Standardization of Assays That Detect Anti-Rubella Virus IgG Antibodies

Abstract

SUMMARYRubella virus usually causes a mild infection in humans but can cause congenital rubella syndrome (CRS). Vaccination programs have significantly decreased primary rubella virus infection and CRS; however, vaccinated individuals usually have lower levels of rubella virus IgG than those with natural infections. Rubella virus IgG is quantified with enzyme immunoassays that have been calibrated against the World Health Organization (WHO) international standard and report results in international units per milliliter. It is recognized that the results reported by these assays are not standardized. This investigation into the reasons for the lack of standardization found that the current WHO international standard (RUB-1-94) fails by three key metrological principles. The standard is not a pure analyte but is composed of pooled human immunoglobulin. It was not calibrated by certified reference methods; rather, superseded tests were used. Finally, no measurement uncertainty estimations have been provided. There is an analytical and clinical consequence to the lack of standardization of rubella virus IgG assays, which leads to misinterpretation of results. The current approach to standardization of rubella virus IgG assays has not achieved the desired results. A new approach is required.

Commentary: Where to Now for Standardization of Anti-Rubella Virus IgG Testing

“…Although rubella IgG tests are calibrated using the World Health Organization (WHO) 1st International Standard for AntiRubella Immunoglobulin (RUBI-1-94) and report results in international units per milliliter, there is a lack of standardization of the results reported by different test kits (2,10). The consequence of this poor standardization is that a patient's results are dependent upon what test kit is used.…”

“…Both analytes have immune "cutoffs" based on these quantitative results; generally 10 IU/ml for anti-rubella IgG and 10 mIU/ml for anti-HBs antigen. Therefore, in the case of some anti-rubella IgG tests, a result of 9.9 IU/ml is deemed negative and a result of 10.1 IU/ml is deemed positive (2). Other test kit manufacturers provide a "gray zone" or equivocal range of about 10% around the cutoff.…”

“…Preferably testing should occur prior to pregnancy, but usually it is performed at the first antenatal visit. A negative or lowpositive result may indicate the need for vaccination, whereas a positive result indicates that the individual has detectable antibodies and therefore is protected against infection (2).…”

“…Only recently has a concerted effort been undertaken to understand, document, and address the issue. Recently, a comprehensive review of standardization of rubella IgG tests was published (2) in an attempt to better understand the reasons for this situation, to highlight the consequences of poor standardization, and to suggest future activities that may alleviate the problem. Indeed, the lack of standardization is not confined to rubella IgG tests but is also experienced in anti-hepatitis B virus surface (antiHBs) antigen and other serological test kits calibrated against an international standard.…”

“…The level assigned to this standard (1,600 IU) was initially measured using test systems that are no longer commonly used: hemagglutination inhibition (HAI), radial hemolysis (RH), and in-house enzyme immunoassays (EIA) (2). HAI and RH tests detect both IgG and IgM, are labor intensive, and are subject to high levels of variability.…”