Reliability of low-avidity IgG and of IgA in the diagnosis of primary infection by rubella virus with adaptation of a commercial test

The detection of IgA and low‐avidity IgG and antibodies in serum is a potentially useful marker of recent infection by a microorganism. We studied the reliability of IgG avidity and presence of IgA for the diagnosis of recent acute infection by rubella virus. Low‐avidity IgG (Avy‐EIA test) was determined with a modified commercial test using 8 molar urea (indirect ELISA, DiaSorin, Italy) and IgA was determined with a homemade indirect ELISA test. Twenty‐five patients with recent primary infection by rubella virus (group I) and 50 healthy subjects (group II) were studied. In group I low‐avidity IgG varied between 100 and 0% (67.3 ± 21.8%); IgA was present in 24 patients (96%). In group II low‐avidity IgG varied from 50.4 to 0% (19.8 ± 16.9%). IgA was present in 2 subjects (4%). The sensitivity of the Avi‐EIA and the IgA test was 92 and 96%, respectively; specificity was 100 and 96%, respectively. We conclude that both low‐avidity IgG and IgA tests are helpful and reliable for the diagnosis of recent primary infection. J. Clin. Lab. Anal. 13:1–4, 1999. © 1999 Wiley‐Liss, Inc.