Kaolin clotting time

The KCT is performed on platelet-poor plasma that is recalcified in the presence of kaolin, a particulate activator that provides a large surface for the assembly of the contact activation factors. Kaolin activates factor XII (Hageman factor), which then triggers the intrinsic coagulation cascade. The test measures the time from recalcification to fibrin clot formation, typically using an automated coagulometer. Because the test is sensitive to phospholipid-dependent reactions, it is particularly useful for detecting lupus anticoagulants, which interfere with the binding of coagulation factors to phospholipid surfaces.

The test is performed by incubating plasma with kaolin and calcium chloride, and the clotting time is recorded. Prolongation of the KCT in the presence of a normal prothrombin time suggests a defect in the intrinsic pathway or the presence of an inhibitor. A mixing study with normal plasma can help distinguish a factor deficiency (corrected by mixing) from an inhibitor (not corrected).

The KCT is one of several assays used to diagnose lupus anticoagulant, along with the dilute Russell's viper venom time (dRVVT) and the activated partial thromboplastin time (aPTT). It is more sensitive than the aPTT for detecting weak lupus anticoagulants, particularly in patients with antiphospholipid syndrome. A prolonged KCT that does not correct in a 1:1 mix with normal plasma indicates the presence of an inhibitor, while correction suggests a factor deficiency.

However, the KCT is not specific for lupus anticoagulant; it can also be prolonged in patients with specific factor inhibitors (e.g., anti-factor VIII) or in the presence of heparin or other anticoagulants. Therefore, confirmatory tests such as the platelet neutralization procedure or the hexagonal phase phospholipid neutralization test are often required to confirm the presence of a lupus anticoagulant.

The KCT was developed in the 1950s as a modification of the whole blood clotting time, using kaolin as a standardized activator to improve reproducibility. It was initially used to investigate hemophilia and other bleeding disorders, but its role shifted toward the detection of lupus anticoagulants in the 1980s when the association between antiphospholipid antibodies and thrombosis became recognized.

The test gained prominence after the description of the "lupus anticoagulant" phenomenon in the 1970s, and it became one of the recommended assays in the 1991 criteria for the classification of the antiphospholipid syndrome. Although newer assays such as the dRVVT have largely replaced it in many laboratories, the KCT remains a valuable tool in specialized coagulation laboratories.

One lesser-known fact is that the KCT is highly sensitive to residual platelets in the plasma sample; even small numbers of platelets can neutralize lupus anticoagulants by providing phospholipid surfaces, leading to false-negative results. Therefore, strict platelet-poor plasma preparation (centrifugation at high speed) is essential.

Another niche application is the use of the KCT in veterinary medicine, particularly in dogs, to screen for inherited coagulation disorders such as factor XII deficiency. Additionally, the KCT has been used in research to study the contact activation system and its role in thrombosis and inflammation. The test is also known to be affected by the source of kaolin, with different preparations yielding slightly different results, which has led to efforts to standardize the reagent.

Lupus anticoagulant

An antiphospholipid antibody that prolongs phospholipid-dependent clotting tests in vitro but is associated with thrombosis in vivo.

Contact activation pathway

The initial phase of the intrinsic coagulation cascade triggered by exposure of blood to negatively charged surfaces, involving factors XII, XI, and prekallikrein.

Platelet-poor plasma

Plasma prepared by centrifugation to remove platelets, typically with a platelet count below 10,000/µL, used for coagulation assays.

Mixing study

A test in which patient plasma is mixed with normal plasma to determine whether a prolonged clotting time is due to a factor deficiency or an inhibitor.