Background The laboratory testing of lupus anticoagulant (LA) is based on the guidelines given by ISTH-SSC and CLSI. Both guidelines advocate using diluted Russel viper venom (dRVVT) test in combination with activated partial thromboplastin time (aPTT) assay and the results should be expressed as ratio to the pooled normal plasma (PNP). Since reagents used for testing may differ between lots as well as between manufacturers, those guidelines recommend normalization of the results. However some authors suggest not using the normalization step to decrease the complexity of already very complicated LA testing.
Objectives To investigate how the normalization of dRVVT and aPTT-ratios based on PNP reference values affects the reference ranges and the number of positive and negative results for the presence of LA.
Methods Double centrifuged plasma samples from healthy subjects (N=49) were analyzed on Behring Coagulation System BCS XP. All samples were analyzed with the dRVVT-based assay, using LA screen and LA confirm from Life Diagnostics, as well as with aPTT assays - PTT-LA from Diagnostica Stago as screen and Actin FS from Siemens Healthcare Diagnostics as the confirm step. Ratios were calculated for both dRVTT and aPTT before and after normalization using mean of run-specific PNP value. Our laboratory reports the dRVVT-ratio and aPTT–ratio as: negative <1.2, weakly positive 1.2–1.4, and positive >1.4. We also investigated 362 previously analyzed samples to figure out how the normalization step influences the results.
Results Ratios for PNP were 1.06±0.01 and 1.26±0.02 for dRVVT and aPTT respectively. Out of 49 healthy subjects without the normalization step, 34 would have been reported as negative (15 as weakly positive) with dRVVT and 3 would have been reported as negative (8 as positive and 38 as weakly positive) with aPTT. After normalization 45 samples would have been reported as negative (3 and 1 as weakly positive with dRVVT and aPTT respectively). The normalization step led to a decrease in the amount of positive diagnosis (from 58% to 44% and 77% to 41% using dRVVT and aPTT respectively) and at the same time the number of samples tested negative increased (7% to 15% and 6% to 34% for dRVVT and aPTT respectively).
Conclusions Our findings indicate that without the normalization step only 6% of healthy subjects would be reported as negative for the presence of LA. The introduction of the normalization step decreases the number of false positive results and this is particularly true for the aPTT based assay. Since false positive results lead not only to unnecessary patients' discomfort, but also generate an increased cost due to prolonged follow up of the patients, an additional step using normalization which is relatively simple and inexpensive is definitely justified for the laboratory diagnosis of LA.
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Disclosure of Interest None declared