Antiphospholipid antibodies (prothrombin, β2-glycoprotein I)

Antiphospholipid antibodies are autoantibodies – defence substances directed against the body's own structures. Despite their name, they usually do not bind to the phospholipids themselves but to blood proteins that dock onto phospholipids. The autoantibody screen measures two of them: antibodies against prothrombin and against β2-glycoprotein I (written „β-GPI“ in the catalogue). They are measured in serum. In the test catalogue they appear in the CFS and PCS autoantibody screens, together with further autoantibodies, for example against MPO, proteinase 3, the thyroid, IL-6, ACE2 and GM-CSF. The background is that changes in blood clotting and autoantibodies are being investigated in studies in long/post-COVID and ME/CFS.

What prothrombin and β2-glycoprotein I do in the body

Prothrombin is the precursor of thrombin, the key enzyme of blood clotting. When clotting is triggered, prothrombin attaches to the surface of activated platelets, which then turn negatively charged phospholipids such as phosphatidylserine outwards. There it is converted into thrombin, which links fibrinogen into fibrin strands. β2-glycoprotein I is a plasma protein that also settles on such negatively charged surfaces – on activated platelets, on dying cells and on microparticles. It is involved in the fine-tuning of clotting and helps to clear away dead cells. Both proteins therefore sit exactly where clotting starts and is held in check.

Antiphospholipid antibodies detected – what happens in the body?

When antibodies bind to β2-glycoprotein I or prothrombin, complexes form on the vessel wall, platelets and immune cells. The cells of the inner vessel wall are activated, monocytes produce more tissue factor, platelets clump more easily and the complement system is set off. At the same time, anticoagulant mechanisms are disturbed. The balance shifts towards clotting: blood clots form more easily in veins or arteries, and in pregnancy the blood supply to the placenta may suffer. If the antibodies persist over months and such events occur, this is called antiphospholipid syndrome. After infections, including after COVID-19, such antibodies have been found more often in studies, frequently only temporarily and in low amounts; their significance in long/post-COVID is still debated.

No antiphospholipid antibodies detected – what does it mean?

If no antibodies against prothrombin and β2-glycoprotein I are found, this is the normal case. It speaks against these two autoantibodies being involved in a tendency to clot at the time of the blood sample. However, antiphospholipid antibodies also include further measures, such as cardiolipin antibodies and lupus anticoagulant, which are not part of the screen. Because the antibodies fluctuate and can appear temporarily after infections, a single result has only limited significance. Together with the other autoantibodies in the screen and further clotting values such as D-dimers, a more complete picture emerges.

The interpretation is given in your report.