Table 2.

Test procedure for lupus anticoagulant (LA) [14]).

Blood collection/pre-analytical factors

-incorporation of information on the patient's anticoagulation in the request is mandatory -be aware that elevated CRP may give false positive LA results

Choice of test/test procedure

-Two tests based on different principles -dRVVT should be the first test considered -The second test should be a sensitive aPTT (suitable PL composition and low concentration) and preferably silica as activator) -LA should be considered positive if one of the two test systems gives a positive result in the three steps (screen-mix-confirm) -Screening tests are performed with dRVVT and aPTT, and regarded to be positive if the normalized clotting time is prolonged beyond the locally established cut-off -Mixing test in a 1:1 proportion of patient: PNP should be used, without pre-incubation -A mixing test with screening reagent is performed if the screening test(s) on undiluted sample is prolonged -Results of mixing test are suggestive of LA when the normalized clotting time is greater than the local cut-off value -Confirmatory test(s) must be performed by increasing the concentration of PL used in the screening test(s). Bilayer or hexagonal (II) phase PL should be used to increase the concentration of PL. -Confirmatory test to be performed if the screening test suggests LA presence, irrespective of the result of the mixing test with screening reagent -Confirmatory test is performed on a mix of 1:1 PP and PNP if the confirmatory clotting time is prolonged

Interpretation

-For paired test LA ratio (screen/confirm) expressed as normalized ratio is calculated -Or the percentage correction [(screen-confirm)/screen x 100] -Results are suggestive for LA if LA ratio (screen/confirm or screen mix/confirm mix) or percentage correction is above the 99th centile -Some of the integrated tests are designed to measure a difference in clotting times on a mixture of plasma

Expression of results

Results should be expressed as ratio of patient-to-PNP run in parallel with the test plasma for all procedures (screening, mixing and confirm)

Cut-off values

-Use in-house cut-off values, do not use cut-off values established elsewhere -Calculate 99th centile on at least 120 normal samples with outlier detection for all normalized ratios -Alternatively, transference of the manufacturer's cut-off values after verification is possible, if manufacturers provide cut-off values established in accordance with guidelines and by appropriate statistical models using a sufficiently large donor population

Post-analytical issues

-It is imperative that testing be repeated after an initial positive result on a second occasion after 12 weeks

Report of results

-LA is reported with a final conclusion as positive/negative -Comments such as borderline or dubious LA are highly discouraged and in these cases the comment should be “suggest re-testing after one week or more”, without suggesting positive or negative LA -Along with the analytical results for the three steps, local cut-off values must be reported -A report with an explanation of the results should be given -Results should always be related to the results of aCL and aβ2GPI to assess the risk profile -Results should be interpreted in a clinical context and knowledge of ongoing treatment Information provided in the request on the patient's anticoagulation status should also be incorporated into the report -A close interaction between the laboratory and the clinician is essential