The quantitative measurement of anticardiolipin (aCL) antibody is important in diagnosing antiphospholipid syndrome (APS).
According to the Second International Anti-cardiolipin Standardization Workshop, the reference ranges of serum aCL are as follow: 
Low positive: Fewer than 20 G phospholipids (GPL), M phospholipids (MPL), A phospholipids (APL) units
Medium positive: 20 or more but fewer than 80 GPL, MPL, APL units
High positive: 80 or more GPL, MPL, APL units
The presence of serum anticardiolipin (aCL) antibody without clinical symptoms does not support a diagnosis of antiphospholipid syndrome (APS). A positive result supports the diagnosis only in the presence of thrombosis or fetal loss. In addition, the test results must be positive at least 2 occasions 12 or more weeks apart to validate the diagnosis.
A higher aCL titer indicates more frequent clinical manifestations among individuals with APS. 
Collection and Panels
Specifics for collection of anticardiolipin (aCL) antibody are as follows:
Container: Red-top tube (serum)
Specimen preparation: Transport minimum of 0.4 mL of serum to transport tube
Storage/transport temperature: Refrigerated at 2-8°C
Unacceptable conditions: Plasma, grossly visible hemolysis and lipemia
Stability (collection to initiation of testing [after separation from cells]): Refrigerated or frozen for 2-3 days
Method: Enzyme-linked immunoassay (ELISA) 
Antiphospholipid syndrome (APS) is an autoimmune disorder characterized by recurrent thrombosis and pregnancy morbidity in the presence of antiphospholipid antibodies (APLA). APS is diagnosed when at least one requirement from both clinical and laboratory criteria are met.
Clinical criteria include the following:
Vascular thrombotic episodes in any tissue or organ
Pregnancy loss (≥1 unexplained loss of a normal fetus beyond the 10th gestational week, ≥1 premature birth before the 34th gestational week due to eclampsia or placental insufficiency, or ≥3 spontaneous abortions before the 10th gestational week)
Laboratory criteria include the following:
Lupus anticoagulant (LA) present in serum
Anticardiolipin (aCL) antibody of immunoglobulin G (IgG) and/or immunoglobulin M (IgM) isotype present in serum (>40 GPL or MPL units, or above the 99th percentile)
Anti–beta2 glycoprotein-I (b2-GPI) antibody of IgG and/or IgM isotype (in titer above the 99th percentile) in serum 
Although not included in the diagnostic criteria, other clinical symptoms, such as livedo reticularis, nephropathy, thrombocytopenia, cardiac valvular disease, and neurological symptoms are commonly associated comorbidities. 
An anionic side of the phospholipids that constitutes the cell membrane normally faces the cytoplasm. In the setting of initial injury, the anionic surface is exposed to the exterior. aCL binds to the anionic phospholipid on the platelet membrane and to the b2-GPI free flowing in the serum. Deposition of this aCL immune complex causes additional exposure of anionic phospholipids and platelet activation by unidentified signal transducing mechanisms. This cascade eventually leads to the formation of thrombosis. APLA refers to other prothrombotic antibodies that behave similarly to aCL. LA and anti–b2-GPI are among the APLAs. 
aCL testing is performed as part of the workup of excessive clotting episodes, along with LA and anti–b2-GPI antibody tests.
In addition, aCL testing is indicated in certain clinical settings.
aCL testing is warranted in young patients with unexplained arterial or venous thrombosis who lack evidence of atherosclerosis or emboli or develop thrombosis at unusual sites.  History of pregnancy loss also justifies aCL testing.
Specific criteria that link pregnancy complications to APS include one or more unexplained death of a morphologically normal fetus beyond the 10th week of gestation, one or more premature births before the 34th gestational week due to eclampsia or placental insufficiency, and three or more consecutive spontaneous abortions before the 10th gestational week. 
aCL assay is also indicated in cutaneous circulation disturbances without clear explanations, such as blue toe syndrome, ulcers resembling pyoderma gangrenosum, and livedo reticularis.
In addition, patients with systemic lupus erythematosus, autoimmune thrombocytopenia, or autoimmune hemolytic anemia should undergo aCL testing since APS can result from these disorders. 
Nonbacterial thrombotic endocarditis without malignancy should also prompt aCL testing for possible APS. 
Although the IgA isotype of aCL is tested together with IgG and IgM isotype, it is not included in the diagnostic criteria of APS since the prevalence of IgA among patients with APS patients is significantly lower than that of IgG/IgM.  However, one study found that the IgA aCL was the only isotype present in 82% of aCL-positive Afro-Caribbean patients.  The level of IgG aCL demonstrates stronger association with the clinical symptoms of APS. Thrombosis, recurrent fetal loss, and thrombocytopenia were found to occur more frequently at higher titer of IgG (>35 GPL units) than the others. 
Although aCL testing has high sensitivity for APS, it has relatively lower specificity, especially when the measured value is low positive (< 20 GPL, MPL, or APL units). Various conditions are associated with low positive results in aCL ELISA, including syphilis, Q fever, hepatitis C, AIDS, tuberculosis, connective-tissue diseases, and drug-induced disorders. In fact, LA reaction is regarded as a more specific test for APS since it is found much less commonly in other disorders.  Additionally, the presence of IgM rheumatoid factor can produce false positive IgM aCL. 
Despite attempts to standardize the testing and reporting practices, multi-laboratory comparative studies and quality assurance programs indicate that different laboratories may report substantially variable aCL values even from the same sample.