You need to be familiar with the coagulation cascade before delving into tests.


There are three common blood coagulation tests, which can all be performed from a single sample of blood. Blood is taken into a tube containing an anti-coagulant Ca2+ chelator, such as sodium citrate and centrifuged to remove the cells (including platelets). In each test Ca2+ is added in excess to overcome the presence of the chelator, a trigger for coagulation is added and the time taken to form a clot is measured. Each of the three tests differs in how coagulation is artificially initiated, which assesses potential defects in slightly different ways.

clotting tests

clotting tests

Activated Partial Thromboplastin Time (aPTT)

Thromboplastin is a word from the past. It was an unidentified substance that could be readily purified from tissues and caused blood to clot. We now know that what we used to call thromboplastin is actually just Tissue Factor and charged phospholipids, the perfect mixture for activating the so-called extrinsic clotting cascade. “Partial thromboplastin” is just charged phospholipids; silica or a similar material is used to activate the intrinsic pathway.

aPPT is useful for testing whether heparin dosing is effective and identifying deficiencies in clotting factors. This may be haemophilia, or reduced production of factors by a failing liver. In sepsis, clotting factors may be exhausted by the procoagulant state of the inflamed tissues.

Prothrombin time (PT)

In this test, Tissue factor is added to trigger the extrinsic coagulation cascade. These days, recombinant Tissue Factor and a standardised quantity of phospholipds are used. This is effectivley thrombinplastin of old, but made in a standardised way, rather than purified from various tissue sources. PT is a good way to test that warfarin dosing is adequate (or excessive) and for Vitamin K deficiency (which has much the same effect as warfarin). As with aPTT, failure of the liver to produce coagulation factors will also be detected by this test.

Thrombin time

In this test an excess of activated thrombin is added to the sample. This should directly clot plasma by cleaving fibrinogen. As such, it’s a good test that fibrinogen is present in sufficient quantities and can form a clot. A mutation in fibrinogen may prevent it clotting as rapidly as it should.

International Normalised Ratio (INR)

The INR was introduced to standardise blood testing between different laboratories using different equipment and different reagents. The INR makes blood tests comparable regardless of where PT testing is done. The sample blood PT is compared with the PT of normal blood, raised to the power of a fudge factor known as the International Sensitivity Index. This ISS is based on the technology and reagents used. If you buy a blood coagulation testing machine, the manufacturer supplies the appropriate ISS to make the INR. In an ideal world where tests were performed identically everywhere, the ISS would be 1.0 (which has no effect on the PT ratio).

inr international normalised ratio

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