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SOP

PT/APTT Mixing (Correction) Study

SOP for the 1:1 plasma mixing study to distinguish factor deficiency from circulating inhibitor in prolonged PT or APTT.

Last verified 1 month ago 3 min read Coagulation #mixing study #correction study #factor deficiency #lupus anticoagulant #inhibitor
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Document Control
Version
1.0
Effective
Aug 30, 2026
Reviewed
Aug 30, 2026
Department
Coagulation
Principle

Pooled normal plasma supplies all coagulation factors in adequate amounts. In a 1:1 mix, a simple factor deficiency is corrected to near-normal because the added normal factors restore activity above the threshold needed to clot. A circulating inhibitor neutralizes the added normal factors in proportion to its strength, so the mix stays prolonged. Time-dependent (progressive) inhibitors require incubation at 37°C before their effect becomes detectable, so incubated mix testing is essential.

Standard Operating Procedure: PT/APTT Mixing (Correction) Study

1. Purpose and Scope

To determine whether a prolonged prothrombin time (PT) or activated partial thromboplastin time (aPTT) is caused by a coagulation factor deficiency (correctable) or by a circulating inhibitor such as a lupus anticoagulant or a specific factor inhibitor (non-correctable), including incubation to expose time-dependent inhibitors.

2. Specimen Requirements

  • Type: Citrated plasma (3.2% sodium citrate, 1:9), double-centrifuged platelet-poor plasma from both the patient and a healthy normal donor/pooled normal plasma.
  • Volume: Sufficient for PT, APTT, mix, and incubation repeats (typically 2-3 mL collected).
  • Filling: Tubes must be filled to the line; underfilling prolongs results.
  • Timing: Test promptly; freeze only if needed and ensure platelet-poor preparation.

3. Safety and Precautions

  • Standard PPE (gloves, lab coat). Blood is potentially infectious.
  • Dispose of sharps and biohazard waste per laboratory policy.
  • Do not mouth-pipette.

4. Equipment and Reagents

  • Automated coagulation analyzer.
  • PT and APTT reagents with the same lot used for the patient run.
  • Pooled normal plasma with a documented normal PT and APTT.
  • Incubation block at 37°C for the 2-hour time-dependent step.

5. Step-by-Step Procedure

  1. Run patient PT and APTT and confirm prolongation above the reference interval.
  2. Prepare a 1:1 mix: one part patient plasma mixed with one part pooled normal plasma; mix gently and test immediately.
  3. Repeat the mix testing after incubation at 37°C for 1-2 hours (essential for lupus anticoagulant and factor VIII inhibitors).
  4. Interpret against a correction criterion: apply the laboratory-established rule (e.g., the mix result within a defined correction window of the reference interval/pooled normal, or the Rosner index / correction index calculation).
  5. When the immediate and incubated mixes both correct, a factor deficiency is suggested; when the incubated mix prolongs, a time-dependent inhibitor is suggested; when neither corrects, an immediate inhibitor (lupus anticoagulant) is suggested.
  6. Proceed to factor assays (PT-derived and APTT-derived) and, if indicated, a lupus anticoagulant panel to localize and confirm.
  7. Release results with a written interpretation.

6. Quality Control

  • Run two levels of PT/APTT control on the day of testing.
  • Validate the pooled normal plasma against reference values daily; check its platelet-free status.
  • Participate in EQA for mixing studies if available; otherwise, maintain competency records for the interpretation protocol.

7. Decision limits

The interpretive thresholds below are clinical decision limits; the canonical adult reference intervals are displayed in the Reference Ranges panel.

  • Correction (immediate mix normalizes or stays within the correction window): factor deficiency — in the setting of an isolated prolonged APTT, one factor, such as factor VIII, IX, XI, or XII, is deficient; multiple-factor deficiency (DIC, liver disease, vitamin K deficiency, warfarin/supra-therapeutic anticoagulant) also corrects.
  • No correction (persistently prolonged mix): circulating inhibitor — lupus anticoagulant prolongs the APTT (often mildly) and does not correct with mixing; a specific factor inhibitor (e.g., anti-factor VIII) is typically time-dependent and lengthens on incubation.
  • Time-dependent inhibitors: a normal immediate mix but prolonged 2-hour incubated mix is classic for anti-factor VIII in acquired hemophilia.
  • Limitations: factor deficiency below roughly 30-50% may still prolong the mix if the factor level is very low; interpretation is part of a multistep algorithm and never a standalone result.

Reference Ranges

Browse all ranges

11–13.5 seconds

25–35 seconds

Frequently Asked Questions

It distinguishes a factor deficiency, which corrects when normal plasma is added, from a circulating inhibitor, which keeps the clotting time prolonged by neutralizing the added normal factors.
Some inhibitors, especially factor VIII inhibitors, act progressively over time. Immediate mixing looks normal but incubation reveals the prolongation, which is why an incubated mix is essential to detection.
Yes, in most cases a 1:1 mix with normal plasma corrects factor deficiencies, because the normal plasma supplies enough of the missing factor to achieve threshold clot activity. Very low factor levels below about 30-50% may not show full correction.