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The success of measles eradication depends upon a laboratory network to rapidly analyze samples obtained as part of surveillance and case investigation. The Pan American Measles Laboratory Network was established in 1995. Major activities of the 22 participating laboratories include the rapid testing of serum samples to diagnose measles, analysis and recommendation of techniques to be used in serologic testing, training in virus isolation, and procurement and distribution of laboratory materials. In addition, a comprehensive quality-control program and an electronic communication network have been developed. Testing for rubella has also been incorporated. The Network has been crucial to the great progress made toward eradicating measles from the Western Hemisphere. The priority given to the laboratories in the Network must continue in order to ensure that the eradication goal is reached and that validation of the interruption of endemic transmission of measles is documented.

Evaluation of the measles virus ELISA kit (Merck) to detect specific IgM as an indicator of primary measles antibody response was carried out. A modification of the manufacturer's cutoff value interpretation was introduced to allow for equivocal results in addition to positive and negative ones. With this modification, the test assayed gave an overall reproducibility of 96.16%. The IgM seropositivity rate for seroneutralization-confirmed measles cases was 100% for naturally infected measles subjects and 90% for primary measles vaccinated subjects. Individuals with positive neutralizing antimeasles antibodies in close contact with a confirmed measles case gave the following measles IgM ELISA results: 54.54% negative, 9.09% positive, and 36.36% equivocal, showing a booster with IgM antibody response on reexposure to the virus. Positive subjects with neutralizing antimeasles antibodies without recent contact with a measles case gave negative IgM results. IgM seropositivity was strongly associated with IgG seroconversion and clinical measles (p < 0.0001). The technique assayed performed adequately for the confirmation of both measles natural infection and primary vaccination and for the differentiation of primary and secondary antibody response, taking into account the modification in the cutoff value interpretation introduced and providing that the serum samples are obtained between days 5 and 30 after onset of rash.

A neutralization enzyme-linked immunosorbent (Nt-ELISA) assay for determination of protective immunity to measles virus was developed and evaluated. This procedure uses the same initial steps as performed to determine antibody titers by seroneutralization (Nt) test. However, a reduction in virus infectivity by neutralizing antibody was determined by quantitation of viral antigen using ELISA. The serum dilution that resulted in neutralization of 50% of infectious virus could be determined from the absorbance values. To be able to screen a large number of specimens, the conditions of the Nt-ELISA test were adjusted such that negative sera for measles antibodies and the positive ones were clearly distinguished on the basis of a single dilution (1:4). This test showed similar sensitivity (88.3%) and equal specificity as the Nt test when screening 136 serum samples from normal subjects. The estimation of protective antibody titers by Nt and Nt-ELISA methods was strongly correlated (correlation coefficient = 0.91). Thus, the measles Nt-ELISA test is rapid, reproducible, sensitive, and specific for detection of protective measles antibodies.