Antiphospholipid Antibodies

Antiphospholipid syndrome (APS) is a systemic autoimmune disease characterized by arterial, venous, or microvascular thrombosis, pregnancy morbidity, or nonthrombotic manifestations in patients with persistent antiphospholipid antibodies (aPL).

In 2023, the American College of Rheumatology (ACR) and the European Alliance of Associations for Rheumatology (EULAR) published an update to the classification of APS.¹ The new criteria include 1 positive aPL test within 3 years of identifying an aPL-associated clinical criterion, followed by additive weighted criteria (score range 1-7 points each) clustered into 6 clinical domains and 2 laboratory domains (Table). Patients accumulating at least 3 points each from the clinical and laboratory domains are classified as having APS.

Compared to the 2006 revised Sapporo classification criteria,² the 2023 ACR/EULAR APS classification had an improved specificity of 99% versus 86% but a reduced sensitivity of 84% versus 99%, respectively.

In conjunction with other clinical findings, these tests help diagnose APS and are used for patients with symptoms of APS that may be secondary to systemic lupus erythematosus (SLE) or SLE-like disorders.^4-6

The assays detect IgG, IgM, and IgA antibodies to cardiolipin and to β₂-glycoprotein I (β2GPI) in either serum or citrated plasma. Results for cardiolipin antibodies are reported in GPL-U/mL, MPL-U/mL, and APL-U/mL for IgG, IgM, and IgA antibodies, respectively.^4-6 Results for β2GPI antibodies are expressed in U/mL for IgG, IgA, and IgM. 

The detection of these antibodies is based on a multiplex flow immunoassay, which involves the use of magnetic beads coupled with an antigen. Results are not equivalent to ELISA-based assay cutoffs for moderately positive (40-79 units) and high positive (>80 units) results cited in the 2023 ACR/EULAR APS criteria.

However, multiplex flow immunoassays may help screen for APS using the original 2006 revised Sapporo classification criteria² vs the 2023 ACR/EULAR APS criteria guidelines¹ given their higher sensitivity in detecting APS (99% vs 86%, respectively). In addition, it should be noted the 2023 ACR/EULAR APS classification criteria are intended for clinical trials and research use only.

Yes, assay cutoffs were established with receiver operating characteristic (ROC) analysis and comparison against commercially available microplate enzyme immunoassays (EIAs) using specimens from patients with APS and healthy blood donors.^4-6 This analysis (performed by the manufacturer) established the assay cutoff at the 99th percentile of a healthy, normal population.^4-6  

The current international consensus statement for the classification criteria of APS suggests confirming positive results after 12 weeks to rule out transient antibodies, which have been described in infectious diseases and are not of clinical significance.² For example, antiphospholipid antibodies, such as lupus anticoagulant and low-titer cardiolipin antibodies, may be transient in children with an acute viral illness.²

Not always, although the correlation is stronger with higher cardiolipin IgG antibody levels. In addition, triple antiphospholipid positivity (ie, positivity for cardiolipin antibody, β2GPI antibody, and lupus anticoagulant) correlates more strongly with both thrombosis and pregnancy morbidity than does single or double positivity.⁷

Anticoagulants do not interfere with results of the cardiolipin and β2GPI antibody testing. In contrast, depending on the level and type of anticoagulant, lupus anticoagulant testing may or may not be affected.^8,9

A thrombotic event will not likely mask an antiphospholipid antibody, because these antibodies are tested on an immunoassay-based platform. However, a positive antibody titer may be detected for the first time after the thrombotic event; thus, testing should be repeated ≥12 weeks after the event to demonstrate antibody persistence.²

The IgA isotype has been implicated in small studies but has not yet been incorporated into the APS diagnostic criteria.² On a very limited basis, IgA has been reported as present in African American patients with SLE. Although this is mentioned as a possible connection in the APS diagnostic criteria,² without further study, it is not known whether this indicates a subgroup of patients with a nonspecific autoimmune disorder who are at risk for specific clinical manifestations.

Phosphatidylserine/prothrombin (PS/PT) antibodies have been associated with antibody-mediated thrombotic events, but they have not been incorporated into the classification criteria for APS.¹ However, because of a high concordance between PS/PT antibodies and the presence of a lupus anticoagulant, testing for PS/PT antibodies may be used as a surrogate for lupus anticoagulant testing if a patient is on an anticoagulant that could potentially cause false-positive lupus anticoagulant (LA) results.¹⁰

In addition, testing for PS/PT may also support the diagnosis of triple-positive APS patients (ie, positivity for cardiolipin antibody, β2GPI antibody, and lupus anticoagulant), and potentially identify a falsely negative LA in double-positive patients (cardiolipin antibody and β2GPI antibody).¹⁰ It should be noted that isolated PS/PT IgM antibodies that are near the cutoff for positivity often reflect an underlying acute-phase response (eg, to an infectious agent) rather than a true positive result and should be correlated with clinical thrombotic manifestations for clinical relevance.

Note: PS/PT antibodies are detected using ELISA methodology and do not have anticoagulant interference.