Improved confirmation of weak lupus anticoagulants by employing sensitive and insensitive reagents to the lupus anticoagulant

Lupus anticoagulant: performance of the tests as recommended by the latest ISTH guidelines

OBJECTIVES

Lupus anticoagulant (LA) is clinically the most relevant among all antiphospholipid antibody tests. Recently, new guidelines for LA detection were published. The objective of this retrospective cohort study was to compare tests recommended under these guidelines with other methods used for LA detection.

METHODS

The study group consisted of 336 subjects suffering from various autoimmune diseases. We used activated partial thromboplastin time (aPTT), diluted Russell viper venom time (dRVVT) and diluted prothrombin time (dPT) tests for LA detection together with a ratio between sensitive and insensitive aPTT reagent. We also tested if LA was dependent on β(2) glycoprotein I (β(2) GPI) using one of the recently described methods.

RESULTS

All LA tests performed were associated with a history of thrombosis. The highest odds ratio (OR) for thrombosis was found for β(2) GPI-dependent LA but sensitivity was low (OR = 8.4; specificity/sensitivity = 98%/15%). All LA tests showed a much stronger association with thrombosis than with pregnancy failure.

CONCLUSIONS

LA tested by aPTT and/or dRVVT (at least one out of two tests positive), as recommended by the guidelines, was associated less strongly with a history of thrombosis (OR = 4.1) than either of these tests separately (OR = 5.0 and 4.3, respectively). With both tests positive ('double LA positivity') the association with thrombosis was stronger (OR = 6.5) compared with only one positive test. In fact, 'double LA positivity', detected by combinations of any of the tests studied, was markedly associated with a history of thrombosis.

Antiphospholipid antibodies--we are not quite there yet

The diagnosis of antiphospholipid syndrome is predominantly made in the laboratory and depends on the persistent presence of antiphospholipid antibodies in individuals with thrombosis or pregnancy morbidity. Correct diagnosis of the syndrome is imperative to prevent unnecessary long secondary thromboprophylaxis. Three antiphospholipid antibody subtypes are included in the classification criteria of the antiphospholipid syndrome: lupus anticoagulants, anticardiolipin antibodies and anti-β2-glycoprotein I antibodies. Only lupus anticoagulants are undisputedly associated with thrombosis, which is why the serological criteria of the antiphospholipid syndrome are under debate. All of the assays used to detect antiphospholipid antibodies are in need of better standardization, although progress has been made in the detection of lupus anticoagulants. The inconsistent association between both anticardiolipin and anti-β2-glycoprotein I antibodies and thrombosis is a cause for alarm. We are in need of better assays to detect those individuals at risk for thrombosis and population-based prospective studies to provide us with accurate risk assessments.

The laboratory diagnosis of lupus anticoagulant in patients on oral anticoagulation

Laboratory-specific cut-off lupus ratios (LR), above which a plasma is judged positive for lupus anticoagulant (LA), were established for both activated partial thromboplastin time-based and dilute Russell viper venom time-based methods. The validity of using these cut-off values to determine the presence of LA in patients on oral anticoagulation (OAC) was assessed. A cohort of 40 patients (23 male and 17 female), aged 22-84 years (mean 52 years) were tested for LA at the time of a thrombotic event. Repeated testing was performed after the same patients were treated with OAC (international normalized ratio 2.0-3.5). For 36 patients (90%), LA status was unchanged pre- and on-OAC. Thirteen of the 40 patients (32.5%) were positive for LA both pre- and on-OAC. Of the 27 patients negative for LA pre-OAC, 23 remained negative on-OAC. The four discordant results were interesting in that LA positivity was demonstrated only after the patient was stable on-OAC. In our cohort of 40 patients, there was a trend for LRs to decrease on-OAC, but this did not reach statistical significance. The subset (4) went against this trend and became positive after the thrombotic event.

