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aPTT
The seconds blood takes to clot via the intrinsic pathway. Checked before surgery, for unexplained bruising and to monitor heparin.
Other names: aPTT, PTT, Activated Partial Thromboplastin TimeSection: Blood clotting
Updated: August 31, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortThe aPTT measures how many seconds blood takes to clot via the intrinsic pathway. A typical range is 25–35 seconds; heparin predictably prolongs it. Prolongation without therapy suggests clotting factor deficiency, shortening suggests hypercoagulability.
Reference ranges
Group
Reference range
Adults
25–35 seconds
The source gives a typical range of 25–35 seconds; blood thinners predictably prolong the time. Reagents differ between laboratories, so the reference on your report may differ. Reference ranges vary between laboratories and assays — use the range printed on your own lab report.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
The aPTT (activated partial thromboplastin time) is the number of seconds a clot takes to form in a test tube after activators of the intrinsic pathway are added. It assesses factors XII, XI, IX and VIII plus the common pathway — X, V, II and fibrinogen.
Doctors order it for unexplained bleeding or bruising, as part of pre-surgical workups, when hemophilia is suspected and to monitor unfractionated heparin. Results are always read together with the INR: different combinations of abnormalities point to different clotting defects.
When it is high
Hemophilia A and BInherited factor VIII or IX deficiency markedly prolongs aPTT with a normal INR.
Heparin therapyUnfractionated heparin prolongs aPTT — the test is used to guide dosing.
Von Willebrand diseaseThe most common inherited clotting disorder; aPTT prolongs when factor VIII falls.
Liver disease and vitamin K deficiencyReduced factor synthesis prolongs both aPTT and INR.
Lupus anticoagulantAutoimmune antibodies prolong aPTT in the tube yet raise clot — not bleeding — risk in the body.
When it is low
HypercoagulabilityA shortened aPTT occurs in thrombophilias, acute inflammation and the early postoperative period.
High fibrinogenExcess factor I speeds up clot formation.
Technical errorVery short times can result from a bad draw or clot activation in the tube — the test is repeated.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Blood is drawn into a citrate tube. No special preparation, but tell the lab about heparin, warfarin and other anticoagulants — they directly affect the result. The tube must be filled to the mark: underfilling distorts the blood-to-citrate ratio.
When a retest makes sense
Before surgery — as the surgeon requires; on heparin — on the dosing schedule. An unexpected abnormality without symptoms is worth rechecking in 2–4 weeks.
Who usually reviews this result
A result outside the reference range is usually reviewed by: hematologist; before surgery — GP or anesthesiologist; on heparin — the prescribing doctor. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: aPTTAPTTPTTActivated partial thromboplastin timePartial thromboplastin time
Units
Measured in: seconds
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
They test two different pathways: the INR (prothrombin time) covers the extrinsic pathway and responds to warfarin; the aPTT covers the intrinsic pathway and responds to heparin. A full coagulation panel reads them together.
Is an aPTT of 40 seconds dangerous?
Modest prolongation without bleeding is often explained by the lab's reagent or mild factor deficiency. Compare with your report's reference and show the result to a doctor — especially with bruising, gum bleeding or upcoming surgery.
Do I need to fast for aPTT?
Not strictly, but most labs recommend a morning fasting draw for coagulation tests — fewer variables affecting the plasma.
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