This study has shown that IgM immunoblot is a valuable tool in the laboratory diagnosis of LB, allowing us to detect 110 patients with “Probable” or “Possible” acute LB over the initial 22.5-month study period that may otherwise have been missed with IgG immunoblot alone. Although false positive results were obtained with the IgM immunoblot, the estimated false positive rate of 48/188 (25.5%) was slightly lower than other recent studies [14, 15]. Introduction of IgM immunoblotting led to an increase in test costs of approximately £34,556. However, if more cases of acute LB are detected and treated early, significant cost savings to the health service could result. The increased risk of developing disseminated and late LB, along with the associated manifestations, in untreated patients has been well described [16,17,18]. Whilst repeat samples are routinely requested in patients with negative serology and recent onset, there is a clear potential for cases to be missed from follow-up. A 2010 study in the Netherlands found that the mean cost of disseminated LB and Lyme-related persisting symptoms was around 5700 Euros per case [19]. Although unlikely, if all 110 of the above patients were missed and progressed to disseminated LB/persisting symptoms, this could equate to 627,000 Euros as well as a huge personal cost.
Introduction of the IgM CLIA produced a much higher false positive rate. Of the 346 sera with an isolated IgM CLIA result, 277 (80.1%) were potentially false positive as they either did not confirm by immunoblot or were assessed as “Not consistent” with acute LB. This highlights that IgM CLIA testing in sera should only be used as part of a robust two-tiered testing protocol with confirmatory testing. The high false positive rate obtained for the IgM CLIA meant that a much higher proportion of samples required immunoblot testing: 22.9% of sera tested by CLIA (April to October 2020) were reactive with IgG and/or IgM CLIA, in contrast with the 8.5% of sera that were reactive with the IgG ELISA in the previous study period. This put extra pressure on the laboratory staff and greatly increased test costs (£28,933). It could be argued that the benefits of IgM CLIA are only marginal and perhaps not cost-efficient; however, 47 patients with probable or possible acute LB were detected, which may otherwise have been missed. Again, although unlikely, if all 47 were missed and progressed to disseminated LB/persisting symptoms, this could equate to 267,900 Euros.
Our results show that there is a significant risk of reporting of inaccurate and misleading results if patients are diagnosed on the basis of an isolated IgM positive result without consideration of clinical details, disease duration and pre-test probability. Prior studies have found that the majority of patients tested for Lyme serology did not meet European or UK clinical case definitions, and recommend that pre-test probability of infection is considered [5, 6, 20]. As a degree of seroprevalence for B. burgdorferi–specific antibodies exists in the population, over testing can lead to high false positive rates. The cost of misdiagnosing someone with acute LB based on false positive results, leading to their inappropriate, ineffective or even harmful treatment with antibiotics, and potentially delaying further investigations into the cause of their symptoms, should not be ignored.
The authors concede that the categorisation of patients into “Probable” or “Possible” LB in this study was flawed as there was limited clinical information available. For this reason, assay sensitivity, specificity and positive predictive values could also not be assessed. Many of the samples tested were not accompanied with adequate clinical details such as symptoms and date of onset. The authors accept this may have influenced the study false positive rate and whilst this may indicate that our testing protocols are suboptimal, this is the real-world scenario for a large number of laboratories where demand for Lyme testing is high and illustrates the challenges faced when interpreting results. The authors also recognise that the study results may have been influenced by the actual assays utilised. It is widely recognised that B. burgdorferi assays lack inter-assay consensus, particularly IgM assays [21, 22]. Interestingly, one of these studies calculated that a small loss of specificity led to an additional 192,716 immunoblot tests required (4,625,183 Euros), with an additional 6191 IgM false positive results.
Due to the issues outlined above, some countries have, or are considering, stopping the use of IgM testing for LB. However, some cases of early disease may be missed and patient confidence in testing regimes will undoubtedly be affected, which could further fuel the controversy around testing. Some manufacturers claim that IgG antibodies to the VlsE antigen of B. burgdorferi can be detected prior to or parallel to the formation of IgM antibodies and are more specific than IgM assays; thus, the use of VlsE/IgG screening assays is sufficient. However, the IgG CLIA used in this study, which contains the VlsE antigen, missed some cases of acute LB. Although complex to implement in laboratories with a high throughput of specimens, perhaps the use of selective testing protocols would be optimal, utilising IgM CLIA only for those patients with an acute onset of specific symptoms within a two-tier testing protocol.