Document 6535618

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hypothyroidism
who was being treated
ment gave the following results:
TSH,
England)
We find that glycine is eluted in large amounts from test
strips used to detect the presence of ketones in urine
(“Ketur-Test”;
Boehringer Corp., Lewes, U.K.). Immersion
of the strip for 1 s (the time recommended by the manufacturer) in 1 mL of water gave a glycine concentration of
approximately 400 pmol/L and for 2 s approximately 1400
zmol/L
when measured in an ion-exchange amino acid
analyzer
(Huger Analytical,
Margate, U.K.). A similar
increase in glycine was found for another brand of test strips
(“Ketostix”; Ames Division, Miles Labs., Slough, U.K.), but
this is not reported because a change in the formulation of
these strips has removed the problem. Addition of glycine in
these amounts would significantly
increase the apparent
endogenous glycine in urine samples screened by amino acid
chromatography,
suggesting such inherited
metabolic diseases as non-ketotic
hyperglycinemia
and some organic
acidurias. A nonspecific
increase
in urinary glycine may
also occur for various reasons. Moderate unexplained
increases in glycine in samples sent to us for amino acid
screening
are found relatively
frequently;
how many of
these are ascribable to test strip contamination
is unknown.
Further investigation
of false-positive
screening tests consumes time and resources, particularly
when reference is
made to a specialist center.
In addition to glycine, the possibility that samples might
be contaminated
by other test strip constituents such as
organic acid buffers should be considered. Aliquots of samples tested with test strips should be discarded.
replace-
Sample date
TSH,
Sample Contamination by Test Strips, J. M. Rattenbury
and Joyce C. Allen (Dept. of Chemical Pathology,
Children’s Hospital, Western Bank, Sheffield SlO 2TH,
by thyroxin
EAIA,
milli-int.units/L
milli-int.units/L
11/84
5/85
7/85
8/85
4.5
19.8
9.0
16.0
9.1
18.7
22.2
Incubation of sample with Protein A-Sepharose
or calf
intestinal alkaline phosphatase to remove or inhibit possible immunoglobulin
interferents
had no effect on the discrepant results. In both assays, sample dilution gave a
linear response. Furthermore,
the EAIA assay buffer contains mouse serum (10 mL/L) to counter any nonspecific
anti-mouse immunoglobulin
reactivity, and an increase in
the concentration
of this component had no effect.
The patient gave a positive test result for anti-mitochondrial antibodies
but negative for rheumatoid factor, the
latter a potential interferent in two-site immunoassays.
In
previous studies (1) no interference by autoantibodies
in the
EAJA was seen.
Incubation of sample (8/85) with goat antiserum
to human 1gM and subsequent
assay with EAIA decreased the
measured TSH to 8.0 milli-int. units/L.
A further serum sample (5/86), assayed for TSH with the
unmodified
EAJA, gave a value of 21.5 milli-int. unitsfL; in
an EAJA incorporating a modified enzyme-antibody
conjugate (Fab’ fragment conjugated to alkaline phosphatase)
it
gave 6.6 milli-int. unitsfL; and with Fab’ fragment and
sample pretreatment
with anti-IgM it gave 8.6 milli-int.
units/L. These results suggest that the discrepant result is
ascribable to the presence of an 1gM that is cross linking the
two mouse monoclonal antibodies in the standard EAJA. The
1gM is not rheumatoid
factor. The binding of the 1gM
appears to be through the Fc position of the monoclonal
antibody, because removal of this region in the preparation
of an Fab’ fragment
conjugate removes the discrepant
result.
The EAIA will be modified by including a suitable conjugate. Interfering
antibodies should be considered in cases
where biochemical results are inconsistent with the clinical
state of the patient.
Reference
1. Clark PMS, Price CP. Enzyme-amplified
and David H. Ellis (I.Q. (Bio) Ltd., Milton Road,
Cambridge, U.K.)
an Infant on Soy Formula, M. H. Cheng, W. Y. Huang,
414 CLINICALCHEMISTRY, Vol. 33, No. 3, 1987
of thyrotropin
a new
Clin Chem
ultrasensitive
1986;32:88-92.
Recently we reported (1) some discrepant results between
an “in-house” immunoradiometric
assay (xius) for thyrotropin (TSH) in serum and an enzyme-amplified
irnmunoassay
(z&) kit (I.Q. (Bio) Ltd., Cambridge, U.K.). In both assays,
mouse monoclonal antibodies are used, the label in the EAIA
being alkaline phosphatase
(EC 3.1.3.1) derived from calf
intestine.
Serum sampled from a patient with primary
assay
immunoassay:
Removal of Interference by lmmunoglobulins in an
Enzyme-Amplified Immunoassay for Thyrotropin in
Serum, Penelope M. S. Clark, Christopher P. Price (Dept.
of Clin. Biochem., Addenbrooke’s Hospital, Cambridge,
U.K.),
14.0
Detection of Bromocriptine-like
evaluated.
Substances in Urine of
and A. I. Lipsey (Clinical Laboratories, Childrens
Hospital of Los Angeles, Los Angeles, CA 90027)
A urine specimen was submitted to our laboratory for
toxicology screen to rule out possible toxin ingestion in a
two-month-old infant brought to the hospital emergency
room for acute gastroenteritis,
diarrhea, and dehydration.
The parent had a history of drug abuse, and the presence of
phencyclidine
(PCP) and cocaine was suspected in this