Test/Range/Collection

Inhibitor screen (1:1 mix), plasma

Negative

Blue

$$

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).

Interpretation

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).

Comments

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]