THROMBOSIS AND HAEMOSTASIS


unwell. For vaccinated patients, this requires a more immediate and vigilant approach. Important initial tests should include platelet count, prothrombin time (PT), activated partial thromboplastin time (APTT), fibrinogen and D-dimer.7 There is growing clinical evidence that ELISA testing is the most appropriate prognostic marker to help guide effective management. While treatment remains uncertain with no therapeutic pathways confirmed, the mainstay of therapy, along with anticoagulation, is now intravenous immunoglobulin, based on preventing antigen-antibody complexes from activating platelets through the FcγII receptor.


NEQAS recommends ELISA Despite the headlines, the actual risk of VITT is the same or even less than might be expected in the general population. While estimates vary, numerous official bodies indicate the risk could be between one in 50,000 and one in 100,000. Further, the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has sought to remind people that the risk of clotting from the vaccine is far lower than not being vaccinated and contracting COVID – for which clotting has become a known symptom, especially those who go on to develop severe respiratory syndrome.7


It is accepted that the risk of


thrombosis from COVID-19 vastly outweighs that from the adenoviral vaccine. However, the need for rapid vaccine roll-out in the UK and other countries like Israel highlights the need to be vigilant for early signs of VITT in at-risk groups rather than relying on clinical thrombotic manifestations. Diagnostic testing is a vital part of this early screening and diagnosis process, highlighting again how collaboration between the clinical teams and laboratory sciences is needed to enable patient care. The NEQAS assessment study indicates that ELISA methods have no false negatives in their VITT samples. Further, it confirms the commutability of liquid and lyophilised patient VITT samples when running ELISA assays. It has now issued guidance to its participating laboratories that confirms best practice would be to run an ELISA, even if that means sending patient samples to a specialist laboratory (Table 1).


Early warning of poor prognosis Several studies have been carried out on the first patients in the UK with VITT. The most recent was published last month in The New England Journal of Medicine.1 Involving 294 patients, it provides chastening reading on the odds of death


Table 1. UK NEQAS-led study identified five criteria for diagnosing VITT.


• Symptoms with five to 30 days of the first vaccination • Low platelet count of 150 x 109


/L


• Excessively high D-dimer levels of over 4000 µg/L • Thrombosis at unusual sites • Raised titres of IgG antiplatelet factor 4 antibodies


Table 2. Level of increased factor risk of death.


• 2.7 (95% confidence interval [CI] 1.4 – 5.2) among patients with cerebral venous sinus thrombosis


• 1.7 (95% CI 1.3– 2.3) for every 50% decrease in the baseline platelet count


• 1.2 (95% CI 1.0– 1.3) for every increase of 10,000 fibrinogen-equivalent units in the baseline D-dimer level


• 1.7 (95% CI 1.1– 2.5) for every 50% decrease in baseline fibrinogen


and identifies early warnings for poor prognosis.


A baseline platelet count and the


presence of intracranial haemorrhage was independently associated with death. This reached 73% among patients with platelet counts below 30,000/cm3


as well


as intracranial haemorrhage. This study found no individual risk factors for VITT. However, it confirmed increased mortality among patients who presented with severe thrombocytopenia, cerebral venous sinus thrombosis, intracranial haemorrhage, laboratory markers of severe coagulation activation, or all of these variables (Table 2).


Confirmatory ELISA


The study used the HIT ELISA to detect the anti-PF4 antibodies rather than rapid non-ELISA tests after finding they often gave false negatives for VITT.8


However,


there were a few patients with coagulation abnormalities indicating probable VITT who still tested negative for anti-PF4 antibodies. In this case, the panel recommended repeat testing with an alternative ELISA method. It is currently unclear if such patients have antibodies to an epitope that is not recognised by currently available assays. The need for a second ELISA test when


VITT is suspected was flagged up by a separate study on VITT patients and published in the Journal of Haematology and Thrombosis in May. Three types of assay were assessed for their ability to detect VITT: rapid assays, IgG-specific ELISAs such as the Stago Asserachrom HPIA IgG ELISA, and poly-specific (IgG, IgA and IgM) ELISAs which included the Asserachrom HPIA. While it ruled out the efficacy of rapid non-ELISA assays, it found diagnostic comparability between the poly-specific ELISAs and those that were IgG specific for detecting VITT. This study indicated that no ELISA would pick up every case of VITT. Therefore, where there were strong clinical indicators even in the face of a single negative ELISA, the findings recommended a second ELISA, or a platelet activation assay.7


It made the


point that clinicians may not be aware that ELISA assays are not widely available in UK laboratories, with only a handful of laboratories globally able to perform platelet activation assay. A third study,6


published in the


The New England Journal of Medicine in May evaluated ELISA testing (including Stago’s Asserachrom HPIA IgG assay) for anti-PF4 antibodies at six UK reference laboratories. Additional testing for anti-


Table 3. Algorithm to detect likelihood of VITT.


• D-dimer <2000 FEU with a normal coagulation screen and fibrinogen levels – VITT unlikely


• D-dimer 2000 – 40000 FEU with strong clinical indication and rising parameters – Suspected VITT • Run HIT ELISA • Avoid heparin anticoagulants


• D-dimer ≥4000 FEU with low to normal fibrinogen and no alternative pathology indicated – VITT indicated • Confirm by running ELISA • If negative, run second ELISA


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