Discussion
APS is an autoimmune disease featuring thrombosis and/or pregnancy morbidity which may lead to severe consequences. Detection of aCL and aβ2GPI as the golden standard in APS diagnosis is not satisfactory in the clinical scenario, and various potential aPLs have been extensively explored.
In this study, the diagnostic value of IgA for aCL or aβ2GPI, IgG/IgM for aANxV or aPS/PT was evaluated in APS patients. In brief, 45.70% and 6.62% of patients with APS were positive for aCL or aβ2GPI IgA respectively, while 30.46% and 24.50% were positive for aAnxV or aPS/PT for at least one antibodies (IgG or IgM). Adding IgA to criterial aPLs could increase the sensitivity in APS diagnosis. Detection of aANxV or aPS/PT, especially aAnxV IgG, could add value to diagnosis. IgG of aANxV or aPS/PT was significantly associated with LA, and IgG aANxV was linked with stroke.
Analysis of the predictive power indicates that although aCL IgA had relatively low specificity, adding IgA to aCL IgG or IgM/aCL or aβ2GpI IgG or IgM test could increase test sensitivity (P < 0.001). The sensitivity (39.07% compared to 29.14%, P < 0.001). and accuracy (60.58% compared to 55.19%, P =0.007) of aAnxV IgG or IgM were both significantly higher than that of aβ2GPI IgG or IgM. Moreover, combination of aCL, aβ2GpI, or aAnxV IgG or IgM had significantly higher sensitivity (47.7% compared to 43.0%, P = 0.016) than that of aCL or aβ2GpI IgG or IgM. Statistic results suggested that adding aAnxV IgG or IgM to aCL or aβ2GpI IgG or IgM would both increase diagnostic value besides criterial antibodies. Meanwhile, there was no significant decrease in specificity (96.67%).
The result was further illustrated with ROC curves for each aPL and their combination. AUC of ACL IgA and aAnxV IgG ranked second and third (0.853 and 0.728) among individual aPLs. Addition of IgA, aAnxV or aPS/PT to aCL or aβ2GpI IgG or IgM would all increase their diagnostic power.
Venn diagram indicated the additive value of new aPLs from another perspective. Positive only for IgA isotype could point out an extra number of patients for both aCL (16, 10.6%) and aβ2GPI (4, 2.6%). Additionally, the number of patients positive for aAnxV IgG, aAnxV IgM, and aPS/PT IgM outperformed those of aβ2GPI, indicating their importance in APS clinical diagnosis. The result suggested that additional tests for extra criteria aPLs could provide unique value in the identification of SNAPS patients.
Besides predictive power, distribution, and comparison of aPLs among different patient groups were also examined. Between PAPS and SAPS, little significant difference was observed except for aCL IgM (p = 0.029) and aβ2GpI IgA (p = 0.043). Between PAPS and SLE, significantly higher titer of IgM aCL, IgA aCL, IgM aPS/PT, IgG AnxV, and IgM AnxV was observed (p<0.001). As for SAPS and SLE, only IgM aPS/PT showed a significant difference (p = 0.015). The results implied that both criterial and non-criterial aPLs had difficulty in distinguishing APS from SLE or APS secondary to SLE. Indeed, baseline information suggested little difference between PAPS and SAPS patients in age and most clinical manifestations (Table 1). It had been estimated in previous studies that around 40% of patients with SLE have aPL, and APS may develop in up to 50-70% of patients with both SLE and aPL(29). Nevertheless, levels of IgG for four aPLs were significantly higher in both PAPS and SAPS group compared to HC, which suggested their diagnostic value.
Finally, the relationship between aPLs and related clinical manifestations was calculated. In this study, no significant association was found between aPLs with any thrombotic events, which was contradictory with results from some previous studies conducted in the Chinese population (24-26). Concerning obstetric complication, aPS/PT IgM was reversely associated with pregnancy loss in women (ORs 0.6, 95% CI 0.5-0.7), which also showed conflicting results (25, 30, 31). For aAnxV, similar to a previous study, no significant relationship was observed(24). The different results might be due to the detection system. ELISA was chosen in this study, and the cut-off value provided by the manufacturer (18 U/ml for all the aPLs) may not reflect real aPL distribution in local population. Indeed, as illustrated in figure 2, 31 patients were negative for all IgG, while as many as 118 patients were negative for all IgM. It could be more suitable if 99th percentile strategy was adopted first to identify cut-off points for each aPLs.
Additionally, the relationship between aAnxV and aPS/PT IgG and LA was confirmed in our studies, and LA was found to be associated with IgG of all four aPLs. Regarding microangiopathy, a series of manifestations had been recorded for the patients (stroke, deep venous thrombosis, pulmonary embolism, etc.), and significant relationship with aPLs (aβ2GPI IgG and aPS/PT IgG) was present for stroke. Previous review has estimated an aPL positivity of 17% in patients with juvenile stroke (<50 years of age) (32). Although detection of aPS/PT alone may have less diagnostic value, it would still be valuable in risk prediction for and prevention of adverse clinical events.
This study has some limitations. Compared to similar studies, the sensitivity for autoantibodies is not very high, which may influence the results of sequence comparison. Since different detection methods and manufacturers vary greatly in antibody measurement, contradictory results could arise(33). In the future, quantitative/semi-quantitative detection methods such as chemiluminescence analysis (CLIA) could be applied to reduce systemic detection error. In addition, both patients and health individual involved in the study were relatively homogenous, and may not reflect real-life condition. A larger sample size and inclusion of patients with a wider range of associated diseases or clinical features could further complement the study.