Haemostasis Flashcards
What is a generic PT test?
The ProThrombin test is an optical mechanical viscosity measurement.
Screening test for the evaluation of the deficiencies in the EXTRINSIC and COMMON pathways of the coagulation cascade.
This part of the pathway involves Tissue Factor III (TF - exposed by injury), factors VII, II (prothrombin), V, X and fibrinogen.
To a centrifuged sample in sodium citrate tube, calcium and thromboplastin are added and the time measured for fibrin clot formation.
International normalised ratio (INR) used to standardise PT results (Thromboplastin is different per batch so a calculation is performed to standardise to an international WHO reference). The first WHO ref was given an International Sensitivity Index (ISI) of 1.0, all others are thromboplastins are calibrated against this.
The INR is the PT ratio of a test sample compared to a normal PT (derived from the log mean normal prothrombin time of 20 normal donors), corrected for the sensitivity of the prothrombin used in the test.
What is a generic APTT Test?
Activated Partial Thromboplastin Time (APTT) evaluates all the clotting factors of the INTRINSIC and COMMON pathways of the coagulation cascade.
Measures the time it takes after adding calcium and activated partial thromboplastin to a spun plasma sample in a sodium citrate tube.
Evaluates factors I (fibrinogen), II (prothrombin), V, VIII, IX, X, XI and XII.
How do mixing studies determine with coagulation factor is the problem?
If both the PT and the APTT are prolonged it suggests the issue is in the common pathway (factor I, II, V and X). Though PT and APTT detect a problem only when the factor decreases by 25-40% of normal.
A mixing study basically involves adding things until we see what was missing.
Addition of normal plasma, absorbed plasma and normal serum in a 50:50 mix to the sample will enable determination of the missing/deficient factors or the presence of an inhibitor as it will correct the clotting time.
Normal plasma contains all clotting factors at normal levels = can determine if there are any inhibitors (e.g. producing an inhibitor against factor 8 concentrate = haemophilia)
Absorbed plasma contains FI, FV, FVIII, FXI, FXII
Normal serum contains FVII, FIX, FX, FXI, FXII
