Guidelines for Lupus Anticoagulant Testing: Preferred Methods and Common Pitfalls
1. Kaolin Clotting Time (KCT) Should Not Be Used for LA Detection
- The International Society on Thrombosis and Haemostasis (ISTH) recommends against using KCT for lupus anticoagulant (LA) testing because of poor reproducibility and problematic behavior in automated coagulometers【1】【2】【3】【4】【5】【6】【7】【8】【9】. Evidence level: moderate.
2. Recommended Screening and Confirmation Strategy
2.1 Screening Tests
- Dilute Russell’s Viper Venom Time (dRVVT) is the first‑choice screening assay; it is the most robust and specific test for detecting clinically significant LA associated with thrombosis risk【1】【3】【5】. Evidence level: high.
- Silica‑activated aPTT with low phospholipid content is a suitable second‑choice screening assay, offering good sensitivity for LA【1】【6】【8】. Evidence level: moderate.
2.2 Tests to Avoid in LA Screening
- Kaolin as activator – problematic in automated systems【1】【6】.
- Ellagic acid as activator – insufficient sensitivity for LA【1】.
- Dilute prothrombin time – variability due to thromboplastin reagents【1】.
2.3 Phospholipid Confirmation
- Perform the same screening assay at high phospholipid concentration to assess correction【1】【4】【6】【7】【8】【9】.
- Percentage correction is calculated as [(screen time – confirm time) / screen time] × 100【1】【4】【6】【7】【8】【9】.
- A result is positive for LA when the percentage correction exceeds the laboratory‑specific cut‑off, which should be established from ≥ 40 healthy donors < 50 years old【1】【4】【6】【7】【8】【9】.
- Because true LA samples may not fully normalize, use percentage shortening rather than absolute normalization to avoid false‑negative results【1】【2】【4】【6】【7】【8】【9】. Evidence level: moderate.
3. Mixing Studies and Interpretation
- Mix patient plasma 1:1 with pooled normal plasma and test within 30 minutes without pre‑incubation【6】【8】【9】.
- Calculate the Index of Circulating Anticoagulant (ICA):
Interpretation: If the clotting time remains prolonged above the 99th percentile or ICA exceeds the locally defined cut‑off, the result suggests LA rather than a factor deficiency【6】【8】【9】.
Additional checks: Perform thrombin time (TT) or review clinical history to exclude heparin contamination or specific factor inhibitors, which cause bleeding rather than thrombosis【2】【3】【7】【9】.
4. Interference, Sample Handling, and Special Populations
4.1 Anticoagulant Interference
- Heparin can be tested if plasma heparin level < 0.8 U/mL using reagents with heparin neutralizers【2】【3】【7】【9】.
- Low‑molecular‑weight heparin (LMWH) may be tested, but results vary with the FXa/FIIa activity ratio of the specific preparation【2】【3】【7】【9】.
- Direct thrombin or FXa inhibitors – effect on LA assays is unknown; testing should be avoided while the patient is on these agents【2】【3】.
- Aspirin and clopidogrel do not interfere with LA assays【2】【3】.
- Hydroxychloroquine may weakly interfere by affecting IgG‑β2GPI complex formation【2】.
4.2 Pregnancy Considerations
- Establish pregnancy‑specific reference ranges because aPTT shortens (elevated factor VIII) and dRVVT may change for unclear reasons【1】【2】【3】【4】【5】【6】【8】.
4.3 Specimen Processing
- Double‑centrifuge plasma to obtain platelet count < 10⁷/mL【1】【2】【3】【4】【5】【6】【8】.
- If testing is not performed immediately, freeze plasma at –70 °C【6】.
- Thaw at 37 °C for 5 minutes by total immersion to prevent cryoprecipitate formation【6】.
5. Common Pitfalls and Reporting Recommendations
- Do not base clinical decisions on KCT results; replace with dRVVT or silica‑based aPTT【1】【2】【3】【4】【5】【6】【7】【8】【9】.
- Do not expect complete normalization on confirmatory testing; rely on percentage correction instead【1】【2】【4】【6】【7】【8】【9】.
- Laboratories must establish their own cut‑offs using the specific reagent/coagulometer combination employed【1】【4】【6】【7】【8】【9】.
- Defer LA testing during acute anticoagulation with novel agents until the medication is discontinued, if feasible【2】【3】.
- For borderline or dubious LA results, either confirm definitively or repeat the test after one week【3】【5】.