Therapeutic anticoagulation inevitably compromises many coagulation assays that are designed to detect a specific abnormality based on the assumption that the patient's coagulation is otherwise normal. Use of heparin neutralisers and mixing studies are available to counteract these effects but they are not without their limitations and accurate detection may not be possible [4, 8, 12]. Whilst it is rarely necessary to investigate for LA in heparinised patients it is relatively often required in those who are receiving OAT . The most frequently used assays to detect LA in the UK are DRVVT and a variety of APTT based assays . Testing on neat plasma in these assays is compromised by OAT giving rise to false-negative and -positive results in patients with and without LA [4, 14, 15] and mixing studies are commonly used to correct the acquired multiple factor deficiency of OAT.
Although the relatively small dilution in 1:1 mixing studies can dilute LA to undetectable levels in a significant number of cases , it remains a useful tool for demonstrating the antibodies in the presence of co-existing coagulopathies providing that the LA is sufficiently potent to prolong the screening test and a confirmatory test is used to demonstrate phospholipid dependence. Fifty six of 80 known LA were detected in the DRVVT mixing tests and 47 of 80 by DAPTT mixing tests, so they clearly have a role to play in detection by commonly used conventional assays. As would be expected, some antibodies were only apparent in one of either DRVVT or DAPTT, more so in DRVVT. However, incomplete normalisation of the OAT effect can be encountered in patients on high dose OAT [1, 16] so diagnosis of the presence of a LA is more reliable when the mixing study confirmatory test corrects back into the reference range whilst the screen remains elevated. Significantly, a very low percentage of DRVVT positive LA (12.5%) generated a mixing study confirmatory test correction into the reference range, in contrast to the 55.3% by DAPTT and 44.8% by TSVT/ET. This is likely due to the specific analyser/reagent/normal plasma combination  with this patient population and would not necessarily be true for other DRVVT/analyser combinations. Incomplete normalisation of the confirmatory test can also be attributable to avid antibodies [17, 18], but providing that additional coagulopathies are excluded, an elevated screen in mixing studies with ≥ 10% correction by a confirmatory test result above the reference range suggests the presence of a LA. Some such antibodies appear to possess a degree of resistance to the swamping effect of high phospholipid confirmatory reagents. This may explain why so few of the TSVT/ET mixing studies with elevated screens did not correct back into the reference range as the ET is a phospholipid independent reagent. Clear positivity in TSVT/ET screening can affirm a suggestive diagnosis in conventional mixing studies. Of note is the 48.3% of TSVT/ET positive LA that were undetectable in mixing studies compared to the 39.3% previously reported for this assay system , further emphasising the limitations imposed by the dilution effect.
Venoms from the Coastal Taipan (Oxyuranus scutellatus) and the Saw-scaled viper (Echis carinatus) contain prothrombin activators capable of activating the des-carboxy prothrombin produced during OAT as well as native prothrombin. The Taipan activator is phospholipid and calcium ion dependent but the Saw-scaled viper activator is not. Together they represent an alternative or adjunct to conventional assay mixing tests in detection of LA patients on OAT. As the DRVVT and DAPTT positive LA were detected using mixing studies, it is unsurprising that more than half were also detectable by TSVT/ET as they would have been potent antibodies and more likely to present in multiple assays. Nevertheless, 27.5% were not detected in TSVT/ET, a clear manifestation of antibody heterogeneity, and likewise, those LA that were positive in TSVT/ET and just one of either DRVVT and DAPTT. Of particular relevance to this study are the 20.0% that were detectable only by TSVT/ET. Irrespective of whether these LA were TSVT/ET detectable only and/or would have presented in DRVVT and/or DAPTT without the OAT effect, there are clear implications for initial diagnostic LA screening on patients receiving OAT. Extrapolating these findings in known LA to the diagnostic setting, use of TSVT/ET would increase detection rates by detecting LA that would be diluted out in DRVVT and DAPTT mixing tests, and also those that are undetectable in conventional assays, probably as a result of antibody heterogeneity and epitope specificity. This should not however engender a false sense of security as some LA that are only detectable in DRVVT or DAPTT will be negative in TSVT and diluted in mixing studies.
Additionally, spontaneous variation of APA has been reported to occur in up to 25% of cases  and TSVT/ET analysis could have a role in disease monitoring for patients on OAT whose antibody presentation or avidity changes over time .