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2242 Part XII: Hemostasis and Thrombosis Chapter 131: The Antiphospholipid Syndrome 2243

COAGULATION TESTS with normal plasma, the presence of an LA should be suspected, espe-
Lupus Anticoagulants cially if the patient does not have bleeding symptoms. The LA needs
to be differentiated from inhibitors of specific coagulation factors and
One of the most intriguing aspects of APS is the LA phenomenon. 288,289 from anticoagulants such as heparin. Besides specific assays to exclude
The various LA tests all report the inhibition of phospholipid-depen- the latter two possibilities, the clinician should check whether the aPTT
dent blood coagulation reactions, but by different detection methods. normalizes when an LA-insensitive aPTT reagent is used or when the
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These include modifications of the aPTT test with LA-sensitive and assay is performed using frozen washed platelets as the source of phos-
LA-insensitive reagents, the kaolin clotting time, the dRVVT, the tis- pholipid, a procedure referred to as the platelet neutralization procedure.
sue thromboplastin inhibition time, the hexagonal phase array test, and The effects of incubation with normal plasma may be helpful in differ-
the platelet neutralization procedure. Readers who are interested in entiating LAs from coagulation factor inhibitors. aPTTs performed on
the latest consensus recommendations on details of the procedures are mixtures of normal plasma and plasma containing a factor VIII inhibi-
directed to reference 257. tor usually show no prolongation immediately after mixing but marked
The results of LA tests can be so variable that even specialized lab- prolongation following incubation for 1 to 2 hours at 37°C, whereas LA-
oratories will disagree as to the results of LA tests. For example, three containing plasmas usually prolong the aPTT immediately after mixing
surveys in the United Kingdom have shown that although most labo- with normal plasma and show no further prolongation with incubation.
ratories agreed on identification of plasmas containing strong positive The clinician should be aware that both types of anticoagulants, LA and
LA activity, they frequently have disagreed about samples with a weak specific coagulation factor inhibitors, may coexist in rare patients and
LA activity. 286 yield confusing laboratory results. Specific coagulation factor inhibitor
Despite these limitations, the presence of a positive LA appears to assays and using an aPTT reagent that is insensitive to LA are helpful
be the strongest predictive diagnostic test for future thrombosis. In a for clarifying most of these cases. LAs may result in artifactual decreases
meta-analysis of the risk for aPL-associated venous thromboembolism in contact activation pathway coagulation factor assays, because these
in individuals with aPL antibodies without an underlying autoimmune assays are based on aPTT. Consequently, these patients are sometimes
disease or previous thrombosis, the mean odds ratios were 1.6 for aCL misdiagnosed as having multiple coagulation factor deficiencies. This
antibodies, 3.2 for high titers of aCL, and 11.0 for LA. In a system- problem can be handled by repeating the coagulation factor assays fol-
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atic literature review, 12 of 12 studies showed significant associations lowing dilution of the plasma samples; this usually results in complete
between LA and thrombosis, with odds ratios from 5.7 to 9.4. LA or partial normalization of coagulation factor levels with progressive
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increased the risks of arterial and venous events to the same extent. Pos- dilution. The use of an aPTT reagent that is insensitive to LA for specific
itivity for both LA and aCL, but not for aCL alone, predicted a higher factor assays is another way to solve this problem.
risk of recurrent thromboocclusive events in patients with first ischemic
stroke. In a prospective study of pregnant women, the PROMISSE
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study (Predictors of Pregnancy Outcome: Biomarkers in Antiphospho- Other Methods for Detecting Lupus Anticoagulant
lipid Antibody Syndrome and Systemic Lupus Erythematosus study), The dilute prothrombin time (dPT) (also known as the tissue throm-
LA was the primary predictor of adverse pregnancy outcome after boplastin inhibition test [TTIT]) is essentially a prothrombin time assay
12 weeks’ gestation in aPL-associated pregnancies.; aCL antibody and done with diluted tissue factor–phospholipid complex. It can be per-
anti-β GPI, did not predict adverse pregnancy outcome if LA was not formed with standard or recombinant tissue factor. 295,296 The results are
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also present. 291 expressed as a ratio of the patient-to-control clotting times.
In patients with SLE as well, the presence of LA activity is more The kaolin clotting time (KCT) depends on the ability of aPL anti-
predictive and more specific for the occurrence of thrombosis or preg- bodies to block the coagulant activity of trace amounts of phospholipid
nancy loss than aCL assays. This was also found in a meta-analysis of present in centrifuged plasma. Some authors maintain that the KCT–LA
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women without autoimmune conditions who had recurrent pregnancy test reflects dependence on prothrombin as a cofactor and is less likely
losses. 293 to be associated with thrombosis than the dRVVT, which appears to be
more dependent on β GPI. 297,298 The hexagonal phase array test is based
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on a prior idea that aPL antibodies recognize phosphatidyl ethanola-
Dilute Russell Viper Venom Time mine directly in the hexagonal phase array configuration but not in the
dRVVT is considered to be one of the most sensitive of the LA tests. The lamellar phase. Although this assay remains in use, the correction of
assay uses Russell viper venom (RVV) in a system containing limiting the prolonged clotting time with hexagonal phase phosphatidyl etha-
quantities of diluted rabbit brain phospholipid. RVV directly activates nolamine is probably similar to the confirmatory step used in the other
coagulation factor X, leading to formation of fibrin clot. LA prolongs LA assays, that is, a result of the excess of phospholipid in the reaction.
dRVVT by interfering with assembly of the prothrombinase complex; The textarin/ecarin test depends on the difference in phospho-
however, the prolongation is reversed by adding excess phospholipid lipid dependence of coagulation mechanisms triggered by two snake
to the reaction (sometimes referred to a “confirmatory test”). To ensure venoms: textarin, which activates prothrombin via a phospholipid-
that prolongation of the clotting time is not a result of a factor deficiency, dependent pathway, and ecarin, which activates prothrombin directly
the procedure includes mixture of patient and control plasmas. Antico- without phospholipid. 296
agulant therapy with heparin, warfarin, or direct thrombin inhibitors
can yield falsely abnormal test results. Annexin A5 Resistance Assay
In addition to the various LA tests, there is a coagulation test that reports
Activated Partial Thromboplastin Time Tests on a thrombogenic mechanism—resistance to annexin A5 anticoagu-
Prolongation of the aPTT detects some LAs, and prolonged aPTTs in lant activity. This assay has been correlated with an immunoassay for
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otherwise healthy individuals are most frequently caused by LAs. The IgG antibodies against domain I of β GPI. The assay has two stages: a
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294
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various commercial aPTT reagents vary widely with regard to sensitiv- first, in which a tissue factor-phospholipid suspension is exposed to test
ity to LA, so it is important to know the characteristics of the particu- plasma, and a second, in which the washed suspension is used to coag-
lar reagent(s) that is (are) being used. When the aPTT of a particular ulate pooled normal plasma in the presence and absence of annexin A5.
plasma sample is prolonged and not correctable by immediate mixture Patients with annexin A5 resistance show a less-than-expected annexin

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