As compared to kaolin clotting time, silica clotting time is a specific and sensitive automated method for detecting lupus anticoagulant.

Lupus anticoagulants (LAs) are antiphospholipid antibodies capable of interfering with the coagulation system and modifying in vitro the phospholipid-dependent clotting tests. Colloidal silica was used as activator to perform an activated partial thromboplastin time (aPTT) in 73 plasma samples with pathologically elevated kaolin clotting time (KCT) values using a photooptical automated coagulometer. Samples were incubated for 5 min with micronized silica, and after recalcification, the clotting times were measured. Pathologically prolonged results were confirmed by a confirmation and a neutralization test adding platelet-poor normal plasma (PPNP) and natural phospholipids, respectively. Silica clotting time (SCT) was abnormally elevated in 72/73 (98.6%) (mean ratio 1.71 +/- 0.28) KCT positive samples and normalized (mean ratio 1.03 +/- 0.16) after adding natural phospholipids to test plasma. The values expressed as SCT and KCT ratios were significantly correlated (r = .92; P < .001). SCT was normal in 40 healthy subjects utilised as controls (mean ratio 0.99 +/- 12). Sensitivity, specificity and diagnostic accuracy of SCT were 98.6%, 100% and 97.6%, respectively. Our data suggest that SCT is a sensitive test for detecting LA with a prolonged KCT in automated photooptical coagulometers. This peculiarity makes it particularly useful, in combination with diluted Russel viper venom time (dRVVT), for large-scale screening tests on LA.