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A Practical Guide to Laboratory Haemostasis

 

Silica Clotting Time [SCT]



Introduction

The Silica clotting time [SCT] is essentially an activated partial thromboplastin time test in which silica replaces kaolin.

Principles

Colloidal silica suspension is mixed with platelet poor plasma and calcium is then added to initiate coagulation. The SCT contains a low dose of phospholipid which makes is very sensitive as a screening test to lupus anticoagulants [LAs.] In the original test [see Ref 2] the greatest sensitivity of the SCT to LAs was not seen when PL was completely absent but at a low PL concentration. This is probably because the SCT without PL is highly variable within each patient group but having a low concentration of PL in the test standardises and maximises its sensitivity. A confirmatory test for a LA can be achieved using the SCT by employing a high PL concentration to neutralise the LA and so demonstrate its PL-dependence.

Method

Colloidal silica suspension is mixed with platelet poor plasma then calcium is added to initiate coagulation.

Reagent Explanation
Platelet poor plasma A source of coagulation factors, particularly thrombin and fibrinogen. 
Colloidal Silica suspension Activator of the intrinsic pathway
Phospholipid Low and High concentration PL are used. A low concentration for the SCT when used a screening test and a high concentration when it is used as a confirmatory test.
Calcium  To initiate coagulation


Heparin will interfere with the test but some commercial kits contain a heparin neutralisation agent e.g. polybrene.

Interpretation

1. In the original test [see Ref 2] - the percentage [%] correction was calculated from the equation:



2. In commercial versions of the test
   a. The SCT ratio for the screening test [low PL concentration] is derived from:

     Patient Screen SCT [s]/Mean of SCT Screen Reference Range [s]

   b. The SCT ratio for the confirmatory test [high PL concentration] is derived from:
    
    Patient Confirm SCT [s]/Mean of SCT Confirm Reference Range [s]

   c. The normalised ratio is derived from:

      Screen ratio/Confirm ratio

A Normalised SCT ratio >1.16 [>1.24 on some machines] indicates the presence of a lupus anticoagulant. A ratio < 1.16 [>1.24 on some machines] suggests a factor deficiency.

Look at the following example:

Sample SCT Clotting Time [s] Ratios
Patient Plasma 210.3 s [Patient Plasma]/[Reference Plasma] = 3.22
Reference Control [Normal] Plasma 65.2 s
Patient Plasma + Phospholipid 62.2 s [Patient Plasma + Phospholipid]/Reference Control [Normal] Plasma +  Phospholipid] = 1.04
Reference Control [Normal] Plasma +  Phospholipid 32.5 s


From the data above: [3.22 - 1.04/3.22] x 100 = 67.7% correction. So in a patient in whom the SCT is prolonged as in this case [210.3s] the 67.7% correction is consistent with the presence of a lupus anticoagulant.

Reference Ranges

The normalised SCT ratio reference range depends on the machine being used for the test but in general a ratio >1.16 [>1.24 on some machines] indicates the presence of a lupus anticoagulant. A ratio < 1.16 [<1.24 on some machines] suggests a factor deficiency.

Interpretation

Commercial versions of this test are relatively insensitive to heparin (up to 0.4 U/ml) because of the presence of polybrene.
The test is also insensitive to oral anticoagulants and gives a normal result in patients with liver disease and abnormal clotting. The test is less effective at distinguishing between a LA and an anti-FVIII antibody. When a LA is present the SCT confirmatory test is prolonged beyond the SCT screening test i.e. the phospholipid dependant nature of the antibody has been demonstrated.

What Test Next

In individuals in whom a LA is identified, the test should be repeated in 12 weeks. It should also be remembered that not all tests including the SCT will identify all LAs and therefore, if the index of suspicion that a specific patients has a LA then other tests should be undertaken. Finally - the causes of a LA should be screened for e.g. check ANA, drugs, viruses etc

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.