Measurement of measles- or rubella-specific IgG can be useful as an additional serologic method for case classification. The most widely used assays for detection of IgG antibodies to measles and rubella are indirect EIAs, although capture IgG assays may also be available for measles testing. IgG testing may be utilized when an equivocal result is obtained for IgM or if a positive IgM result is questioned due to clinical or epidemiologic information that is inconsistent with a case of measles or rubella.

A seroconversion is demonstrated with paired serum specimens tested together in the same assay in which the acute phase sample has a negative result for IgG and the second or convalescent specimen has a positive IgG result. Demonstration of a diagnostically significant rise in IgG titre (by a validated, quantitative, indirect IgG EIA) confirms a positive IgM result. This method relies on the availability of two specimens, usually collected 10-21 days apart, that are tested together in the same assay. The specific criteria for documenting an increase in titre depend on the parameters for the particular commercial assay and may require multiple serial dilutions of the serum specimens. The EIA optical density (OD) values are not titres, and increases in OD values do not directly correspond to titre increases.

In order to successfully demonstrate a seroconversion or a diagnostically significant rise in titre, the first (acute) sample should be collected soon after rash onset (ideally within 7 days). If the first specimen is IgG negative, the second serum can be collected at any time on or after 10 days post rash. True cases of measles or rubella should have detectable IgG in serum collected ≥10 days after rash onset. The absence of detectable virus-specific IgG in both serum specimens is acceptable evidence to rule out/discard a suspected case.