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I.
MEASLES SEROLOGIC TECHNIQUES |
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In
areas with a low incidence of measles, the diagnosis of measles by clinical
presentation is often complicated because of the sporadic nature of
the disease and the widespread occurrence of other rash-causing illnesses.
In addition, many measles cases in previously vaccinated or immunosuppressed
individuals do not meet the clinical case definition. Therefore, confirmation
of measles virus infection must be made using laboratory-based methods.
Antibody detection is the most versatile and commonly used method for
measles diagnosis. In acute, uncomplicated measles, a significant rise
in measles-specific IgG antibodies between acute- and convalescent-phase
serum specimens is generally considered diagnostic. A positive test
result for specific IgG antibodies in a single serum specimen indicates
past infection with measles virus or measles vaccination, but does not
ensure protection from infection or re-infection. Detection of specific
IgM antibodies in a single serum specimen collected within the first
few days of rash onset can provide a good presumptive diagnosis of current
or recent measles virus infection. Therefore, the IgM assay is the test
of choice for rapid diagnosis of measles cases.
II.
SEROLOGIC SPECIMEN HANDLING |
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Blood
Collection
Blood for serologic testing is collected by venipuncture or by finger/heel
stick into a serum-gel separator tube. Do not freeze the tube before serum
has been removed. Centrifuge the tube to separate serum from clot. Aseptically
transfer serum to a sterile tube that has an externally threaded cap with
an o-ring seal. Fresh, sterile serum can be shipped overnight on wet ice
pack or at ambient temperature. Frozen serum is shipped on dry ice for
next-day delivery. Hemolyzed and lipemic serum and plasma are noted and
tested; usually without apparent interferences.
Arrival,
Tracking, Reporting
Serum specimens for measles IgG, IgM, and neutralization testing arrive
at CDC through the Data and Specimen Handling Section (DASH) from international,
state, and local health departments, occasionally from doctors' offices
and from contract health clinics working on special study or vaccine protocols
with the National Center for Infectious Diseases or the National Immunization
Program and from PAHO and WHO reference laboratories. A completed DASH
form (appendix: CDC 50.34 rev 11/92 ) must be submitted for specimens
from state health department laboratories (US only). All specimens accepted
are by prior approval of Dr. William J. Bellini, Chief of the Measles
Virus Section. Specimens are tracked by DASH and results are reported
back through DASH to state health departments or directly to the principal
investigators, contract clinics, or PAHO and WHO. Raw data of all test
results are kept on file. All results are reported through the laboratory
director.
Storage
Frozen serum is thawed at room temperature 1-2 hours or in refrigerator
overnight just prior to testing. Serum may be kept at +4 C for several
days to complete retesting before returning to -20 C for long-term storage.
As quantities permit, all serum samples tested are kept in long-term storage
within the measles laboratory.
To ensure
optimal test performance, it is essential that all test procedures be
followed exactly as described in the package inserts:
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Working
dilutions of the assay reagents must be prepared identically each time.
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Incubation
times and temperatures must be strictly observed.
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Test
reagents must be handled in the prescribed manner. To minimize freeze-thaws
and to avoid contamination, use sterile technique to dispense reagents
in small volumes but not volumes less than 50 ul.
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Lyophilized
reagents, especially diluents, should be reconstituted to volume described
and mixed vigorously on a mechanical mixer.
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High
background signal.
a. High background signal in scattered plate wells indicates poor washing
technique or reagent cross-contamination. Clean washer and repeat test.
b. High background in all plate wells may indicate improper reagent
dilution or contamination of test reagents or diluents. Repeat test
with new products.
c. High background signal in uninfected cell control wells of a particular
test specimen suggests unidentified serum antibody reactions with test
reagents. The specimen may be retested in serial dilution.
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Special
care should be taken to match patient identifiers with serum dilution
tubes, with location of sera on test plates and with calculated O.D.
values.
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A record
should be kept of each assay as the test is performed. Note the reagents,
lot numbers, and expiration dates. Record how dilutions are made and
the timing of each step.
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A continuous
record of all control values should be kept to monitor changes in assay
performance over time.
IV.
RESULTS AND INTERPRETATION |
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IgM
Result |
IgG
Result |
Previous
Infection
History |
Current
Infection |
Comments |
+ |
+
or - |
not
vaccinated, no
history of measles |
recent
1st MMR |
seroconvert* |
+ |
+
or - |
not
vaccinated, no
history of measles |
wild-type
measles |
seroconvert*,
classic measles |
+ |
+
or - |
Previously
vaccinated, primary vaccine failure |
recent
2nd MMR |
seroconvert* |
- |
+ |
previously
vaccinated,
IgG+ |
recent
2nd MMR |
IgG
level may stay same or boost |
+ |
+ |
previously
vaccinated,
IgG+ |
wild-type
measles |
may
have few or no symptoms** |
+ |
+ |
recently
vaccinated |
exposed
to
wild-type measles |
cannot
distinguish if vaccine or wild-type, evaluate on epidemiologic grounds*** |
+
or - |
+ |
distant
history of measles |
wild-type
measles |
may
have few or no symptoms**, if clinically compatible may have been
misdiagnosed initially |
* |
IgG
response depends on timing of specimen collection (4) |
** |
If
so, do not consider contagious unless clinical presentation is consistent
with measles |
*** |
If
IgM negative, helpful to rule out wild-type measles infection |
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Cremer
NE, CK Cossen, G Shell, J Diggs, D Gallo, and NJ Schmidt. (1985) Enzyme
immunoassay vs plaque neutralization and other methods for determination
of immune status to measles and varicella-zoster viruses and vs complement
fixation for serodiagnosis of infections with those viruses. J. Clin.
Microbiol. 21:869-873.
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Erdman
DD, LJ Anderson, DR Adams, JA Stewart, LE Markowitz, and WJ Bellini.
(1991) Evaluation of monoclonal antibody-based capture enzyme immunoassays
for detection of specific antibodies to measles virus. J. Clin. Microbiol.
29:1466-1471.
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Hummel
KB, DD Erdman, J Heath, and WJ Bellini. (1992) Baculovirus expression
of the nucleoprotein gene of measles virus and utility of the recombinant
protein in diagnostic enzyme immunoassays. J. Clin. Microbiol. 30:2874-2880.
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Helfand
RF, JL Heath, LJ Anderson, EF Maes, D. Guris, and WJ Bellini. (1997)
Diagnosis of measles with an IgM capture EIA: The optimal timing of
specimen collection after rash onset. J. Infect. Dis. 175:195-199.
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Ratnam
S, G Tipples, C Head, M Fauvel, M Fearon, and BJ Ward. (2000) Performance
of indirect immunoglobulin M (IgM) serology tests and IgM capture assays
for laboratory diagnosis of measles. J. Clin. Microbiol. 38:99-10.
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