The silica clotting time (SCT) is a PTT-like assay that uses colloidal silica as its negatively charged particulate activator. It was first described in Chantarangkul V, Tripodi A, Arbini A, Mannucci PM. SCT as a screening and confirmatory test for detection of the lupus anticoagulants.Thromb Res 1992;67:355-65.
The current version, HemosIL SCT, marketed by Instrumentation Laboratory Company, utilizes synthetic phospholipid for the cephalin reagent component instead of the original bovine cephalin. The kit contains a low concentration phospholipid screening reagent, SCT Screen, and a high concentration phospholipid confirmatory reagent, SCT Confirm. Like the PTT, a calcium chloride activator is also provided. The calcium chloride solution contains polybrene to minimize heparin interference up to 0.4 U/mL.
To report the SCT, the operator generates and computes reference intervals with their means for both SCT Screen and SCT Confirm. Next, the patient SCT Screen and SCT Confirm results are divided by their respective means to produce normalized SCT Screen and SCT Confirm ratios. Finally, the patient SCT Screen ratio is divided by the patient SCT Confirm ratio to generate the normalized SCT ratio.
The combination of silica and phospholipid activate and sustain intrinsic coagulation upon addition of ionic calcium. When a lupus anticoagulant is present, SCT Confirm becomes prolonged beyond the SCT Screen. Normalization renders the SCT insensitive to warfarin.
Although the normalized SCT ratio decision point is generated locally and varies by instrument, a cutoff of 1.2 is typical.
The clinical efficacy of HemosIL SCT is equivalent to low phospholipid PTT, as documented in Grypiotis P, Ruffatti A, Pango V, et al. Use of a new SCT for diagnosing lupus anticoagulant in patients who meet the clinical criteria for antiphospholipid syndrome. J Clin Lab Anal 2006;20:15-8 and in Devreese KMJ. Evaluation of a new SCT in the diagnosis of lupus anticoagulants. Thrombos Res 2007;120:427-38.
The HemosIL SCT functions well with Beckman Coulter, Inc ACL instrumentation, and is useful when used in a lupus anticoagulant profile that includes at least one additional test system such as the dilute Russell Viper Venom time assay, kaolin clotting time, or the dilute prothrombin time assay