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Read the full lessonHemostasis Disease States and Laboratory Determinations

Hematology, quick reference 4 of 10

04 / 10 · Hematology

PT and APTT patterns

Screening patterns, specimen checks, and mixing studies

Sources checked Reviewed by Alexan Berberian, MS, MLS(ASCP) ·

Prothrombin time (PT) and activated partial thromboplastin time (APTT) measure the clotting time of citrated plasma after activation by different reagents. Some factors are measured by both tests, so one time or both can be prolonged.1

PT · tissue factor / extrinsic arm
VII

APTT · contact / intrinsic arm
XII → XI → IX + VIII

Common: X + V → thrombin (activated II) → fibrin formation

Assay schematic. Roman numerals identify the factors. Factors V and VIII are cofactors. Thrombin cleaves fibrinogen to fibrin. Contact activation also needs prekallikrein and high-molecular-weight kininogen.1

Patterns and possible explanations2

Screening pattern Possibilities to investigate Distinguishing findings
PT prolonged
APTT in range 		VII deficiency/inhibitor; early vitamin K deficiency or warfarin. Also possible with liver disease or direct factor Xa inhibitors.
APTT prolonged
PT in range 		VIII, IX, XI, or contact-factor deficiency/inhibitor; heparin; lupus anticoagulant. Contact-factor deficiency usually causes no bleeding. Lupus anticoagulant can accompany thrombosis.
Both prolonged Common/multiple-factor deficiency: vitamin K deficiency, liver disease, or disseminated intravascular coagulation; anticoagulants. Compare with platelets, fibrinogen, medications, and targeted tests.

Specimen and method checks

An underfilled tube leaves excess citrate for the plasma volume and may prolong times. A hematocrit above 55% also reduces the plasma volume, so adjust the citrate volume as the procedure directs. Check for clots, heparin from a line draw, delays, and optical interference.3

Anticoagulant effects depend on drug, concentration, and method. Normal times cannot exclude a direct oral anticoagulant.2

What a mixing study shows

Equal volumes of patient and normal plasma supply missing factors. If the time corrects by the method's criteria, deficiency is more likely. Failure to correct suggests an inhibitor or drug. Some inhibitors become apparent only after incubation.1

Mixing also dilutes inhibitors, so a weak inhibitor may be missed. Anticoagulant drugs can mimic an inhibitor pattern.1

Bleeding with normal PT/APTT: investigate von Willebrand disease, platelet dysfunction, mild factor deficiencies, or factor XIII deficiency.2

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