Kaolin clotting time (KCT) is a sensitive test to detect lupus anticoagulants.^[2] There is evidence that suggests it is the most sensitive test for detecting lupus anticoagulants.^[3] It can also detect factor VIII inhibitors but is sensitive to unfractionated heparin as well.^[4]

The KCT on whole blood is known as the "Activated Clotting Time" (ACT) and is widely used in various instruments during surgery such as cardiac bypass to monitor heparin.^[5]

KCT was first described by Dr. Joel Margolis in 1958.^[6] Later on, it was found to be very sensitive to lupus anticoagulants but was only reliable when test plasmas were mixed with normal plasma in various proportions.^[3] It became the preferred method for lupus anticoagulant testing after Dr. Wilhelm Lubbe showed it to be a good marker for recurrent fetal loss.^[7]

KCT is similar to the activated partial thromboplastin time test, except it does not use exogenous phospholipid.^[2] Thus, a confirmatory test that uses excess phospholipid is needed to validate the presence of lupus anticoagulants.^[2] Otherwise, diluting the test plasma in normal plasma before testing provides characteristic mixing patterns.^[8]

Kaolin is the surface activator, and the test also requires small amounts of cell fragments and plasma lipids to provide the phospholipid surface required for coagulation.^[2][4] Therefore, the sample quality is important for the validity of the screening test.^[2]

The test combines a test plasma with kaolin, and after a brief pre-incubation and the addition of calcium chloride, the time to clot (in seconds) is measured.^[6] Mixes of patient plasma with normal plasma are recommended for testing.^[9]

The KCT test/control ratio of greater than or equal to 1.2 indicates that a defect is present.^[4] If the test/control ratio is between 1.1 and 1.2, the test is equivocal.^[4]

A good way of expressing the result using mixes is to calculate the Rosner index.^[10] If A is the KCT of normal plasma, B is that of the 1:1 mix and C is that of the patient plasma, then the Rosner index is 100x(B-A)/C. Values above 15 indicate a positive result but in most cases labs set their own cutoff values.^[9]

If the KCT is less than 60 seconds, this suggests that the test plasma is contaminated with platelet fragments; therefore, the test is not valid.^[4]

• Dilute Russell's viper venom time
• Partial thromboplastin time
• Activated clotting time