ALT
blank
channel.
Thus we use an AST test
cartridge, our own AST blank cartridge, and AST reagents in a system
Blank Correction for Metronidazole
Interference with Continuous-Flow
Measurement of Aspartate Aminotransferase
configured as ALT and ALT blank. By
using the “Expected Range” (ER) command, it is a simple
procedure
to
change the test name from ALT (or
SGPT) to AST (or SGOT) on the printout.
Unfortunately,
because of negative
To the Editor:
Many patients and outpatients
are
being treated with metronidazole (Flagyl; Searle & Co.,) as prophylaxis
against infection by obligate anaerobes
and for trichornoniasis.
As is well
known, this drug decreases results for
aspartate aminotransferase
(AST, EC
2.6.1.1)
as deter-
AIR
activity
-
SAMPLE
-
‘!
grflwhto?3
u
AIR
BUFFER
FLOWCELL
serum
CONNECTOR
ASSEMBLY
kkO32-1
BUFFER
in
55Iy
reared
24 fl
DIALYZER
L-T0
jI
changes in absorbance
in AST and
ALT chemistries
and the fact that a
“blank” is usually subtracted from the
test result,
use of the above system
alone did not solve the problem completely. In addition, the blank-channel
absorbance must be added to the negative-test-channel
absorbance to correct
for metronidazole
interference.
This involves a minor change in the disc program for the ALT channel, which was
made by Mr. P. Desfetes, product engineer
for Technicon
International
(France), at our request.
The correction this simple modification provides is almost absolute, allowing for minor variations
in dialysis
rates, etc. (Table 1). Note that the
primer must contain rnetronidazole
to
WASTE
LJ
ISO
080
TO
PULLTHROUGI4
WASTE
DIRECT
COLORMETRY
340 nm
Simplified flow diagram
rectionfor interference
of blank-channel
corby metronidazole in
the assay for aspartate aminotransferase
Numbersindicateflow rates, in mL/min.
Table 1. Aspartate Aminotransferase Values for Pairs of Sera from
Five Patients Not Receiving Metronidazole (Typical Results; Single
Determinations)
AST, U/L at 37 #{176}C
1. Janata J, Huber RJ. Chemically
sensitive field effect transistors.
Chap. 3 in Ion-
Selective Electrodes in Analytical Chemistry, 2, H Freiser, Ed., Plenum, London,
1980, pp 107-174.
2. Bergveld P. Development of an ion-sensitive solid-state device for reurophysiological measurements. iEEE Trans Bio,ned
Eng BME-17, 70-71 (19701.
CLINICAL
concentration
factitiously
lower results.
A simple solution to this problem is
to provide a blank channel (Figure 1)
with phosphate buffer, volumes, flow
rate, and dialysis pathlength
the same
as for the test channel. The Technicon
7RU
References
406
above-normal
A. K. Covington
E. A. Cooper’
C/tern, and
Univ. of Newcastle
Newcastle
upon Tyne,
UK
noted this effect, observ-
of 10 mg/U ofserum decreased the AST
(measured
at 37 #{176}C)
by approximately
25 U/L. Patients
with normal
AST
activity,
when given doses of metronidazole (drug concentration
in serum,
6-12 mg/L), will show AST values of
less than 0 U/U, and patients
with
BS 5724
because clotting of-the 23-gauge intravenous “butterfly” cannula was apparent after about 12 mm.
The first 13 sequential K4 ChemFET analyses gave a mean value of
3.80 mrnollL (SD 0.06 mmol/L). After
blood sample was taken for alternative
discrete analysis; however, the application of a tourniquet
and sampling
through a separate needle by syringe
was considered necessary; any resulting hemolysis would lead to a higher
potassium concentration in these samples. The mean for the ChemFET analysis (3.80 mmol/L) was in fact slightly
lower than that found by flame photometry (Corning 450: 4.1 mmol/L) and
by an ion-selective electrode instrument (Corning 902: 4.05 mmol!L) but
this could also be related to the standards chosen for use with the three
instruments.
we also have
field
CHEMISTRY,
No added metronidazole
Kinetic
Metronidazole added (final concn 25 mg/L)
Kinetic
method
SMA II with
blank channel
method
43
100
17
312
198
43
96
19
310
209
48
99
17
299
194
Vol. 29, No. 2, 1983
SMA II with
blank channel
SMA II without
blank channel
45
97
17
302
<0
26
<0
243
197
137