IgM tests present a high rate of false positive results. This can lead to misdiagnosis, inappropriate treatments, and lack of treatment for the true aetiology (Landry ML. 2016). In the following table, I have collected several well-documented cases of IgM tests falsely positive for an infectious disease. In most of these studies, the cause of the false positive result was found to be either another acute infection or autoantibodies, including rheumatoid factor (RF), an IgM that binds to the Fc region of IgG. So it might be important to determine the exact origin of a false positive IgM test, in cases where a diagnosis is hard to find: it could be the clue that ultimately leads to the true aetiology.
|IgM falsely positive for:||True aetiology||N. of cases||Reference|
Nombre virus IgM
|Adenovirus||1||Landry ML. 2016|
|Measles virus IgM||Sulfa drug allergy||1||Landry ML. 2016|
|HAV IgM||CHF||1||Landry ML. 2016|
|HEV IgM||HAV (IgM+)||1||Landry ML. 2016|
|HSV IgM||VZV (IgM+)||1||Kinno R. et al. 2015|
|VZV (IgM+)||11/50||Ziegler T. et al. 1989|
|Parvovirus B19 (acute)||5/65||Costa E. et al. 2009|
|RF||9/50||Ziegler T. et al. 1989|
|RF||1||Pan J. et al. 2018|
|Anti-HDF||2/50||Ziegler T. et al. 1989|
|HSV-2 IgM||HSV-1||1||Landry ML. 2016|
|VZV||Anti-HDF||5/74||Ziegler T. et al. 1989|
|HSV (IgM+)||8/54||Ziegler T. et al. 1989|
|RF||8/54||Ziegler T. et al. 1989|
|EBV VCA IgM||CMV (IgM+)||1||Landry ML. 2016|
|CMV (IgM+)||7/50||Aalto MS et al. 1998|
|B. burgdorferi (IgM+)||2||Pavletic A. Marques AR. 2017|
|HEV (IgM+)||33,3%||Hyams C et al. 2012|
|CMV IgM||HEV (IgM+)||24,2%||Hyams C et al. 2012|
|Anti-HDF||10/75||Ziegler T. et al. 1989|
|RF||3/75||Ziegler T. et al. 1989|
|WNV IgM||HSV-2||1||Landry ML. 2016|
|B. burgdorferi IgM
(OspC and/or BmpA)
|HSV 2 (IgM+)||1||Strasfeld L. et al. 2005|
|VZV (acute)||5/12||Feder HM. et al. 1991|
|EBV (acute)||14/58||Goossens HA. et al. 1998|
|CMV (acute)||13/58||Goossens HA. et al. 1998|
|Mycoplasma IgM||WNV (IgM+)||1||Landry ML. 2016|
List of abbreviations. CHF, congestive heart failure; CHIK virus, Chikungunya virus; CMV, cytomegalovirus; EBV, Epstein-Barr virus; HAV, hepatitis A virus; HDF, human diploid fibroblast cells; HEV, hepatitis E virus; HIV, human immunodeficiency virus; HHV-6, human herpesvirus type 6; HSV, herpes simplex virus; RF, rheumatoid factor; VZV, varicella-zoster virus; WNV, West Nile virus.
Cross-reactivity with other pathogens
One possible cause for false positive results is cross-reactivity between antigens that belong to different pathogens. A little-known example of this phenomenon comes from the research on ME/CFS: in the study that ultimately ruled out the involvement of XMR virus in the pathogenesis of ME/CFS, antibodies to that pathogen were found in about 6% of both cases and healthy controls, whereas the molecular testing turned out to be negative in all participants (Alter HJ. et al. 2012). So, sera reactivity to XMRV is likely due to a relatively common pathogen that has an antigen similar to another one belonging to XMRV.
In one case of false positive HSV IgM due to VZV infection, the serum/CSF IgM ratio as a function of time had the same profile for both the viruses, suggesting cross-reactivity (Kinno R. et al. 2015). Cross-reactivity between HSV IgM and VZV IgM seems quite common, with both false positive HSV samples due to reactivity to VZV and false positive VZV IgMs due to IgM against HSV (Ziegler T. et al. 1989).
Interestingly enough, although OspC is considered to be a highly specific antigen of B. burgdorferi, OspC IgM is often positive in patients with active EBV or CMV infections (Goossens HA. et al. 1998). If cross-reactivity was responsible for false positive OspC IgM in infectious mononucleosis, we would expect false positive IgM for EBV and CMV in early Lyme disease. And this is is exactly what has been found in two cases of acute Lyme disease, where falsely positive IgM to VCA has been documented (Pavletic A. Marques AR. 2017).
Latent infections reactivation
It has been described a rise of EBV VCA IgM titers in CMV primary infections. This is likely due to EBV reactivation in many cases. This can lead to a misdiagnosis of a primary EBV infection, instead of a primary CMV infection. This error can have serious consequences during immune suppression or pregnancy, when CMV infections are health-threatening (Aalto MS et al. 1998).
Rheumatoid factor interference
As mentioned in the introduction, rheumatoid factor (RF) is an autoantibody – mainly of the IgM subclass – that is found in most of the patients with rheumatoid arthritis (Hermann E. et al. 1986). It binds the constant region (Fc region) of human IgGs and thus can bind the enzyme-linked immunoglobulins often used in serologic assays, leading to falsely positive results (Pan J. et al. 2018).
Cross-reactivity with autoantigens
Another possible cause of falsely positive IgM tests to a pathogen is the presence of autoantibodies other than RF. Autoantibodies to fibroblast cells have been found to be the cause of a false positive IgM test for HSV and CMV (Ziegler T. et al. 1989). This kind of reactivity to self-antigens is probably non-specific of a particular autoimmune disease, and it has been found for instance in pretibial myxedema, Graves’ disease, and Hashimoto’s thyroiditis (Arnold K. et al. 1995).
The effect of prevalence on the rate of false positive results
The likelihood of a false positive result is inversely correlated with the prevalence of the pathogen in the specific population considered. In other words, the rarer the disease, the more likely a positive test for that disease is a false positive. This can be easily seen introducing the predictive positive value (PPV), which is the probability that a positive test is really a true positive (Lalkhem AG. et McCluskey A. 2008). PPV is given by
In the following figure, you can see how PPV increases as the prevalence increases. This diagram has been plotted considering a sensitivity of 67% and a specificity of 53%.