4.1.1. Distribution of patients according to age and sex
There were 118 (92.97%) women in SLE patient group with an average age of 31.1 9.46, of which, the majority of patient were aged 16-45 (92.9%). These results are similar to previous researched results, suggesting that the primary epidemiological characteristics of SLE are more likely to occur in women of childbearing age.
4.1.2. Age of SLE onset
The majority of patients develop SLE at age of 15 - 45 (85.2%), average age of disease onset is 25.93 9.74. Many epidemiological studies have shown that SLE is more likely to develop in women of 15-45 age group, the period with the highest activity of female sex hormones.
4.1.3. Family history of SLE
Six SLE patients (4.69%) have at least one relative with SLE.
Many epidemiological studies have shown that the prevalence of SLE in relatives of SLE patients is usually 10 to 20 times higher than that of the general population (4-12%), suggesting the role of genetic factor in the pathogenesis of SLE.
4.1.4. SLE manifestations
The most common SLE manifestations are hair loss - 53.1%, nephropathy - 45.3%, acute/subacute cutaneous lupus - 38.3%, leukopenia - 38.2% and low complement - 62.2%. These results are quite consistent withthose of other authors, however, the prevalence of serositis, neurologic disorders and thrombocytopenia are relatively low, which may be due to differences in the choices of samples and differences in SLE clinical characteristics across races.
4.2. Prevalence and SLE diagnostic utility of autoantibodies 4.2.1. Antinuclear antibody (ANA)
Prevalence in SLE patients: 85.9% of SLE patients had positive ANA at the first visit. In previous studies, the prevalence of ANA in SLE patients ranged from 60 - 100%. The cause of this wide oscillation may be due to differences in the choice of samples, study design, testing techniques and positive cut points for antibody, the seroconversion of ANA.
SLE diagnostic utility: The sensitivity and negative predictive value of ANA were 85.94% and 72.31%, respectively, which is quite low compared to the previous study results. The cause of this difference may be due to the high proportion of SLE patients with negative ANA in this study (14.1%), the differences in the choice of controls and characteristics of these subjects. The specificity of ANA for diagnosis of SLE is quite low (46.15%) when SLE cases were compared with other autoimmune diseases, which is due to the fact that ANA may be found in many other autoimmune diseases.
4.2.2. Anti-dsDNA antibodies
Prevalence in SLE patients: 64.1% of SLE patients in this study have positive anti-dsDNA antibodies, which agrees with the previous study results (37-85%). The most significant factors influencing this value were the level of disease activity and organ damage of studied subjects, the testing technique and the cut-off point for the antibody evaluation.
SLE diagnostic utility: The specificity of anti-dsDNA antibodies in SLE diagnosis when SLE cases were compared with healthy controls and other autoimmune diseases was 93.33% and 97.44%, respectively. These values are quitely similar to previous
research results (95 - 100%). Contrary to the specificity, the sensitivity of anti-dsDNA antibodies to SLE is generally not high and widely variable (13-86%). The frequent fluctuation of this antibody in the course of disease and the different sensitivity of immunoassay techniques may be the cause of this variations.
4.2.3. Anti-C1q antibody
Prevalence in SLE patients: The prevalence of anti-C1q antibody in SLE patients widely ranged from 15 to 63% in the previous studies, our results were 25% at the first visit. In 3 large sample size studies of Julkunen H (2012), Mok CC (2010) and Orbai AM (2015), the prevalence of anti-C1q antibody in SLE patients were relatively low with little differences.
SLE diagnostic utility: The anti-C1q antibody has the worst value in SLE diagnosis with an area under the ROC curve of 0.676 and a sensitivity of 25%. The low prevalence and frequent fluctuations of anti-C1q antibody in SLE patients may be the main cause of decreased sensitivity of this antibody in disease diagnosis.
4.2.4. Antinucleosome antibodies (AnuA )
Prevalence in SLE patients: The prevalence of AnuA in SLE patients in this study was 81.3%. In previous studies, this value widely ranged from 45% to 100%, which was due to the differences in race and levels of disease activity of studied subjects.
SLE diagnostic utility: Many studies have shown high specificity (90-100%) of AnuA in SLE diagnosis, which agrees to our results (98.7%). The possible cause of this high specificity is that nucleosome is a self-antigen with an important role in the pathogenesis of SLE. The sensitivity of AnuA to SLE in this study was 81.25%, higher than that of anti-dsDNA antibodies (64.06%).
4.3. Relationship between autoantibodies with clinical characteristics and activity of SLE
4.3.1. Antinuclear antibody (ANA)
The positive ANA (≥ 1.2 OD) in SLE patients was significantly related to the occurrence of low complement and other autoantibodies. The association between this antibody with the clinical manifestations and activity of SLE in previous studies is unclear due to the lack of reliable evidences.
4.3.2. Anti-dsDNA antibodies
The positive anti-dsDNA antibody in SLE patients is significantly associated with acute/subacute cutaneous lupus, leukopenia and low complement, but not with lupus nephritis. The serum levels and prevalence of anti-dsDNA antibodies in the flares are higher than those in the stable periods of disease, however, this antibody is not constantly related to both SLE and lupus nephritis activity, especially with flares of disease.
4.3.3. Anti-C1q antibody
The positive anti-C1q antibody in SLE patients is significantly associated with the occurrence of nephropathy and low complement.
The specificity of this antibody in predicting lupus nephropathy is higher than that of the remaining antibodies and its concentration is closely inversely correlated with both C3 and C4 complement concentrations (p < 0.0001). The prevalence and mean concentration of anti-C1q antibody tends to increase with the level of disease activity. The correlation between anti-C1q antibody concentration with SLICC renal activity score and the specificity of this antibody in predicting lupus nephropathy are also better than the remaining antibodies.
4.3.4. Anti-nucleosome antibody (AnuA)
The positive AnuA in SLE patients is associated with chronic cutaneous lupus, nephropathy, leukopenia and low complement.
Sensitivity in predicting lupus nephropathy of AnuA is higher than that of remaining antibodies. The correlation with C3, C4 complement concentrations and SLEDAI score of this antibody is also closer than the remaining antibodies.