Measurement of measles virus-specific neutralizing antibodies: Evaluation of the syncytium inhibition assay in comparison with the plaque reduction neutralization test

Plaque reduction neutralization (PRN) is the "gold-standard" for the measur ement of measles-specific neutralizing antibodies. However, it is a complic ated assay and tends to be operator-dependent. It has been suggested that t he simpler syncytium inhibition assay (SIA) can give results comparable to the PRN test. We compared these two assays using 544 serum or plasma sample s obtained from children at various times after natural infection, primary measles immunization, and measles revaccination. The results of the two ass ays correlated well overall (r = .86; p < 0.0001). The strain of challenge virus (wild-type versus vaccine strain) did not significantly influence SIA titers and the assay performed equally well with serum and plasma. PRN tit ers greater than or equal to 120 and >800 are thought to indicate protectio n against clinical illness and infection respectively. The equivalent SIA c ut-off values using 125 plaque-forming units as the challenge inoculum were greater than or equal to 16 and >128 respectively. At low PRN titers (<200 ), the correlation between PRN and SIA values was reasonable (r = 0.60; p < 0.001) when a challenge inoculum of 12.5 plaque-forming units was used. At the lowest PRN titers (<100), 15% of the samples gave divergent results. T hese data confirm the utility of the SIA in the determination of measles-sp ecific neutralizing antibodies when antibody titers are high. However, the PRN assay remains the test of choice when maximum sensitivity at low titers is required. (C) 1999 Elsevier Science Inc.