Kaolin Clotting Time (KCT)

Plasma (platelet poor), frozen

2 mL

1 mL

Blue-top (sodium citrate) tube

Do not draw from an arm with a heparin lock or heparinized catheter.

The KCT is a sensitive screening test of lupus anticoagulants (LA) in patients with a history of thrombosis.^6-10

This test was developed and its performance characteristics determined by Labcorp. It has not been cleared or approved by the Food and Drug Administration.

In the kaolin clotting time (KCT) test, kaolin, a negatively-charged particulate activator, is incubated with test plasma to activate the contact factors and the intrinsic system.¹¹ Calcium chloride is added and the mixture is timed until a clot forms.

The intrinsic mechanism requires the presence of all coagulation factors except factor VII and XIII. In the KCT, unlike TTIT, dRVVT, and aPTT, no reagent phospholipid is added, so plasma phospholipids are required for the reaction to proceed. For this reason, the KCT is sensitive to the presence of lupus anticoagulants, when if present, will generally prolong the KCT. All clot-based tests are potentially affected by factor deficiencies. The KCT is prolonged in any except factor VII or XIII deficiency. Lupus anticoagulants are nonspecific antibodies that extend the clotting time of phospholipid-dependent clotting assays such as the KCT.^7,10 Unlike specific factor antibodies, LA are usually associated with venous thrombosis, pulmonary embolism, arterial thrombosis, and recurrent fetal loss.¹² LA do not specifically inhibit coagulation factors; rather they neutralize anionic phospholipid-protein complexes that are involved in the coagulation process. Prolongation of clot-based assays is highly dependent on the sensitivity of the reagent employed. Reagents with reduced amounts of phospholipid, such as KCT, aPTT-LA, and dilute Russell viper venom (dRVVT), have enhanced sensitivity for LA.⁷ Testing for lupus anticoagulant (LA) and the antiphospholipid syndrome that is associated with these antibodies is described in more detail in the APS section. The KCT is sensitive to deficiency or inhibition of factors in the intrinsic pathway. These include the contact factors; high molecular weight kininogen (HMWK); prekallikrein; and factor XII along with procoagulant factors XI, IX, VIII, X, V, prothrombin, and fibrinogen.^10,12-14 An extended KCT can be seen in acquired deficiencies of intrinsic factors II, IX, and X that result from vitamin K deficiency or the use of oral anticoagulants that block vitamin K-dependent production of procoagulant factors. These conditions also affect the level of factor VII, an extrinsic pathway factor. Since factor VII has a short half-life relative to the vitamin K-dependent factors of the intrinsic pathway, vitamin K-dependent factor deficiency can often result in an extended PT with a normal aPTT and KCT. Consumption coagulopathy, such as disseminated intravascular coagulation (DIC), can produce an extended KCT due to depletion of intrinsic factors. The KCT can also be extended in conditions that reduce the production of procoagulant factors (eg, severe liver disease or malnutrition). Inhibitors, both factor specific and nonspecific, can also prolong the KCT.