Based on the most recent criteria, classification with APS requires one clinical and one laboratory manifestation:
Clinical:A documented episode of arterial, venous, or small vessel thrombosis — other than superficial venous thrombosis — in any tissue or organ by objective validated criteria with no significant evidence of inflammation in the vessel wall
1 or more unexplained deaths of a morphologically normal fetus (documented by ultrasound or direct examination of the fetus) at or beyond the 10th week of gestation and/or 3 or more unexplained consecutive spontaneous abortions before the 10th week of gestation, with maternal anatomic or hormonal abnormalities and paternal and maternal chromosomal causes excluded or at least 1 premature birth of a morphologically normal neonate before the 34th week of gestation due to eclampsia or severe pre-eclampsia according to standard definitions, or recognized features of placental insufficiency
Laboratory:Anti-cardiolipin IgG and/or IgM measured by standardized, non-cofactor dependent ELISA on 2 or more occasions, not less than 12 weeks apart; medium or high titre (i.e., > 40 GPL or MPL, or > the 99th percentile)
Anti-β2 glycoprotein I IgG and/or IgM measured by standardized ELISA on 2 or more occasions, not less than 12 weeks apart; medium or high titre (> the 99th percentile)
Lupus anticoagulant detected on 2 occasions not less than 12 weeks apart according to the guidelines of the International Society of Thrombosis and Hemostasis.There are 3 distinct APS disease entities: primary (the absence of any comorbidity), secondary (when there is a pre-existing autoimmune condition, most frequently systemic lupus erythematosus, SLE), and catastrophic (when there is simultaneous multi-organ failure with small vessel occlusion).According to a 2006 consensus statement, it is advisable to classify APS into one of the following categories for research purposes:

I: more than one laboratory criterion present in any combination;
IIa: lupus anticoagulant present alone
IIb: anti-cardiolipin IgG and/or IgM present alone in medium or high titers
IIc: anti-β2 glycoprotein I IgG and/or IgM present alone in a titer greater than 99th percentileThe International Consensus Statement is commonly used for Catastrophic APS diagnosis.
These tests must be carried out on a minimum of two occasions at least 6 weeks apart and be positive on each occasion, demonstrating persistent positivity, to allow a diagnosis of antiphospholipid syndrome. This is to prevent patients with transient positive tests (due to infection etc.) being diagnosed as positive.Distinguishing a lupus antibody from a specific coagulation factor inhibitor (e.g. : factor VIII) is normally achieved by differentiating the effects of a lupus anticoagulant on factor assays from the effects of a specific coagulation factor antibody. The lupus anticoagulant will inhibit all the contact activation pathway factors (factor VIII, factor IX, factor XI and factor XII). Lupus anticoagulant will also rarely cause a factor assay to give a result lower than 35 iu/dl (35%) whereas a specific factor antibody will rarely give a result higher than 10 iu/dl (10%). Monitoring IV anticoagulant therapy by the PTT ratio is compromised due to the effects of the lupus anticoagulant and in these situations is generally best performed using a chromogenic assay based on the inhibition of factor Xa by antithrombin in the presence of heparin. Anticardiolipin antibodies
Anti-cardiolipin antibodies can be detected using an enzyme-linked immunosorbent assay (ELISA) immunological test, which screens for the presence of β2glycoprotein 1 dependent anticardiolipin antibodies. A low platelet count and positivity for antibodies against β2-glycoprotein 1 or phosphatidylserine may also be observed in a positive diagnosis.