CoagulationCoagulation laboratory testsAbnormal PT and PTT - causes Show Last author update: 1 December 2020 Last staff update: 21 May 2021 Copyright: 2002-2022, PathologyOutlines.com, Inc. PubMed Search: Abnormal PT and PTT - causes Page views in 2021: 10,012 Page views in 2022 to date: 9,838 Cite this page: Seasor T, Moser KA. Abnormal PT and PTT - causes. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/coagulationabnormalPTPTT.html. Accessed December 23rd, 2022. Definition / general
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Board review style question #1 Which of the following is the most likely possible cause of combined PT and aPTT prolongation?
Board review style answer #1 D. Underfilled collection tube. Underfilling the blood collection tube will result in a prolongation of aPTT and PT due to the increased concentration of citrate. High hematocrit (not low hematocrit) can also result in a relative excess of citrate due to increased red blood cells and a relatively smaller volume of plasma in the tube. Lupus anticoagulant affects aPTT to a greater degree than PT due to the high concentration of phospholipids in PT reagents. Factor VII deficiency would be expected to prolong the PT but not the aPTT. Comment Here Reference: Abnormal PT and PTT - causes Board review style question #2 A 68 year old man has an aPTT of > 150 s (reference interval 24 - 35 s) and PT of 12 s (reference interval 12 - 15.5 s). A 1:1 aPTT mixing study result is 33 s. He has no personal or family history of a bleeding disorder. What is the most likely cause of his aPTT prolongation?
Board review style answer #2 C. Prekallikrein deficiency. Prekallikrein deficiency causes prolonged aPTT that corrects in a 1:1 mixing study and is not associated with clinical bleeding. The other contact factors (factor XII and high molecular weight kininogen) give a similar picture. Factor IX deficiency can also cause a prolonged aPTT that corrects in a mixing study but it is associated with a bleeding diathesis (hemophilia B). Factor XIII deficiency does not prolong the PT or aPTT (recall that factor XIII crosslinks fibrin but the PT and aPTT reaction endpoint is the formation of fibrin and these tests do not measure the effect of factor XIII). Some types of von Willebrand disease may be associated with prolonged aPTT (e.g. type 2N, type 3) due to significantly decreased factor VIII activity but the degree of aPTT prolongation is typically less than was seen in the case in this question. Comment Here Reference: Abnormal PT and PTT - causes Back to top What happens if you underfill a sodium citrate tube?Any citrate tube filled less than 90 percent of its stated volume will yield falsely lengthened coagulation results and can result in the physician adjusting the patient's anticoagulant dosage down- ward to a degree that risks serious complications including blood clots and stroke.
What happens if a coagulation specimen collection tube is underfilled?Evacuated collection tubes must be filled to completion to ensure that a 9:1 blood-to-anticoagulant ratio is achieved. Under-filling of citrate collection tubes results in an increased anticoagulant-to-blood ratio and can extend clot-based coagulation assays.
What will happen if a tube is underfilled?Underfilled tubes, defined as less than 90 percent of the fill volume, may result in prolongation of calcium-dependent, clot-based testing such as the PT and activated partial thromboplastin time (APTT) assays. Citrate's anticoagulant effect is due to chelation of calcium in the specimen.
How does a short draw affect PT & APTT?The short draw affected the PTTs but had no effect on the chromogenic assays. The problem with accepting the short draw is if you must perform additional clottable assays such as lupus anticoagulant profiles or factor assays. In this case, you might be giving out incorrect data.
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