Influence of Aspirin on in Vitro Direct Potentlometry of
C1 In Serum, Ryszard Lewandowski,
Tomasz Sokaiski,
and Adam Hulanicki (Dept. of Chem., Warsaw
University, ul. Pasteura 1,02-093 Warsaw, Poland)
The common use of potentiometric methods of electrolyte
determination in body fluids, especially in the automated
mode (1-5), can involve errors caused by the presence of
some drugs or their metabohites in the investigated media.
The first step in estimating the influence of those substances is to make in vitro measurements.
We investigated effects of some drugs (aspirin, procainamide, digoxin, streptomycin sulfate, tetracycline, erythromycin, hidocaine, diazepam, quinidine sulfate) on direct
potentiometry of potassium, sodium, calcium, and chloride
in SERONORM control serum (Nyegaard & Co., Diagnostic Division, Oslo, Norway). We used a potentiometric
clinical analyzer “Microlyte” (KONE Corp., Finland) and
the following procedure: A known amount of the drugs
corresponding
to 10-fold the therapeutic concentrations
was dissolved in doubly distilled water or in ethanol:water
(1:99 by vol), and a constant amount of SERONORM was
added. The pH was adjusted to 7.4 with dilute sulfuric acid
or Tris.
Results of determination
of potassium, sodium, and chloride in SERONORM agreed well with the producer’s certifled values. This certification for calcium is for total content, whereas our measurements refer to ionized calcium.
Thus results for potassium, sodium, and chloride were
compared
with the certified values, whereas those for
calcium were matched with an average of 47 measurements in pure SERONORM. No significant effect of the
studied drugs on the determination of cations was stated,
but we observed a strong effect of aspirin on determination
of chloride. Measurements made directly after addition of
aspirin were in error by 5%. This positive error of chloride
determination gradually increased and finally reached a
very large value (>38%), beyond the measuring range of
the analyzer.
Figure 1 presents calibration graphs of the chloride
ion-selective electrodes in the presence of aspirin, hydrolyzed aspirin, sodium acetate, and sodium sahicyhateof the
same molar concentrations. For the aspirin the interference increases with time, finally causing a loss of sensitiv200
E(mV)
4,5
-3
-2
-1
0
log
Fig.1.Calibration
graphsforchloride
ionselective
electrode in
aqueousKCI (1) and in thepresenceofacetate
(2), non-hydrolyzed
aspirin(, hydrolyzedaspirin(, and salicylate( at16.7mmol/L
and pH 7.4
2146
CLINICAL CHEMISTRY, Vol. 35, No. 10, 1989
of the Cl electrode for chloride ions in the range
investigated. The same effect is caused by sodium salicylate
of the same molar concentration. Sodium acetate shows no
significant
influence. The evaluated selectivity coefficients
are
=
160, KAC = 0.20, and K
=
1.5, indicating that the effect is mainly due to the salicylate, the
principal metabohite of aspirin. However, in the presence of
a concentration corresponding to the normal therapeutic
concentration for aspirin, the bias was within the limits of
measurement precision. Evidently, only in cases of excessive aspirin intake may the result for chloride determination be erroneous.
ity
We thank KONE Corp. for supplying the “Microlyte” analyzer
for performing these experiments. The financial support of the
project CPBP 01.17 is acknowledged.
References
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Chem 1984;30:1720.
2. Ng RH, Altaffer M, Ito R, Statland BE. The TechniconRA-1000
evaluated for measuringsodium,potassium, chloride, and carbon
doixide. Chin Chem 1985;31:435-8.
3. West P. An evaluation of the AM 721 ion-selectiveelectrode
system for the estimation of sodium and potassium in plasma,
urine and whole blood.J Autom Chem 1983;5:182-7.
4. White JK, Chan DW. Evaluation of Beckman Electrolyte 2
Analyzer using ion-selective
sodiumand potassium electrodes. J
Clin Lab Autom 1984;4:23-7.
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CritIcal Evaluation of an Immunoradiometrlc Assay of
Sex-Hormone Binding Globulin, G. E. Wieringa and A.
Crawford (Regional Immunoassay
Laboratory,
Withington Hospital, West Didsbury, Manchester, M20
8LR, U.K.)
The U.K.
market
for kit methods
for sex-hormone-
binding globulin (SHBG) determination
iscurrently
dominated by a liquid-phase immunoradiometric
assay (IRMA)
from Farmos Diagnostica, Oulunsalo, Finland (used by
60% of all laboratories). Although the method is reliable
and rapid, a major drawback of itisthe requirement
that
all specimens be diluted 100-fold, a potential source 0:
inaccuracy and between-batchimprecision.
A recentaddition to the market is an amplified IRMA (AMIRA) from
DiagnosticProducts Corporation (DPC, Los Angeles, CA),
which involves two ligand(biotin)-hinked
monoclonal antibodies (MAb) to SHBG and a 1251-labeled monoclonal
antibody to SHBG. After incubation of all three MAbs with
10 pL of undiluted specimen, avidin is added to form a
bridge between the ligand-coated tube and the SHBGMAb-ligand complex. Separation is achieved by decanting
excess (unbound) tracer, followed by two wash steps.
We evaluated the new method (y) and compared its
performance with the Farmos method (x). Specimens from
50 patients were analyzed in duplicate by both techniques
and results showed a correlation coefficient of 0.98, y =
0.98x + 3 nmolJL, and standard errors of slope and intercept = 0.03 and 2.17, respectively. Paired duplicate analysis for the DPC kit showed a CV of 2.3% for patients’ values
between 4 and 20 nmol/L (n = 19 pairs), 3.2% between 21
and 80 nmol!L (n = 47), and 2.9% between 81 and 180