A qualitative or quantitative measurement of the level of IgG antibodies directed against cardiolipin, a phospholipid found in mitochondrial membranes and platelets. These antibodies are part of the antiphospholipid (aPL) group, a key laboratory marker for the diagnosis of antiphospholipid syndrome (APS). Their presence is associated with an increased risk of blood clots (venous or arterial) and pregnancy complications such as recurrent miscarriage.