SAGE IVD recommended conditionally including the measles IgG antibody test category in the third EDL:
• as a disease-specific IVD for use in clinical laboratories (EDL 3, Section II.b, Vaccine-preventable diseases);
• using an immunoassay format;
• to aid in the diagnosis of clinically suspected measles infection; pending the submission of further evidence to show that the test has utility in confirming cases when used with IgM.

Measles is a disease of public health concern, and it is important to detect outbreaks early. In clinical settings the measles IgG test is mostly used in combination with IgM or PCR tests to confirm cases. A single positive or negative result of measles IgG does not rule out the presence or absence of measles infection; so it is not diagnostic of measles on its own. The test is known to be useful in diagnosing some complications of measles, such as SSPE, and in determining immune status in specific groups, including transplant patients. It is also known to be a useful test for determining the immunity of a person pre- and post-vaccination, and thus could be used to study seroprevalence in a population. But the test was not submitted as a screening tool for immunization; moreover, the WHO representative confirmed that IgG is not recommended for seroprevalence. Nevertheless, the IgG test is part of the case definition for measles and forms part of the WHO algorithm for diagnosing the disease in an elimination setting. In part because of this, there is not much data on the test’s clinical accuracy or performance. And very few data were submitted on the test’s usefulness. SAGE IVD noted a definite need for more evidence on how the measles IgG test has made a difference to show that it is an essential test. Other limitations of the test include the fact that it requires paired sera and cannot be used as a POC test. It requires a laboratory with ambient environmental conditions and a regular power supply; it may also prove costly in LMICs and resource-constrained settings. SAGE IVD noted that the application listed an EIA format for the test, but highlighted the existence of avidity assays, which could also be useful in diagnosing acute measles.

As an IgG response is part of the standard confirmation of a measles case, it is difficult to assess test accuracy; inevitably, the reference standard used will incorporate an IgG test in some cases. There are no studies comparing the results of this test against an independent reference standard (given that it is effectively part of the case definition). The test is embedded in WHO protocols for testing measles. Evidence of the value of the IgG test could be better established by looking at the numbers of measles cases that were diagnosed by an IgG when other methods failed. The study from the Spanish outbreak indicates that a small number of cases were only identified by IgG testing following a negative IgM test. Some evidence exists showing that there are measles cases detected only by use of the IgG test, particularly showing reinfection or infection in the vaccinated population.

World Health Organization. (‎2021)‎. The selection and use of essential in vitro diagnostics: report of the third meeting of the WHO Strategic Advisory Group of Experts on In Vitro Diagnostics, 2020 (‎including the third WHO model list of essential in vitro diagnostics)‎. World Health Organization. https://iris.who.int/handle/10665/339064