AntiCardiolipin Antibody
Antiphospholipid syndrome (APS) is an acquired autoimmune thrombotic tendency that is identified by the presence of abnormal antiphospholipid laboratory tests in patients who have a history of recurrent arterial and venous thrombosis, recurrent fetal loss, thrombocytopenia, neurologic events including transient ischemic attack (TIA) and stroke, and dermatologic disease, , primarily livedoreticularis.
In addition to testing for lupus anticoagulant, immunologic methods can be used to detect anticardiolipin antibody.
Diagnostic criteria for the antiphospholipid syndrome include both clinical and laboratory findings.
Clinical criteria
1. Vascular thrombosis One or more clinical episodes of arterial, venous or small vessel thrombosis
2. Pregnancy morbidity
(a) One or more unexplained deaths of a morphologically normal fetus at or beyond the 10th week of gestation
(b) One or more pre-term births of a morphologically normal neonate before the 34th week of gestation because of: (i) eclampsia or severe pre-eclampsia or (ii) recognized features of placental insufficiency
(c) Three or more unexplained consecutive spontaneous miscarriages before the 10th week of gestation, with maternal anatomic or hormonal abnormalities and paternal and maternal chromosomal causes excluded
Laboratory criteria
At least 2 positive tests for antiphospholipid antibody 12 weeks apart
1. Lupus anticoagulant (LA) present in plasma, on two or more occasions at least 12 weeks apart
2. Anticardiolipin (aCL) antibody of immunoglobulin (Ig)G and/or IgMisotype in serum or plasma, present in medium or high titre
(i.e. 40GPL units or MPL units, or the 99th centile), on two or more occasions, at least 12 weeks apart
3. Anti-b2–glycoprotein I antibody of IgG and/or IgMisotype in serum or plasma (in titre the 99th centile), present on two or moreoccasions at least 12 weeks apart
Antiphospholipid antibody syndrome (APS) is present if at least one of the clinical criteria and one of the laboratory criteria are met
IgG and IgMαCL are expressed in international standardized GPL and MPL units, respectively. GPL and MPL units are defined as binding observed with 1 μg of affinity-purified polyclonal IgG and IgMαCL
Direct measurement
ELISA (enzyme-linked immunosorbent assay)
Principles:
The three characteristics of relevance for direct (i.e. ELISA) detected antiphospholipid antibodies are target, isotype and titre.
To be significant the antiphospholipid antibodies must be:
a) Directed against either cardiolipin (aCL) or β2-glycoprotein-I (anti-β2GPI)
b) IgG and/or IgM
c) Present in medium or high titre (i.e. >40 GPLU or MPLU for aCL or >the 99th percentile for either aCL or anti-β2GPI.
Cardiolipin is found almost exclusively in the inner mitochondrial membrane where it performs an essential role in regulating enzymes involved in mitochondrial energy metabolism. The term 'cardiolipin' originates from the fact that it was first isolated from bovine heart in the early 1940s and formed the basis of the Wasserman test for syphilis.
β2-glycoprotein I (also known as Apolipoprotein H) is a multifunction protein which, in addition to binding to cardiolipin and inducing functionally relevant conformational change, also interferes with platelet aggregation by inhibiting serotonin release and interferes with various steps in the coagulation pathway.
The subset of antibodies that bind β2GPI correlate strongly with the thrombotic complications of the APS.
Indirect measurement
Through their effect on phospholipid-dependent coagulation assays
Principles:
Antiphospholipid antibodies can affect the phospholipid used in some laboratory tests of coagulation (e.g. the APTT) and produce a prolonged clotting time. This effect may be overcome by adding sufficient excess of phospholipid that the antibody is overwhelmed and the in vitro clotting time shortens.
Demonstration of the presence of an antiphospholipid antibody by coagulation tests requires:
a) Prolongation of a phospholipid dependent coagulation testAND
b) Correction of that prolongation by adding an excess of phospholipid or otherwise eliminating the effect of any APLOR
c) comparison to a phospholipid independent confirmatory test.
