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Coagulation
Activated partial thromboplastin time (aPTT)
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This is the date the draft was written, not a medical review date.
The aPTT assesses the intrinsic and common pathways (factors XII, XI, IX, VIII, X, V, II and fibrinogen) and is used to monitor unfractionated heparin and screen for factor deficiencies.
Reference interval
| Conventional (seconds) | SI (seconds) | |
|---|---|---|
| Adults | 25–35 seconds | 25–35 seconds |
Specimen: Citrated plasma · Conventional × 1 = SI
Causes of high values
Unfractionated heparin, haemophilia A and B, von Willebrand disease, factor XI or XII deficiency, liver disease, DIC, vitamin K deficiency and lupus anticoagulant (prolongs aPTT without bleeding, may be associated with thrombosis).
Causes of low values
A shortened aPTT is usually an artefact or reflects elevated factor VIII (acute phase) and has little diagnostic value.
Pre-analytical and method notes
The reference interval is reagent- and analyser-dependent (often 25-35 s, up to about 40 s) and each laboratory defines its own; heparin therapeutic ranges (usually expressed as a ratio to control) are set locally. A prolonged aPTT that does not correct on a 1:1 mixing study suggests an inhibitor. Delayed processing, underfilled tubes and difficult sampling affect results.
References
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Elsevier — Tier 3 (textbook)
- Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
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