Here is a question that appeared on the Clinical Laboratory Science (CLS) Educators’ list: “We are trying to figure out the principle, procedure for the tissue thromboplastin inhibition (TTI) test. We are aware that it seemingly is no longer performed, but feel there is a good chance that the ASCP could potentially ask questions about it on their certification exam. Can anyone clarify this test for us? Somehow we are having trouble understanding what we are finding in the literature.
I provided this answer for the list:
The TTI test is synonymous with the dilute prothrombin time (DPT) test. Here is the description from my old textbook, Corriveau DM, Fritsma GA. Hemostasis and Thrombosis in the Clinical Laboratory. Lippincott, 1988:
“In the tissue thromboplastin inhibition test, PT reagent is diluted 1:50 or 1:500 with saline. The dilution is prewarmed, then 0.1 mL of dilute reagent is mixed with 0.1 mL of the test plasma and the mixture is incubated for 3 minutes. CaCl2 (0.025 molar) is forcibly added and a timer started. PNP is tested at the same time, and the results are compared. If the ratio of patient plasma interval to PNP interval is 1.3 or greater, lupus anticoagulant is presumed to be present. A ratio of 1.1 or less is considered to be normal, and lupus anticoagulant is absent.”
The test was developed in Dr. Doug Triplett’s Midwest Hemostasis laboratory, and is now usually called the DPT. Some still use it, although in 2009 the ISTH discouraged its use as being poorly reproducible in Pengo V, Tripodi A, Reber G, Rand JH, Ortel TL, Galli M, De Groot PG; Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibody of the Scientific and Standardisation Committee of the International Society on Thrombosis and Haemostasis. Update of the guidelines for lupus anticoagulant detection. Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibody of the Scientific and Standardisation Committee of the International Society on Thrombosis and Haemostasis. J Thromb Haemost. 2009;7:1737-40. However, those who continue to use it swear by it for its sensitivity.