Antibodies to the enterobacterial common antigen: standardization of the passive hemagglutination test and levels in normal human sera.

The passive hemagglutination test for antibodies against the enterobacterial common antigen (ECA) of Kunin was standardized for diagnostic purposes. Human erythrocytes were coated with a soluble ECA+ preparation from Salmonella typhimurium or, as specificity controls, with a similar ECA- preparation from congenic ECA-negative bacteria with saline, and the hemagglutination assay was performed on microtiter plates. The specificity of the test was ascertained further by inhibition assays with purified ECA and with crude ECA+ and ECA- preparations. The reproducibility of the tests was 96.4%; on this basis, a fourfold or larger difference in titers was regarded as significant. The anti-ECA titers in 649 serum samples from healthy persons ranged from less than 4 to 8,192. The titers increased with age, so that th geometric mean titers were 57 at 1.5 years of age and 201 at 45 years of age. After this, the titers decreased again, to a low of 52 in persons more than 70 years old. Women had higher titers than men up to the age of 40 years.