Coagulation

• Mixing studies are typically used to investigate abnormal clotting time results
• Mixing studies help distinguish clotting time prolongation due to a coagulation factor deficiency or an inhibitor (specific or nonspecific)
• Mixing study may direct further coagulation testing but it is not by itself diagnostic

Contributed by Tori Seasor, M.D. and Karen A. Moser, M.D.

Differential diagnosis of prolonged PT or aPTT incorporating mixing study results (correction versus noncorrection)
Clotting time prolonged	Mixing study result	Key differential diagnoses
Prothrombin time (PT)	Corrects	Factor deficiency (VII, X, V, II) Warfarin Vitamin K deficiency Liver disease
	Does not correct	Specific factor inhibitor (VII, X, V, II) Direct Xa inhibitor
Activated partial thromboplastin time (aPTT)	Corrects	Factor deficiency (VIII, IX, XI, contact factors) Rarely, von Willebrand disease (if VIII is low enough to prolong aPTT)
	Does not correct	Specific factor inhibitor (VIII, IX, XI, contact factors) Heparin Direct thrombin inhibitor Lupus anticoagulant
PT and aPTT	Corrects	Common pathway factor deficiency (X, V, II) Warfarin (high doses) or superwarfarin Vitamin K deficiency Disseminated intravascular coagulation Liver disease
	Does not correct	Common pathway factor inhibitors (X, V, II) Direct thrombin inhibitors Direct Xa inhibitors Lupus anticoagulant (rarely) Heparin (very high doses)

Prothrombin time (PT) and activated partial thromboplastin time (aPTT) are performed on a 64 year old man. The patient is taking no anticoagulants or other medications. The results of a 1:1 mixing study are

Patient aPTT: 51 s (reference range 38 - 42 s)
Immediate 1:1 aPTT mixing study: 36 s

Which of the following is the next best test to order?

1. 4:1 mixing study
2. Factor VIII activity
3. Factor X activity
4. von Willebrand factor activity

B. Factor VIII activity. The mixing study indicates a corrected result which suggests a factor deficiency as a cause of the prolonged aPTT. The next step would be to investigate which factor deficiency is causing this, which can be accomplished with factor VIII, IX and XI activity testing. Factors VIII, IX and XI are factors in the intrinsic coagulation pathway that can prolong the aPTT and are associated with clinical bleeding. Lupus anticoagulant testing could also be considered, as some weak lupus anticoagulants may show apparent correction in mixing studies.

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Reference: Mixing studies

Coagulation laboratory tests are drawn on a 35 year old woman with the recent diagnosis of an autoimmune disease. The physician ordered a 1:1 mixing study after the original results revealed a prolonged activated partial thromboplastin time (aPTT).

Patient aPTT: 51 s (reference range 38 - 42 s)
Immediate 1:1 aPTT mixing study: 50 s

Which of the following do these laboratory results suggest?

1. Factor V Leiden mutation
2. Factor VII deficiency
3. Lupus anticoagulant
4. Vitamin K deficiency

C. Lupus anticoagulant. The aPTT was prolonged in the patient sample and the addition of NPP in the immediate mixing study did not correct the aPTT to the normal reference range. This result suggests an inhibitor that overcomes the normal coagulation factors in the patient sample as well as the NPP. Lupus anticoagulants behave as nonspecific inhibitors in aPTT mixing studies and are a common cause of unexpected aPTT prolongation. The recent history of an autoimmune disorder diagnosed in a young woman would also raise concern for a lupus anticoagulant as an explanation for her prolonged aPTT.

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Reference: Mixing studies