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.
Hi George,
I am in favor of using “aPTT”, with a small letter a. Perhaps, the confusion partly began when people started using the capital A, ignoring that the activated state in biochemistry/enzymology would be represented with the small letter “a”. Similarly we use aFX (but not AFX) for the activated state of Factor X in coagulation to differentiate it from FX zymogen. Unfortunately, the tradenames in the past have violated these principles or at least did not help the situation. For instance, ACT (activated clotting time) and APTT-SP, commercialized with capital A or sometimes even without a/A such as PTT Automate for ease of use.
BTW, although the clinical application of PTT has almost stopped in North America, research works and publications are still done using PTTs, reagents without activators (non-activated PTT or procoagulant phospholipid formulations). So, the authors in these circumstances could rightfully use the PTT term instead of a defined complex mixture of phospholipids, buffers, preservatives, etc and differentiate their reagents from aPTTs (with activator).
With all due respect, I think your proposal about omitting “a” from aPTT and instead using PTT might help clinicians but it could also confuse researchers in the field with procoagulant phospholipids.
Best regards, -Ali
Hi George,
the term PTT is certainly favored in North America. I prefer the term aPTT or APTT to denote it’s an activated PTT. Agree that North American labs no longer offer an ‘unactivated’ PTT, but the term PTT remains ambiguous and has always been in use, so at what stage should we say North American labs switched from an ‘unactivated’ PTT to an activated PTT? Of course, the aPTT is used for more than as a screen for factor deficiencies such as FVIII and FIX. In some labs it is also used to monitor unfractionated heparin, since it is more commonly available than the anti-Xa. Also, in some labs, it is used as a part of the investigation for lupus anticoagulant (LA). Such a multi-purpose assay, and aPTT reagents are variously sensitive to these, and so selecting the right aPTT reagent for you lab can be problematic. For guidance on this, see my STH ‘blog’ (PubMed ID: 30630206). Happy to share a copy with anyone that asks me (best option is by email: [email protected]). Cheers, E