How to read coagulation tests (PT, INR, aPTT)
Other platforms explain it with text and video. In VMS you practice it on the patient.
Clotting tests guide anticoagulant doses and warn of bleeding before procedures. Knowing which test reflects which pathway makes the results useful.
Step by step
Check the sample
The blue-top tube must be filled to the line. Do not draw from a line that has heparin running through it. [1]
Read PT and INR
PT and INR measure the extrinsic pathway. They rise with warfarin, liver disease and vitamin K deficiency. [2]
Use INR for warfarin
Most patients on warfarin aim for an INR of 2 to 3. Some mechanical heart valves need a higher range. [2]
Read the aPTT
The aPTT measures the intrinsic pathway. It rises with heparin, hemophilia and some factor inhibitors. [2]
When both are high
Both PT and aPTT high suggests severe liver disease, DIC, massive transfusion or a large anticoagulant effect. Check platelets and fibrinogen. [2]
Remember the newer drugs
Direct oral anticoagulants affect these tests in unpredictable ways. A normal result does not prove the drug has worn off. [2]
Report and act
Report out-of-range results and bleeding signs. Hold doses only as ordered and follow the reversal protocol. [1]
Common errors
- Drawing an aPTT from a line with heparin running.
- Using the INR to monitor heparin.
- Assuming a normal INR rules out a direct oral anticoagulant effect.
Typical adult ranges; use your lab's ranges [2]
| Test | Typical normal | Common target on treatment |
|---|---|---|
| PT | About 11 to 13.5 seconds | Read as INR on warfarin |
| INR | About 0.8 to 1.1 | 2 to 3 for most warfarin uses |
| aPTT | About 25 to 35 seconds | About 1.5 to 2.5 times control on heparin, per protocol |
Practice it in VMS
In VMS you do this on the patient, not on paper: every step goes on the checklist with a score, and in Teach mode Blocky tells you what comes next.
VMS cases that train this skill
Sources
Facts and steps follow AAMC Core Entrustable Professional Activities (Core EPAs); NCCPA PANCE Content Blueprint. VMS is not affiliated with these organizations. Updated: 2026-10-11.