Clotting Screen

The clotting screen comprises the prothrombin time (PT, reported also as the international normalised ratio, INR), the activated partial thromboplastin time (aPTT), the thrombin time (TT) and the Clauss fibrinogen, measured on platelet poor citrated plasma.

• The PT tests the extrinsic arm and the shared final steps of the cascade (factors VII, X, V, II and fibrinogen).
• The aPTT tests the intrinsic arm and the shared steps (contact factors, XI, IX, VIII, X, V, II and fibrinogen).
• The TT tests the conversion of fibrinogen to fibrin.
• The Clauss assay measures functional fibrinogen.

It answers whether a deficiency of coagulation factors, an inhibitor or an anticoagulant effect is present. It does not test platelet function, von Willebrand factor function, factor XIII or fibrinolysis, so a normal screen does not exclude a mild or platelet based bleeding disorder.

The clotting screen is one of the most commonly requested tests. In patients without a bleeding history or anticoagulant exposure, routine pre-procedure screening has low yield, and a structured bleeding history predicts surgical bleeding better.

Indications. Bleeding or easy bruising, suspected DIC, liver disease, vitamin K deficiency, warfarin and heparin monitoring, major haemorrhage and massive transfusion, trauma, sepsis, obstetric haemorrhage, snake envenomation, unexplained thrombosis, family history of a bleeding disorder, and bleeding in a neonate.

Specimen.

• 3.2% sodium citrate in a ratio of 9 parts blood to 1 part citrate, with the tube filled to at least 90% of its nominal volume. Underfilling prolongs results.
• Clean venepuncture without prolonged tourniquet. A sample taken from a heparinised line needs an adequate discard.
• Haematocrit over 0.55 needs a reduced citrate volume.
• Transport at room temperature, centrifuge within 4 hours (within 1 hour for unfractionated heparin monitoring), and do not refrigerate.

Adult reference ranges are given. They are reagent- and analyser specific, and local ranges take precedence.

Prothrombin time and INR. Range: PT about 10 to 14 seconds, INR 0.8 to 1.2 in people not taking warfarin.

Prolonged:

• warfarin
• vitamin K deficiency (malabsorption, biliary obstruction, poor intake, antibiotics, newborns)
• liver disease
• DIC
• deficiency of factors VII, X, V or II
• direct oral anticoagulants (more with rivaroxaban, apixaban and edoxaban than dabigatran)
• massive transfusion
• severe hypofibrinogenaemia
• some lupus anticoagulants
• factor inhibitors

Shortened: raised factor VII in pregnancy or with oral contraceptives, and sampling artefact.

Factor VII has the shortest half life (about 6 hours), so the PT is the first test to prolong in vitamin K deficiency, early warfarin therapy and acute liver failure. The INR is calibrated for stable warfarin therapy and is not valid for other states. The usual warfarin target is 2.0 to 3.0, and 2.5 to 3.5 for some higher risk indications. In chronic liver disease a prolonged INR does not measure bleeding risk, because reductions in anticoagulant proteins rebalance haemostasis.

Activated partial thromboplastin time. Range: about 25 to 35 seconds, or a ratio to control of 0.8 to 1.2.

Prolonged:

• unfractionated heparin
• direct thrombin inhibitors (dabigatran, argatroban, bivalirudin)
• deficiency of factors VIII, IX, XI or XII (haemophilia A, B and C)
• von Willebrand disease with low factor VIII
• prekallikrein and high molecular weight kininogen deficiency
• lupus anticoagulant
• acquired factor inhibitors (acquired haemophilia A)
• DIC
• liver disease
• vitamin K deficiency
• severe hypofibrinogenaemia
• high dose direct oral anticoagulants

Shortened:

• acute phase response with high factor VIII and fibrinogen
• pregnancy
• early DIC
• traumatic sampling

A therapeutic range for unfractionated heparin is usually 1.5 to 2.5 times the control value, calibrated for each reagent against anti-Xa activity (0.3 to 0.7 IU/mL). Factor deficiency usually prolongs the aPTT only when activity falls under about 30 to 40%, so mild haemophilia and mild von Willebrand disease can be missed. Factor XII, prekallikrein and kininogen deficiency prolong the aPTT without causing bleeding.

