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]

Inhibitor screen (1:1 mix) is a sample topic from the Guide to Diagnostic Tests.

To view other topics, please or .

Anesthesia Central is an all-in-one web and mobile solution for treating patients before, during, and after surgery. This collection of drugs, procedures, and test information is derived from Davis’s Drug, MGH Clinical Anesthesia Procedures, Pocket Guide to Diagnostic Tests, and PRIME Journals. .