Required section · Section 2 of 6
What the screen actually tests
PT and aPTT are conventional clot-based tests performed on citrated platelet-poor plasma. Each test triggers clotting through a different starting point and times how long the plasma takes to clot in a mechanical clot-detection system. A prolonged result means something in the tested pathway is slower than the reference population, and nothing more specific than that until follow-up narrows it down.
The reusable model for this whole module is a short sequence: check the specimen and the drug history, compare the screen to its reference interval, and if the result is prolonged and unexplained, run a 1:1 mix of patient plasma with normal pooled plasma. The mix is a localization tool. Persistent correction in a validated mixing procedure supports a factor-deficiency pattern but does not prove it, and failure to correct can reflect an inhibitor, an anticoagulant effect, or a mixed abnormality that needs targeted follow-up.
Immediate and incubated mixing results require a locally validated interpretation method, its own controls, and locally set decision limits. Time-dependent anti-factor VIII activity can emerge only after incubation, but a high-titer inhibitor can also inhibit immediately, so both timepoints carry information and neither alone settles the question.
Treat the mix as one more piece of localizing evidence, not a verdict, and read it against the specimen and medication facts you already collected.
The reusable sequence behind every case: from screening result to next action.
Check specimen and collection history
Confirm citrate fill, absence of visible clot or fibrin, transport and processing history, and whether the draw came from a line that could carry anticoagulant contamination.
Obtain medication and dosing history
Get the medication name, last dose, and renal function context, because these can change the meaning of a clot-based result and must be known before escalation.
Compare PT and aPTT to the reference interval
Run the clot-based screen on the validated method and compare it to the laboratory's established interval, not a value carried over from another system.
Mix if prolonged and unexplained
When specimen and drug review do not explain a prolonged result, perform a 1:1 patient and normal-pooled-plasma mix under the validated immediate and incubated procedure.
Interpret the correction pattern
Read the immediate and incubated results against the local correction limit; correction favors a factor-deficiency pattern, failure to correct raises inhibitor, anticoagulant, or mixed causes.
Choose the test that most separates the leading explanations
Select the confirmatory test, such as a factor assay or lupus anticoagulant panel, that best distinguishes the top two explanations rather than ordering a broad panel by habit.
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