The tests used for this purpose include:
a) Dilute Russell Viper venom test (DRVVT)
b) Silica clotting time (SCT)
c) Kaolin clotting time (KCT)
d) The Textarin/Ecarin time
e) Taipan venom time (TVT)
f) Factor V ratio
No one test will detect all lupus anticoagulants and for any given test detection may vary between manufacturers. At least two different tests should therefore be used.
- Anticardiolipin antibody and lupus anticoagulant tests are discordant in approximately 30% of patients. Positive results with either of these assays should be regarded as independent risk factors for a thrombotic tendency.
- Positive results with both tests do not indicate a greater risk of thrombosis than a single positive test.
- The anticardiolipin antibody test is more useful than the lupus anti-coagulant to follow up the patients undergoing immunosuppressive treatment.
- A close relationship has been established between thrombotic events and the presence of anticardiolipin antibodies (ACA).
There is strong evidence linking ACA with venous thromboembolism in patients with SLE, however evidence linking ACA with venous thromboembolism (VTE) in patients without SLE is limited.
It has also been suggested that non-SLE patients with ACA and VTE are resistant to usual intensities of warfarin therapy, which has prompted more intense anticoagulation of these patients by many physicians.
IgA phospholipids (cardiolipin) antibody result >15 APL with negative IgG and IgM phospholipids (cardiolipin) antibody results are not diagnostic for APS.
Detection of phospholipid (cardiolipin) antibodies is not affected by anticoagulant treatment.
Reference range is 0 - 20 GPL units and 0 - 10 MPL units.
Specimen requirement is one SST tube of blood
COMMENTS
1. It is important to remember that APLs may be transiently detected after illness, especially infection and no conclusions should be drawn until a second test has been performed.
Transient antiphospholipid antibodies are only very rarely associated with thrombosis.
2. APS may occur in association with other disorders (secondary APS) most notably autoimmune disorders, especially systemic lupus erythematosis [SLE] and scleroderma.
3. Individuals with both a lupus anticoagulant and moderate/high titre anticardiolipin antibodies show a greater risk of thrombosis than those in whom only a single test is positive.
4. The detection of a LA in patients receiving vitamin K antagonists [VKAs] can be difficult. Discontinuation of the VKA for 1-2 weeks and when the INR is <1.5 and then screening for a LA is recommended. Alternatively if the INR is between 1.5-3.0 a 1:1 mix of patient plasma and normal plasma can be made and then used to screen for a LA. However the interpretation of results may be still difficult and a weak LA that may be clinically significant may be missed by the dilution. Other tests e.g. Textarin/Ecarin clotting times can be used although these are not currently recommended by the ISTH SSC working party on Lupus Anticoagulants.
5. The effect of LMWHs on screening for a LA is variable. Some commercial kits for screening for a LA contain heparin-neutralising reagents e.g. polybrene.
Reference
- Levine JS, Branch DW, Rauch J. The antiphospholipid syndrome. N Engl J Med. 2002;346:752-763
- David Keeling,IanMackie,Gary W. Moore,Ian A. Greer,Michael Greaves and British Committee for Standards in Haematology Guidelines on the investigation and management of antiphospholipid syndrome British Journal of HaematologyVolume 157,Issue 1,pages 47–58,April 2012
- Bill Giannakopoulos, Freda Passam, YiannisIoannou, and Steven A. Krilis How we diagnose the antiphospholipid syndromeBlood2009113:985-994
- Bill Giannakopoulos, Freda Passam, SoheilaRahgozar, and Steven A. Krilis Current concepts on the pathogenesis of the antiphospholipid syndrome Blood2007109:422-430
- Danowski A, Rego J, Kakehasi AM, Funke A, Carvalho JF, Lima IV, Souza AW, Levy RA.Guidelines for the treatment ofantiphospholipidsyndrome.Rev Bras Reumatol. 2013 Apr;53(2):184-92.
- Rand JH, Wolgast LR.Dos and don'ts in diagnosing antiphospholipidsyndrome. Hematology Am Soc HematolEduc Program. 2012;2012:455-9
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