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Inhibitor screen (1:1 mix)


Inhibitor screen (1:1 mix), plasma




Fill tube completely.

Physiologic Basis

Test is useful for evaluating a prolonged PTT and/or PT. (Presence of heparin should first be excluded.)

Patient’s plasma is mixed with pooled normal plasma (1:1 mix) and PTT and/or PT are performed. If the patient has a factor deficiency, the post-mixing PTT/PT will be normal (correction). If an inhibitor is present, the post-mixing PTT/PT will remain prolonged (no correction) immediately and/or after incubation (at 37°C for 1 or 2 hours).


Positive in: Presence of inhibitor: Antiphospholipid antibodies (lupus anticoagulant, LAC), factor-specific antibodies, or both.

Negative in: Factor deficiencies.

See evaluation of isolated prolongation of PTT (Figure 9–28).


Lupus anticoagulant is a nonspecific inhibitor, which prolongs PTT on inhibitor screen (1:1 mix study) both immediately and after 1–2 hours' incubation. 1-hour to 2-hour incubation period is often needed to detect factor-specific antibodies with low in vitro affinities (eg, post-mixing PTT is normal immediately, but is prolonged after incubation).

Dependent on the result of the 1:1 mix study, follow-up testing is often performed, ie, dilute Russell’s viper venom time (DRVVT) if lupus anticoagulant is suspected or factor assays if factor deficiency is considered.

Ortel TL. Antiphospholipid syndrome: laboratory testing and diagnostic strategies. Am J Hematol 2012;87:S75.  [PMID: 22473619]

Sakurai Y et al. Acquired hemophilia A: a frequently overlooked autoimmune hemorrhagic disorder. J Immunol Res 2014;2014:320674.  [PMID: 24741588]

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McPhee, Stephen J., et al. "Inhibitor Screen (1:1 Mix)." Guide to Diagnostic Tests, 7th ed., McGraw-Hill Education, 2017. Medicine Central, im.unboundmedicine.com/medicine/view/GDT/619202/all/Inhibitor_screen__1:1_mix_.
McPhee SJ, Lu CM, Nicoll D. Inhibitor screen (1:1 mix). Guide to Diagnostic Tests. 7th ed. McGraw-Hill Education; 2017. https://im.unboundmedicine.com/medicine/view/GDT/619202/all/Inhibitor_screen__1:1_mix_. Accessed March 20, 2019.
McPhee, S. J., Lu, C. M., & Nicoll, D. (2017). Inhibitor screen (1:1 mix). In Guide to Diagnostic Tests. Available from https://im.unboundmedicine.com/medicine/view/GDT/619202/all/Inhibitor_screen__1:1_mix_
McPhee SJ, Lu CM, Nicoll D. Inhibitor Screen (1:1 Mix) [Internet]. In: Guide to Diagnostic Tests. McGraw-Hill Education; 2017. [cited 2019 March 20]. Available from: https://im.unboundmedicine.com/medicine/view/GDT/619202/all/Inhibitor_screen__1:1_mix_.
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TY - ELEC T1 - Inhibitor screen (1:1 mix) ID - 619202 A1 - McPhee,Stephen J, AU - Lu,Chuanyi Mark, AU - Nicoll,Diana, BT - Guide to Diagnostic Tests UR - https://im.unboundmedicine.com/medicine/view/GDT/619202/all/Inhibitor_screen__1:1_mix_ PB - McGraw-Hill Education ET - 7 DB - Medicine Central DP - Unbound Medicine ER -