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MLT Registration: Coagulation

MLT Registration · Work through your exam, one topic at a time.

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Coagulation

Coagulation items on the MLT exam are preanalytical as often as interpretive. You are expected to know why a citrate tube fails, which pathway each screening test measures, how a mixing study separates deficiency from inhibitor, and how to monitor heparin and warfarin without being fooled by interference.

Specimen rules

  • Use 3.2% sodium citrate at a 9:1 blood-to-anticoagulant ratio; fill to the etched line, because a short draw leaves excess citrate and falsely prolongs clotting times.
  • Draw a discard tube first when collecting from a line or butterfly to clear dead space; never draw a citrate tube after an EDTA tube, since carryover chelates calcium.
  • Hematocrit above roughly 55% requires reduced citrate volume per a standard formula.
  • Centrifuge to platelet-poor plasma and test within about 4 hours at room temperature; freeze plasma at -20 degrees C for short storage and at -70 degrees C for factor assays.

What each test measures

  • PT reflects factor VII plus the common pathway (X, V, II, fibrinogen). INR = (patient PT / mean normal PT) raised to the ISI power.
  • aPTT reflects factors VIII, IX, XI, XII plus the common pathway and is heparin sensitive.
  • Thrombin time and reptilase time assess fibrinogen conversion; a prolonged thrombin time with a normal reptilase time points to heparin.

Pattern recognition

  • Prolonged aPTT with normal PT: hemophilia A or B, factor XI or XII deficiency, von Willebrand disease, heparin, or lupus anticoagulant.
  • Prolonged PT only: factor VII deficiency, early warfarin effect, early liver disease or vitamin K deficiency, since factor VII has the shortest half-life.
  • Both prolonged: common pathway defect, severe deficiency, disseminated intravascular coagulation, liver failure, or a direct oral anticoagulant.
  • Mixing study: correction immediately and after 2 hours at 37 degrees C means factor deficiency; failure to correct means an inhibitor such as a factor VIII inhibitor or lupus anticoagulant.

Worked example

aPTT 78 s (reference 25-35), PT 12.4 s (reference 11-14). A 50:50 mix corrects to 33 s immediately and to 34 s after 2-hour incubation. Factor VIII is 4%, vWF antigen is 55%, platelet count is normal. Reasoning: correction proves a deficiency rather than an inhibitor; a normal PT localizes the defect to the intrinsic pathway; a low factor VIII with normal vWF antigen supports hemophilia A rather than von Willebrand disease. Report and refer for factor replacement planning.

Traps that cost marks

  • Interpreting a prolonged INR in liver disease or on a direct oral anticoagulant as a valid warfarin dose guide.
  • Calling a lupus anticoagulant a bleeding risk; it is associated with thrombosis.
  • Missing heparin contamination when thrombin time is prolonged but reptilase time is normal.
  • Accepting an underfilled citrate tube.