Abnormal Coagulation Tests
PT and/or PTT prolonged
All Pathways
PathologyDiagnostic
Coagulation Disorder Workup Algorithm (ISTH)
Coagulation Disorder Workup Algorithm (ISTH): Abnormal Coagulation Tests → Pre-Analytic Check → Pattern of Abnormality → Isolated PT Prolongation → Mixi...
Pathway Overview
11 steps
Algorithm Steps
11 total
01Start 02Action Pre-Analytic Check
Rule out artifacts
- Proper fill (9:1 blood:citrate)
- Hemolysis, lipemia, icterus
- Sample age (<4 hours for PTT)
- Anticoagulant contamination
- Consider repeat if questionable
03Decision Pattern of Abnormality
PT, PTT, or both
- Isolated prolonged PT
- Isolated prolonged PTT
- Both PT and PTT prolonged
- Thrombin time if needed
04Action Isolated PT Prolongation
Extrinsic pathway
- Factor VII deficiency (early, short half-life)
- Warfarin therapy
- Liver disease (early)
- Vitamin K deficiency (early)
- Consider mixing study if unexpected
05Decision Mixing Study
Mix 1:1 with normal plasma
- Corrects: Factor deficiency
- Does not correct: Inhibitor present
- Partial correction: Weak inhibitor or low factor
- Incubate for lupus anticoagulant
06Action Mixing Study Corrects
Factor deficiency
- Order factor assays based on pattern
- PTT only: VIII, IX, XI, XII
- PT only: VII
- Both: II, V, X, fibrinogen
- Consider von Willebrand panel
07Outcome Final Interpretation
Diagnosis with clinical correlation
08Action Mixing Study Does Not Correct
Inhibitor present
- Immediate non-correction: Lupus anticoagulant
- Time-dependent: Factor VIII inhibitor
- LA testing: dRVVT, hexagonal phase
- Bethesda assay for factor inhibitors
- Consider heparin contamination
09Action Lupus Anticoagulant Workup
ISTH criteria
- Screen: dRVVT, sensitive aPTT
- Mixing study: No correction
- Confirm: Correction with phospholipid
- Must confirm on 2 occasions 12 weeks apart
- Associated with thrombosis, not bleeding
- Path rejoins step 07Shared downstream outcome
10Action Isolated PTT Prolongation
Intrinsic pathway
- Heparin effect most common
- Factor VIII, IX, XI, XII deficiency
- Lupus anticoagulant
- von Willebrand disease
- Contact factor deficiency
- Path rejoins step 05Shared downstream outcome
- Path rejoins step 09Shared downstream outcome
11Action Both PT & PTT Prolonged
Common pathway or multiple defects
- DIC (consumptive)
- Liver disease (advanced)
- Vitamin K deficiency (advanced)
- Factor II, V, X deficiency
- Massive transfusion
- Anticoagulant overdose
- Path rejoins step 05Shared downstream outcome
Guideline Source
ISTH Guidelines for Coagulation Testing
Clinical Safety Information
Clinical Decision Support — Not a Substitute for Clinical Judgment
Individual patient factors may require deviation from these recommendations.
Known Limitations
- Pre-analytical variables affect results
- Anticoagulant interference must be considered
- Some rare deficiencies need specialized testing
- Platelet function testing varies by institution
- Mixing studies interpretation requires experience
Applicable Regions
USAUUKEU
AU: RCPA haemostasis guidelines
UK: BCSH coagulation testing standards
US: ISTH and CAP guidelines
Version 1Next review: 2028-01-01
Next steps
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Related Resources
Frequently Asked Questions
What is the Coagulation Disorder Workup Algorithm (ISTH)?
The Coagulation Disorder Workup Algorithm (ISTH) is a diagnostic clinical algorithm for Pathology. It provides a structured decision tree to guide clinical decision-making, based on ISTH Guidelines for Coagulation Testing.
What guideline is the Coagulation Disorder Workup Algorithm (ISTH) based on?
This algorithm is based on ISTH Guidelines for Coagulation Testing (DOI: 10.1111/jth.14214).
What are the limitations of the Coagulation Disorder Workup Algorithm (ISTH)?
Known limitations include: Pre-analytical variables affect results; Anticoagulant interference must be considered; Some rare deficiencies need specialized testing; Platelet function testing varies by institution; Mixing studies interpretation requires experience. Individual patient factors may require deviation from these recommendations.
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