01Purpose and principlesWhat the assessment is for and the core concepts behind it.
PT and APTT measure the time to fibrin formation in platelet-poor plasma after different laboratory triggers. They are simplified probes of a complex cellular haemostatic system, not direct simulations of surgery or spontaneous bleeding. Interpretation begins with the clinical phenotype: mucocutaneous bleeding suggests primary haemostasis, whereas deep muscle, joint and delayed procedural bleeding suggests coagulation-factor deficiency. A patient with no bleeding history and an incidental isolated abnormality follows a different pathway from a septic patient with bruising, thrombocytopenia and falling fibrinogen.
The pattern localises the next question. Prolonged PT alone directs vitamin K, warfarin, liver and factor VII assessment. Prolonged APTT alone directs heparin exposure, lupus anticoagulant and intrinsic-factor testing. Both prolonged directs fibrinogen, thrombin time, liver, DIC, common factors and anticoagulants. The thrombin time is sensitive to heparin, direct thrombin inhibitors and low or abnormal fibrinogen, while reptilase or anti-Xa testing may help through the specialist laboratory. Results depend on reagent and reference interval, so ratios or seconds cannot be transferred casually between laboratories.
Pre-analytical and medicine effects are frequent. Blood drawn from a heparinised line, an incompletely filled citrate tube or a sample containing clot may produce a false pathway. Warfarin, unfractionated heparin, low-molecular-weight heparin and direct oral anticoagulants require agent-specific interpretation. Mixing tests and factor assays should follow laboratory discussion because residual drug can mimic an inhibitor. In active bleeding, haemostatic resuscitation and source control proceed in parallel; the laboratory refines targeted replacement and reversal rather than delaying it.
Key points
- A coagulation screen is not a global bleeding-risk test. Normal PT and APTT do not exclude von Willebrand disease, platelet dysfunction, factor XIII deficiency or an anatomical bleeding source.
- Check sample quality first: underfilled citrate tubes, high haematocrit, heparin contamination, delayed processing and a clot in the tube can create misleading prolongation or consumption.
- PT mainly assesses the tissue-factor and common pathways and is used as INR for vitamin K antagonist monitoring; APTT assesses contact, intrinsic and common pathways but varies by reagent.
- An isolated prolonged APTT can reflect heparin, lupus anticoagulant, factor VIII, IX or XI deficiency, or non-bleeding contact-factor deficiency. Bleeding history determines significance.
- An isolated prolonged PT suggests vitamin K deficiency, early liver dysfunction, warfarin or factor VII deficiency, while combined prolongation raises common-pathway deficiency, severe liver disease, DIC or anticoagulant effect.
- A mixing study that corrects supports factor deficiency; failure to correct supports an inhibitor or drug effect. Time-dependent inhibitors may require incubation and specialist interpretation.
- Falling fibrinogen, thrombocytopenia, prolonged clotting and high D-dimer in the right clinical context supports DIC, but trends and the precipitating illness matter more than one borderline value.
- Direct oral anticoagulants can variably affect routine tests, and a normal PT or APTT does not reliably exclude clinically important drug concentration. Establish agent, dose and last administration time.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Warfarin, vitamin K deficiency, early liver synthetic dysfunction and factor VII deficiency are leading mechanisms; medicine and nutrition history guide focused testing.
Heparin, lupus anticoagulant and intrinsic-factor deficiency cause this pattern. A personal bleeding history separates haemophilia from many non-bleeding inhibitor or contact-factor findings.
DIC, severe liver disease, marked vitamin K deficiency, common-pathway factor loss, massive dilution or anticoagulant effect can prolong PT and APTT together.
A falling platelet count and fibrinogen with rising D-dimer and prolonged clotting in sepsis, trauma, obstetric catastrophe or malignancy supports DIC.
Mucosal or procedural bleeding with normal PT and APTT raises platelet dysfunction, von Willebrand disease, factor XIII deficiency or a local structural source.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Repeat PT, APTT, fibrinogen and thrombin timeFirst step - Why
- Confirm the pattern on a correctly collected peripheral sample and assess common mechanisms.
- Interpretation and limitations
- Check citrate fill, clot, collection site and processing. Pair with platelets, haemoglobin, liver function and D-dimer when consumption or major bleeding is possible.
- 02
Medication and timing reconstruction - Why
- Identify anticoagulant exposure and whether routine tests are interpretable.
- Interpretation and limitations
- Record exact agent, dose, last administration, renal function and line flushes. A normal routine screen does not exclude an important DOAC concentration, and residual drug can distort mixing and factor assays.
- 03
Mixing study - Why
- Distinguish probable factor deficiency from inhibitor or anticoagulant effect.
- Interpretation and limitations
- Correction immediately and after incubation supports deficiency; non-correction suggests inhibitor or drug. Interpretation is laboratory specific and requires exclusion of heparin or DOAC interference.
- 04
Factor and von Willebrand assays - Why
- Confirm the specific inherited or acquired haemostatic defect suggested by phenotype and screen.
