2. Biomarkers

A biomarker can be defined as an alteration in a cellular, molecular, genetic, epigenetic, biochemical, biological, or other body events that specifically accompanies a disease or condition and is amenable for quantitative and qualitative analysis. Therefore, a biomarker can be used for diagnostic, prognostic, and theranostic purposes, and the more specific it is for a disease, the more reliable it becomes [5, 42]. In SLE, the search for novel and specific biomarkers is highly crucial because as a disease of a huge range of clinical and serological manifestations, it is challenging in so many levels including accurate diagnosis, predicting disease progression, identification of disease flares, directing proper therapy, and for the discovery of new treatments [5]. Owing to this tremendous heterogeneity, it is therefore expected that no single biomarker for SLE can satisfy all the above purposes but the continuous efforts in understanding SLE pathogenesis should accumulate informative data for the discovery of novel and reliable biomarkers [43].

The most commonly employed SLE biomarkers are antinuclear antibodies (ANAs) and anti-DNA antibodies [43]. However, as will be described below, ANAs possess low specificity to SLE due to their coexistence in other autoimmune diseases as well as in some healthy individuals [44]. On the other hand, anti-DNA antibodies, despite being the most specific biomarkers for SLE [27], have poor predictive values as their levels do not always parallel disease activity [45]. Therefore, the search for novel biomarkers for SLE never ceased, and with the emergence of newer detection technologies and the advances achieved in understanding SLE pathogenesis, new biomarkers have emerged from collective efforts of genetic, epigenetic, transcriptomic, and proteomic studies as shall be described [46].
