Evaluation of fully automated assays for the detection of Rubella IgM and IgG antibodies by the Elecsys^® immunoassay system

Highlights

• The Elecsys Rubella assays were compared in four European clinical laboratories.

• Performance data of the Elecsys Rubella assays were evaluated.

• Low IgM reactivity was demonstrated in samples from previously infected patients.

• Low incidence of IgM false positives caused by potentially cross-reacting infections.

• Availability of results in approximately half the time required for employed assays.

Abstract

Screening for acute rubella infection in pregnancy is an important element of antenatal care. This study compared the sensitivity, specificity and reproducibility of two new, fully automated Elecsys^® Rubella IgM and IgG immunoassays designed for the Elecsys 2010, Modular Analytics E170, COBAS e-411 and COBAS e-601 and e602 analytical platforms, with current assays using serum from patients with primary rubella infections, vaccinated patients, patients with potentially cross-reacting infections and on routine samples in clinical laboratories in France, Germany and Italy. Both assays showed good within-run and within-laboratory precision. A sensitivity of 79.8–96.0% was demonstrated for Elecsys IgM in primary, early acute infection, consistent with existing assays. In samples obtained from routine antenatal screening, the Elecsys Rubella IgM assay revealed high specificity (98.7–99.0%). A significantly (p < 0.0001) lower reactivity was demonstrated in samples from previously infected patients where acute rubella infection was excluded, and the incidence of false positives in patients with potentially cross-reacting infections was lower with Elecsys Rubella IgM compared with other. The Elecsys Rubella IgG assay exhibited a relative sensitivity of 99.9–100.0% and specificity of 97.4–100.0% in samples from routine antenatal screening. The Elecsys Rubella IgM and IgG assays allow convenient, rapid and reliable determination of anti-rubella antibodies. Sensitivity, specificity and reproducibility were comparable with existing assay systems. Assay results were available in approximately half the time required for currently employed methods and the assays are compatible with widely used analytical platforms.

Introduction

Acute rubella infection is often associated with symptoms such as erythematous rash and low fever, and is regarded generally as a mild and transient infection. The consequences are more serious if infection occurs during pregnancy with an 80% risk of transmitting the virus to the developing foetus if infection occurs during the first trimester (Miller et al., 1982). Foetal infection may result in miscarriage or stillbirth, or in the child being born with congenital rubella syndrome (CRS). Children born with CRS may suffer a range of developmental problems including visual or auditory impairment, growth retardation and encephalopathy resulting in mental retardation (Best et al., 2004, World Health Organisation, 2000a).

The incidence of rubella varies from country to country and is influenced by the effectiveness of vaccination programmes. Over 120 countries incorporate rubella vaccination into their healthcare plans and have reduced the incidence of acute rubella infection and of CRS in newborns; between 1990 and 2008 immunisation and surveillance programmes coordinated by the Pan American Health Organisation resulted in a 98% reduction in the number of cases of rubella (World Health Organisation, 2000b). Coverage of populations is incomplete; however, even in developed countries with efficient programmes, and imported rubella cases amongst immigrant populations from countries without a vaccination programme remains a concern (Reef et al., 2002). Consequently, testing according to the national guidelines for acute rubella infection in pregnancy is an important element of antenatal care because of the potential risk of CRS associated with maternal infection, particularly in the first trimester (Banatvala and Brown, 2004, Edlich et al., 2005, Pustowoit and Liebert, 1998). On the other hand, in countries with a high level of vaccination, it is more likely that a positive rubella IgM will be due to a false positive rather than a true positive result. For this reason high specificity of rubella specific IgM assays in antenatal testing is absolutely necessary. Whilst there is no in utero treatment available, current guidelines recommend that infected pregnant women should be offered counselling as to the possible risk of vertical transmission and be offered pregnancy termination, especially if infection occurs prior to 16 weeks gestation (Dontigny et al., 2008).

Serology is the most common method of confirming the diagnosis of rubella. The presence of stable anti-rubella IgG antibody levels usually signifies existing immunity, due either to previous infection or to vaccination (Enders and Knotek, 1989, Skendzel, 1996) but can also indicate the later stages of a current acute infection, however, a single positive value for rubella IgG cannot discriminate between acute infection and existing immunity. Serial sampling is required to detect increasing levels of rubella-specific IgG which might be associated with acute infection. The presence of rubella-specific IgM in patients who have not received recent vaccination requires careful interpretation before it can be considered diagnostic of active infection, as rubella-specific IgM can persist for up to a year or more after natural infection or vaccination (Best et al., 2002). The presence of rubella-specific IgM may also be due to polyclonal stimulations of the immune system (World Health Organisation, 2000a, Cremer et al., 1984). Concurrent measurement of rubella IgM and clinical history will confirm whether an acute infection is present. Absence of detectable anti-rubella antibodies (either IgM or IgG) does not exclude the possibility of primary rubella infection in patients receiving immunosuppressive therapy or with immunosuppressive conditions such as HIV infection (Kaplan and Benson, 2009).

In this paper, the validation of two new, automated assays for rubella-specific IgM and IgG is reported for use in the routine testing of blood samples from subjects undergoing pregnancy screening or from pregnant women in antenatal care programmes. With these assays results are available after 18 mins, which is considerably shorter than currently available methods, and the assays are designed to run on the used widely Elecsys^® 2010, Modular Analytics E170, COBAS^® e411 and COBAS^® e601 and e602 analytical platforms and are easy to integrate into routine laboratory testing programmes. The Elecsys assays use non-infectious recombinant rubella-like particles (RLP) as antigen, as a validated alternative to authentic rubella virus (Pustowoit et al., 1996). RLPs contain the three structural proteins of the rubella virus and are morphologically and immunologically identical to wild type virions, however, the higher antigen concentration and improved antigen purity obtained with RLPs have been shown to improve the specificity and measuring range of RLP-based assays (Grangeot-Keros et al., 1995). In addition, the Elecsys Rubella IgG assay contains a recombinant fragment of the rubella envelope protein E1, which has been engineered to combine high solubility with excellent antigenicity (Scholz et al., 2008). In the present study, the Elecsys assays were run in parallel with currently available assays for the assessment of a range of defined and routine samples in independent clinical laboratories in Europe.