- Interpretation and limitations
- Use specialist panels rather than indiscriminate testing. Factor VIII can be low in haemophilia or von Willebrand disease and can rise during inflammation, pregnancy and stress.
- 05
Anti-Xa or drug-specific anticoagulant testing - Why
- Estimate relevant anticoagulant activity when emergency bleeding or procedure decisions require it.
- Interpretation and limitations
- Use the assay calibrated for the actual agent and local availability. Sampling time and renal clearance are essential; an unfractionated-heparin anti-Xa result cannot be read as a DOAC concentration.
- 06
Liver profile, blood film and DIC trend - Why
- Identify synthetic failure, fragmentation and consumption in systemic disease.
- Interpretation and limitations
- DIC is a clinicopathological trajectory, not a single score. Repeat platelets, PT, APTT, fibrinogen and D-dimer while treating the precipitant and assessing bleeding or thrombosis.
04Clinical next stepsHow the result changes management or prompts escalation.
01BleedingStabilise before fine classificationFirst stepActive significant bleeding accompanies an abnormal screen or anticoagulant exposure.+
- 1Use ABCDE, secure access, identify the bleeding source, send FBC, coagulation, fibrinogen, group and screen or crossmatch, renal and liver tests, and activate the major-haemorrhage pathway when indicated.
- 2PreferredAlternativePreferred haemostatic treatment is source control plus agent- and defect-specific reversal or component support advised by haematology and transfusion; an alternative empirical product is justified only within the major-haemorrhage protocol.
- 3EscalationEscalate intracranial, obstetric, postoperative or haemodynamically significant bleeding immediately and repeat clinical and laboratory endpoints after each major intervention.
02IncidentalWork through an isolated prolongationPT or APTT is prolonged in a stable patient without current major bleeding.+
- 1Verify the sample and medication history, repeat from a clean peripheral venepuncture and assess personal and family bleeding, previous surgery and thrombosis.
- 2PreferredFor isolated APTT, the preferred next step is laboratory-guided thrombin time, anti-Xa or mixing assessment after drug exclusion; for isolated PT, review warfarin, vitamin K, liver and factor VII mechanisms.
- 3EscalationEscalate persistent unexplained abnormality before an invasive procedure or with a positive bleeding history to haematology; routine surgery should not be cancelled solely on an unverified artefactual result.
03ConsumptionRecognise and follow DICSystemic illness accompanies thrombocytopenia, prolonged clotting, falling fibrinogen or high fibrin-degradation markers.+
- 1First lineTreat the precipitant as first-line management, obtain serial haemostatic and organ tests, inspect for bleeding, thrombosis and skin or line changes, and involve haematology early.
- 2AlternativeUse blood components for clinically significant bleeding or procedural need according to the transfusion protocol; prophylactic correction of every abnormal number is an alternative only when specific guideline criteria are met.
- 3EscalationEscalate worsening fibrinogen, platelets, bleeding, thrombosis or organ failure to major-haemorrhage and critical-care support, reassessing for liver failure, TMA and anticoagulant effects.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Trend PT or INR, APTT, fibrinogen, platelets and clinical bleeding at a frequency matched to active consumption, anticoagulant reversal or procedure urgency.
- For anticoagulants, document agent, last dose, renal and liver function, reversal product, timing and the assay used to judge residual effect.
- After vitamin K or factor replacement, repeat the relevant test at the product- and protocol-defined interval and assess haemostasis, not numerical correction alone.
- In DIC, monitor the precipitating disease, perfusion, renal and liver function and thrombosis as well as bleeding because consumption can produce both.
- Close specialist factor, inhibitor and von Willebrand results through a named clinician, especially before planned surgery, pregnancy or neuraxial procedures.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Normal screens miss primary haemostasis
Platelet disorders and von Willebrand disease commonly have normal PT and APTT. The bleeding pattern and dedicated tests remain decisive.
Lupus anticoagulant is prothrombotic
Despite prolonging phospholipid-dependent clotting in vitro, lupus anticoagulant often signals thrombosis risk rather than a spontaneous bleeding disorder.
Factor twelve does not predict bleeding
Contact-factor deficiency may produce a striking APTT prolongation without clinical bleeding, illustrating why phenotype must accompany the laboratory result.
DOAC normality can deceive
Routine reagents vary in sensitivity. A normal PT or APTT cannot reliably declare the absence of apixaban, rivaroxaban or dabigatran effect.
Fibrinogen is an acute-phase protein
Inflammation can keep fibrinogen apparently normal early in consumption. A falling serial value may be more informative than one result.
07Common pitfallsFrequent interpretation and management errors.
- 01
Using PT and APTT as universal preoperative bleeding screens without a bleeding history.
- 02
Ignoring an underfilled citrate tube or heparin contamination from a central line.
- 03
Assuming a prolonged APTT means the patient will bleed when lupus anticoagulant or factor XII deficiency is possible.
- 04
Using a normal routine screen to exclude clinically relevant DOAC exposure.
- 05
Calling one abnormal panel DIC without the precipitating illness and serial trend.
- 06
Giving plasma merely to normalise numbers in a non-bleeding patient without a supported indication.