A prolonged aPTT with a normal PT needs a mixing study with 1 part patient plasma to 1 part normal plasma. Correction suggests factor deficiency. Failure to correct suggests an inhibitor, which is immediate for lupus anticoagulant and time dependent (after 1 to 2 hours of incubation) for factor VIII inhibitors.

Thrombin time. Range: about 14 to 19 seconds, within about 3 seconds of the control. Prolonged: unfractionated heparin, direct thrombin inhibitors (very sensitive to dabigatran), hypofibrinogenaemia (under about 1 g/L), dysfibrinogenaemia, high levels of fibrin degradation products, paraproteins, amyloidosis, and neonatal samples. Shortened: no clinical significance.

A normal TT excludes a dabigatran effect. Reptilase time is unaffected by heparin, so a prolonged TT with a normal reptilase time points to heparin, and both prolonged points to a fibrinogen disorder.

Fibrinogen (Clauss). Range: 1.5 to 4.0 g/L. Raised: acute phase response, pregnancy (about 4 to 6 g/L at term), diabetes, smoking, obesity, oral contraceptives, nephrotic syndrome, malignancy. Lowered: DIC, liver failure, fibrinolytic therapy, major haemorrhage and massive transfusion, snake envenomation, asparaginase, congenital hypofibrinogenaemia and afibrinogenaemia, and dysfibrinogenaemia (low functional level with normal antigen).

A value under 1.5 g/L predicts severe bleeding in major haemorrhage, and under 1.0 g/L carries a high spontaneous bleeding risk. In obstetric haemorrhage a value under about 2 g/L predicts progression to severe haemorrhage. Very high heparin concentrations and some direct thrombin inhibitors can distort the Clauss assay.

Critical values and findings needing immediate action. Triggers vary by laboratory.

• INR over 5.0 in an anticoagulated patient, or any prolongation with active bleeding.
• aPTT over about 100 seconds, or a ratio over 3.0, in a patient not receiving heparin.
• Fibrinogen under 1.0 g/L.
• Prolonged PT and aPTT with low fibrinogen, low platelets and raised D-dimer (DIC).
• Prolonged PT with encephalopathy or rising bilirubin (acute liver failure).
• A markedly prolonged TT in a patient with an unknown anticoagulant exposure.

Interferences and pitfalls.

• Underfilled tubes, high haematocrit, heparin contamination from lines, traumatic venepuncture (which can shorten the aPTT), delayed processing, and cold storage all alter results.
• Haemolysis, lipaemia and icterus interfere with optical endpoints.
• Reagent sensitivity to factor deficiency, heparin and lupus anticoagulant differs between manufacturers.
• Direct oral anticoagulant effect on PT and aPTT is reagent dependent, and normal values do not exclude clinically relevant drug levels. Calibrated anti-Xa assays or the dilute thrombin time are needed.
• The INR is not valid for direct oral anticoagulants, liver disease or other non-warfarin states.
• Mild factor deficiency, von Willebrand disease and platelet function disorders can coexist with a normal screen.

Special groups.

• Neonates and infants: PT and aPTT are longer, and vitamin K-dependent factors are low at birth. Age specific ranges apply.
• Pregnancy: fibrinogen and factors VII, VIII and X rise, and the PT and aPTT shorten slightly.
• Children: routine pre-operative screening without a bleeding history has low yield.
• Older adults: increased sensitivity to warfarin.
• Liver and kidney disease: results reflect complex haemostatic change and do not predict bleeding alone.

Follow up tests. Mixing studies, individual factor assays (VIII, IX, XI, XII, VII, X, V, II), von Willebrand screen, lupus anticoagulant, D-dimer, anti-Xa assay, reptilase time, fibrinogen antigen, factor XIII assay, platelet function tests, full blood count and film, liver function tests, vitamin K-dependent factor assays.