Can low molecular weight heparin (LMWH) interfere with antiphospholipid antibody testing?

From the Guidelines

Yes, low molecular weight heparin (LMWH) can interfere with antiphospholipid antibody testing, particularly with clotting-based assays like the lupus anticoagulant (LA) test, as it affects the APTT and can cause false-positive results 1.

Key Considerations

• LMWH therapy, such as enoxaparin (Lovenox), dalteparin (Fragmin), or tinzaparin (Innohep), should ideally be considered when interpreting antiphospholipid antibody test results.
• The interference of LMWH with LA testing is primarily due to its effect on factor Xa and thrombin, which can prolong clotting times in these assays 1.
• However, LMWH generally does not significantly interfere with solid-phase immunoassays that detect anticardiolipin antibodies or anti-β2-glycoprotein I antibodies.
• If testing must be performed while a patient is on LMWH, this information should be clearly noted on the laboratory requisition, and results should be interpreted with caution.

Testing Recommendations

• Patients on LMWH therapy should ideally have antiphospholipid testing performed before starting these medications or after discontinuing them for at least 24-48 hours.
• In cases where discontinuing anticoagulation is not feasible, specialized testing methods that are less affected by LMWH may be available at reference laboratories.
• Sampling during the trough period of low-molecular-weight heparin (LMWH) or using alternative tests like TSVT/ET may be considered in certain situations 1.

Clinical Implications

• The use of LMWH in patients with antiphospholipid syndrome (APS) or those at risk of thrombosis requires careful consideration of the potential interference with antiphospholipid antibody testing 1.
• Prophylactic anticoagulation therapy with heparin or LMWH may be recommended in asymptomatic aPL-positive patients during assisted reproductive technology (ART) procedures to prevent thrombotic complications 1.

From the Research

Interference of Low Molecular Weight Heparin with Antiphospholipid Antibody Testing

• Low molecular weight heparin (LMWH) can interfere with antiphospholipid antibody testing, as it affects clotting assays and can lead to false-negative or false-positive results 2.
• The interference of LMWH with antiphospholipid antibody testing can be mitigated by avoiding testing while patients are on anticoagulants, temporarily switching to an anticoagulant with less assay interference, testing for antiphospholipid antibodies at nadir levels of anticoagulants, or using anticoagulant neutralizers 2.
• LMWH is commonly used as an anticoagulant in patients with antiphospholipid syndrome, and its use can be effective in preventing recurrent thromboembolic episodes 3, 4.
• However, LMWH can also cause heparin-induced thrombocytopenia, a rare but dangerous condition, especially in patients who are being exposed to it for the first time 5.

Effects of Pre-Analytical Variables on Antiphospholipid Antibody Testing

• Pre-analytical variables, such as blood to anticoagulant ratio, can affect the results of antiphospholipid antibody testing, particularly when using the activated partial thromboplastin time (aPTT) assay 6.
• The anti-activated factor X (anti-FXa) assay appears to be less affected by pre-analytical variables and may be a better method for monitoring heparin therapy in patients with antiphospholipid syndrome 6.

Clinical Implications

• Clinicians should be aware of the potential interference of LMWH with antiphospholipid antibody testing and take steps to mitigate this interference, such as using alternative anticoagulants or anticoagulant neutralizers 2.
• The use of LMWH in patients with antiphospholipid syndrome requires careful monitoring and management to minimize the risk of adverse effects, such as heparin-induced thrombocytopenia 5, 4.