Usefulness of synthetic phospholipid in measurement of activated partial thromboplastin time: a new preparation procedure to reduce batch difference

Commercial activated partial thromboplastin time (APTT) reagents prepared with phospholipid extracted from animal or plant sources often differ in their response to heparin and coagulation factors and in their reference values. It is also known that there are variations in phospholipid composition and preparation procedure. The present study attempted to demonstrate that an APTT reagent based on synthetic phospholipids (phosphatidylethanolamine, phosphatidylcholine and phosphatidylserine), which are substances of high purity and homogeneity, reduces batch-to-batch difference when compared with two traditional APTT reagents derived from rabbit brain and soybean. Three types of APTT reagent (SYN-APTT, RBT-APTT, SOY-APTT), prepared respectively from synthetic phospholipid, rabbit brain, and soybean, were tested. The total batch-to-batch difference was coefficient of variation (CV) 0.7-2.4% in the five reagents prepared from synthetic phospholipid (SYN-APTT), but CV 1.5-10.3% in the two traditional reagents (RBT-APTT and SOY-APTT). Additionally, high-performance liquid chromatography (HPLC) analysis showed clear variation in the phospholipid composition of the RBT-APTT and SOY-APTT reagents. In conclusion, the SYN-APTT reagent derived from synthetic phospholipid was shown to reduce batch-to-batch difference, and we therefore suggest that synthetic phospholipid is a substance useful in the preparation of APTT reagent and could contribute to stability of supply and uniform diagnosis.

Lupus anticoagulant testing using plasma spiked with monoclonal antibodies: performance in the UK NEQAS proficiency testing programme

We report here results from a United Kingdom National Quality Assessment Scheme (UK NEQAS) exercise in which both plasma spiked with monoclonal antibodies and plasma from a patient known to have lupus anticoagulant (LA) were distributed to 245 hemostasis laboratories with a request for them to test for possible LA using their routine screening procedure. In general, good agreement was seen in the diagnosis of samples spiked with monoclonal antibodies against beta2-glycoprotein 1 (beta2GP1) and prothrombin, the LA-positive patient sample, and a normal pooled plasma; over 87% of centers correctly identified each sample. However, methods employing platelet neutralizing procedures were associated with a higher proportion of false-negative responses with the antiprothrombin-spiked sample, and it is important to recognize that sensitivity and responsiveness of different methods may vary between artificial plasmas and different LA-positive patient plasmas.

Comparison of four commercially available activated partial thromboplastin time reagents using a semi-automated coagulometer

Large numbers of activated partial thromboplastin time (aPTT) reagents are sold in the market. The phospholipid content and its source, nature and the amount of activators are highly varied in different aPTT reagents. The present study was undertaken to evaluate which of the four aPTT reagents commonly used is suitable as an all-purpose reagent for a modest haemostasis laboratory. Four aPTT reagents (reagent A, Platelin LS; reagent B, Silimat; reagent C, Actin FSL; reagent D, CK Prest) were tested against 75 different plasmas obtained from normal patients as well as from patients with different haemostatic problems. All the tests were conducted by one of us (S.S.) in duplicates. Different aPTT reagents missed different proportions of mild factor VIII and factor IX deficiency (36.4, 18.2, 4.6 and 13.6% for reagents A, B, C and D, respectively) and showed abnormal results with normal plasmas (i.e. more than 5 s prolongation) (29.2, 25, 8.3 and 12.5% for reagents A, B, C and D, respectively). All the reagents faithfully picked up moderate and severe factor VIII and factor IX deficiency. There was no difference among the four aPTT reagents regarding their ability to prolong aPTT to therapeutic dosage of heparin or in their ability to give comparable factor VIII or factor IX levels in one-stage aPTT-based assays. There were differences in aPTT reagents in their ability to pick up mild deficiency of coagulation factor VIII and factor IX. Some reagents showed abnormal aPTT results in mild cases of factor VIII and factor IX deficiency without producing a large number of falsely prolonged aPTT with normal plasma.