Introduction: It is essential for the management of the pregnant patients with a recent rash or in contact with a rubelliform illness that differential diagnosis of primary rubella infection from reinfection could be accomplished. The presence of rubella specific IgM has been taken in the past as a reliable indicator of primary infection, but in some cases the presence of IgM can be due to other causes. In this study, considering the fact that IgG avidity in primary rubella is much lower than in reinfection, the main objective was to use a convenient serological method for rubella IgG Avidity assay.

Method: For this purpose 72 serum samples from measles negative exanthematous patients were investigated. These samples were divided in two categories of A and B according to the serological markers (HI test results and IgM detection) and also the age of patients, presence of low avidity IgG for group A and high avidity IgG for group B was expected. An avidity IgG ELISA test was performed by elution method for each group using Radim commercial kit and 6M urea (a mild protein denaturant) and avidity index was calculated.

Results: Using Splus software CART method (classification and regression tree), the borderline achieved for separation the high avidity IgG from low avidity one was 47.5% and using Roc curve in SPSS software specificity and sensitivity of the test were calculated.

Conclusion: The results show that, this method is suitable for differential diagnosis of primary rubella infection from reinfection.

Hakim Research Journal 2005 8(3) 38-44.