Rapid enzymologic anti-neuraminidase antibody microtest.

A micro-neuraminidase-inhibition-technique (the Essen NIT) using a glycoprotein as substrate and employing the method of Aminoff for the determination of free N-acetyl neuraminic acid has been developed by our group. The main difference between the micro NI-test and the WHO-method is the mathematical evaluation of AB titer (TI 50) from the investigation of a single dilution only. This procedure is possible, because it could be proved that this reaction follows enzyme-antienzyme kinetics of the Michaelis-Menten type. On the basis of the microliter system and the Michaelis-Menten reaction, the Essen-NI-test has the following economical and technical advantages over the WHO-method: (a) use of only 1/25th volume of the expensive substrate (glycoprotein) per serum in comparison to the WHO-test, (b) investigation of up to 90 sera per technician and day instead of a few sera per technecian and 2 days and (c) testing of nasal washings without further dilution. Regression analysis of test results of antibody determination evaluated with both methods showed a linear correlation with a regression coefficient of about 1. Titers with the WHO-method were 5-10 times higher than with micro NI test, most possibly due to the shorter reaction times. Nevertheless, positive sera were found positive with either test. Strain specificity of neuraminidase could be equally well evaluated with the WHO-method and the Essen-NIT when 5 influenza virus A isolates from Berlin of 1968-1975 were tested against influenza virus X/15 HK immune serum.