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Laboratory Testing in the Identification of Antiphospholipid Syndrome
Laboratory Testing in the
Antiphospholipid syndrome (APS) is an autoimmune disease characterized by one or both of the following: 1) vascular thrombosis that may be arterial, venous, or both; and 2) pregnancy morbidity.1 Other features include cardiac valve disease, cerebral ischemia, stroke, renal nephropathy, livedo reticularis, and thrombocytopenic purpura.1 Although the prevalence of APS in the general population is unknown, the disease affects 5% to 15% of patients who have recurrent arterial and venous thrombosis.2,3 APS may be more prevalent in women than in men, with onset typically occurring in the fourth or fifth decades of life.4 APS criteria have also been associated with stroke in young adults, who often lack other stroke risk factors.3
Laboratory Identification of APS
According to the 2006 international consensus statement on APS classification criteria, identification of APS requires the presence of vascular thrombosis and/or pregnancy morbidity, along with at least 1 of the following antiphospholipid antibodies: lupus anticoagulant (LA) antibodies, cardiolipin IgG and IgM antibodies, and β2-glycoprotein I (β2-GPI) IgG and IgM antibodies.1 Assays for the 3 antibody types are often performed in parallel. Detection of more than 1 antibody type is associated with more severe APS than the detection of a single antibody.2 If none of the 3 antiphospholipid antibodies are detected, APS is unlikely.1
Lupus anticoagulant and β2-GPI antibodies appear to be more specific and less sensitive than cardiolipin antibodies for identification of APS.1,3,4 Cardiolipin and β2-GPI IgM and IgA antibodies are less strongly associated with APS than are IgG antibodies.1,6,7,11 Cardiolipin and β2-GPI IgA antibodies may only be associated with thrombosis in specific patient subgroups and are excluded from the international consensus APS laboratory criteria.1 However, in the absence of definitive studies showing that cardiolipin and β2-GPI IgM and IgA antibodies are not associated with APS, all 3 isotypes (IgG, IgM, and IgA) are routinely assayed in APS diagnostic testing.7 The algorithm in the Figure shows first-line serologic testing for classification of APS, based on the 2006 international consensus classification criteria.1
Additional, non–first-line, serologic markers that have been associated with APS symptoms in limited studies include cardiolipin IgA, β2-GPI IgA, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylcholine, phosphatidylglycerol, phosphatidic acid, prothrombin, and annexin V antibodies. These antibodies are excluded from the classification criteria to preserve diagnostic specificity.1 However, they may be used to identify patients who are at risk of developing APS clinical symptoms but who test negative for LA, cardiolipin, and β2-GPI antibodies.2 For example, phosphatidylserine antibodies are associated with stroke and both phosphatidylserine and phosphatidylinositol antibodies are associated with pregnancy morbidity.8,9 Annexin V antibodies have also been linked to recurrent pregnancy loss.10 Some cases of APS are seronegative—they lack detectable antiphospholipid antibodies and cannot be identified by laboratory testing.7
First-line and non–first-line antibody tests used for APS classification are presented in the Table.
|Content reviewed 12/2011|
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