Diagnostic Use
The APTT is a useful and sensitive test to screen for deficiencies/abnormalities of the intrinsic and common pathways of coagulation cascade and for monitoring of heparin therapy.
If APTT is prolonged and the cause is not due to heparin contamination or other pre-analytical problems, correction studies with normal plasma and testing with alternate APTT reagents may be required:
1. Complete correction suggests factor deficiency.
2. No or partial correction suggests heparin, lupus anticoagulant or presence of factor inhibitor (antibody).
3. Addition of protamine sulphate corrects heparin defect.
Requesting physician will be contacted if more complex and expensive testing is indicated.
Interpretation
APTT results are often interpreted with results of the PR in determining which condition may be present.
Causes of prolonged APTT include:
Haemophilia A (factor VIII deficiency), haemophilia B (Christmas disease, factor IX deficiency), von Willebrand Disease, factor XII deficiency (not usually associated with abnormal bleeding), factor XI deficiency, liver disease, disseminated intravascular coagulation, heparin therapy, factor inhibitors (most commonly against F VIII), lupus anticoagulant.
Short APTT usually reflects factor activation from difficult collect or recent trauma/surgery.
Reference Intervals
| Age |
Triniclot RI |
Actin RI |
| Birth – 1 day |
39 – 51 |
39-47 |
| 2 – 5 days |
36 – 54 |
37-50 |
| 6 – 30 days |
35 – 50 |
35-46 |
| 31 – 90 days |
32 – 46 |
32-43 |
| 3 – 6 months |
33 – 41 |
33-38 |
| 6 months – 1 year |
32 – 49 |
32-45 |
| 1 year – Adult |
25 – 38 |
24-35 |
Therapeutic – Units: seconds
| Age Range |
Either Sex |
| All |
60 – 100[1] |
[1] Triniclot APTT Therapeutic Range for unfractionated heparin.Therapeutic range based on CLSI Guidelines H47 A2 (2008) correlates to 0.3 – 0.7 IU/ml.
Test Method
Assay Method: LabPLUS has changed to TriniCLOT aPTT S for APTT testing. The APTT reference interval remains unchanged, and the UFH therapeutic range remains unchanged at 60-100 seconds. Anti-Xa remains primary for UFH dose adjustment.
Method effective from 06/05/2026.
Limitations / Interference
APTT may not detect mildly reduced coagulation factor levels of 30 - 50%. If mild haemophilia is suspected, even in the presence of a normal APTT, specific factor assays should be performed.
Collection artifacts such as heparin contamination, clots, incompletely filled tubes (excess citrate), polycythemia (citrate/plasma ratio inappropriate), high lipid levels, TPN may spuriously prolong APTT.
Uncertainty of Measurement
4%