I (Geo) prefer PTT to APTT or aPTT because the “A” or “a” is superfluous. Laboratory scientists no longer offer an “unactivated” PTT. The “partial thromboplastin” reagent consists of a phospholipid or combination of phospholipids that may include animal, plant, or synthetic sourced phosphatidylserine, phosphatidylcholine, phosphatidylinositol, sphingomyelin, and/or phosphatidylethanolamine. Accompanying the phospholipid is one of several particulate activators including silica, kaolin, ellagic acid, polyphenols, or Celite (diatomaceous earth). The PTT reagent is warmed to 37°C and mixed 1:1 with 37°C test plasma. The mixture is incubated for a set period after which 37°C 0.025 M calcium chloride (CaCl2) solution is added and the reaction is timed to clot formation. This assay is typically automated.
A typical PTT reference interval is 25–35 seconds. Laboratory scientists employ the PTT as a screen to detect a procoagulant deficiency, often FVIII or FIX deficiency, and manufacturers calibrate their PTT reagent so that the reaction becomes prolonged at a specified FVIII and FIX level. Our September Quick Question asks, “At what FVIII activity level should the PTT become prolonged?” The actual number, in U/dL may be debated as we are concerned about the percentage of false negatives and false positives, so please comment on your answer in the space provided.
September 5: In response to Dr. Favaloro’s comment, I’m comfortable using APTT or aPTT, though I continue to consider the “A” or “a” superfluous same as the “III” or “3” as in ATIII for antithrombin, which most authors have dropped in favor of AT. The PTT was introduced in 1953 and an activator was added in the 1970s. I have a fuzzy memory of tediously performing the original PTT in 1967 as a callow MLS student by “tilt-tube,” when the normal would reach ~90 seconds. The “unactivated” PTT was never used after 1980.
Here’s the citation for Dr. Favaloro’s referenced article: Favaloro EJ, Kershaw G, Mohammed S, Lippi G. How to optimize activated partial thromboplastin time (APTT) testing: solutions to establishing and verifying normal reference intervals and assessing APTT reagents for sensitivity to heparin, lupus anticoagulant, and clotting factors. Semin Thromb Hemost. 2019;45:22–35. doi: 10.1055/s-0038-1677018. PMID: 30630206. If you receive your copy from Dr. Favaloro, it offers clues to help you answer the September 2024 Quick Question.
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