Rheumatology (Oxford)
The reliability of immunoassays to detect autoantibodies in patients with myositis is dependent on autoantibody specificity.
Abstract
Objectives
In order to address the reliability of commercial assays to identify myositis-specific and -associated autoantibodies, we aimed to compare the results of two commercial immunoassays with the results obtained by protein immunoprecipitation.
Methods
Autoantibody status was determined using radio-labelled protein immunoprecipitation for patients referred to our laboratory for myositis autoantibody characterization. For each autoantibody of interest, the sera from 25 different patients were analysed by line blot (Euroline Myositis Antigen Profile 4, EuroImmun, Lübeck, Germany) and dot blot (D-Tek BlueDiver, Diagnostic Technology, Belrose, NSW, Australia). Sera from 134 adult healthy controls were analysed.
Results
Overall commercial assays performed reasonably well, with high agreement (Cohen's κ >0.8). Notable exceptions were the detection of rarer anti-synthetases with κ < 0.2 and detection of anti-TIF1γ, where κ was 0.70 for the line blot and 0.31 for dot blot. Further analysis suggested that the proportion of patients with anti-TIF1γ may recognize a conformational epitope, limiting the ability of blotting-based assays that utilize denatured antigen to detect this clinically important autoantibody. A false-positive result occurred in 13.7% of samples analysed by line blot and 12.1% analysed by dot blot.
Conclusion
The assays analysed do not perform well for all myositis-specific and -associated autoantibodies and overall false positives are relatively common. It is crucial that clinicians are aware of the limitations of the methods used by their local laboratory. Results must be interpreted within the clinical context and immunoprecipitation should still be considered in selected cases, such as apparently autoantibody-negative patients where anti-synthetase syndrome is suspected.
Related Questions
In a patient with low titer +anti-SAE antibody and known ILD, but no other clinical features of dermatomyositis, how would you approach further testing or would you treat the patient as dermatomyositis associated ILD?
When someone with ILD has an isolated biomarker without other clinical features associated with that biomarker, I have to ask myself these questions: first, is the biomarker simply a false positive because I have tested a plethora of biomarkers and second, is ILD the initial or only manifestation as...
What is your approach to elevated CK in patients without clinical or serologic evidence of immune mediated myopathy?
Thanks for your great question: This is especially important because we are much more likely to see patients sent for "elevated" CPK than patients with myositis.Unfortunately, I think that the answer to the approach depends a lot the circumstances, including (1) how elevated is the CPK, and (2) how ...
What is the negative predictive value of a negative extended myositis panel in dermatomyositis patients?
The short answer is "not great, but it depends" since the more nuanced answer is contingent on: the constellation of myositis-specific antibodies (MSA) included on the panel, the nitty-gritty of how the commercial lab is checking the MSA included (this detail can change between vendors, or even ...