Page 342 - Essential Haematology

328 / Chapter 24 Platelets, blood coagulation and haemostasis

citrated plasma. The normal time for clotting is after which small incisions are made in the fl exor

10 – 14 s. It may be expressed as the international surface forearm skin. Bleeding stops normally in
normalized ratio (INR) (see p. 375) . 3 – 8 minutes. It has largely been replaced by specifi c
The activated partial thromboplastin time platelet aggregation tests, platelet adhesion assays

(APTT) measures factors VIII, IX, XI and XII in and the platelet function analysis - 100 (PFA - 100)

addition to factors X, V, prothrombin and fi brino- test (see below). The bleeding time is prolonged in

gen. Three substances – phospholipid, a surface thrombocytopenia but is normal in vascular causes
activator (e.g. kaolin) and calcium – are added to of abnormal bleeding.
citrated plasma. The normal time for clotting is

approximately 30 – 40 s.
Tests of p latelet f unction
Prolonged clotting times in the PT and APTT
because of factor deficiency are corrected by the The most valuable investigation is platelet aggrego-

addition of normal plasma to the test plasma (50 : 50 metry which measures the fall in light absorbance

mix). If there is no correction or incomplete correc- in platelet - rich plasma as platelets aggregate. Initial
tion with normal plasma, the presence of an inhibi- (primary) aggregation is caused by an external
tor of coagulation is suspected. agent, the secondary response by aggregating agents
The thrombin (clotting) time (TT) is sensitive released from the platelets themselves. Th e fi ve

to a deficiency of fibrinogen or inhibition of external aggregating agents most commonly used

thrombin. Diluted bovine thrombin is added to are ADP, collagen, ristocetin, arachidonic acid
citrated plasma at a concentration giving a clotting and adrenaline. The pattern of response to

time of 14 – 16 s with normal subjects. each agent helps to make the diagnosis (see Fig.
25.10 ). Flow cytometry is now increasingly used in
routine practice to identify platelet glyoprotein
Specifi c a ssays of c oagulation f actors
defects.
Most factor assays are based on an APTT or PT in In the PFA - 100 test, citrated blood is aspirated
which all factors except the one to be measured are through a capillary tube onto a membrane coated
present in the substrate plasma. This usually requires with collagen/ADP or collagen/adrenaline. Blood

a supply of plasma from patients with hereditary flow is maintained. Platelets begin to adhere and

deficiency of the factor in question or artifi cially aggregate, primarily via VWF interactions with

produced factor - deficient plasma. Th e corrective GPIb and GPIIb/IIIa, resulting in occlusion of the

effect of the unknown plasma on the prolonged aperture.

clotting time of the deficient substrate plasma is The PFA - 100 analysis may give false negative

then compared with the corrective eff ect of normal results with relatively common platelet defects. Full
plasma. Results are expressed as a percentage of platelet function tests and VWF screening may be
normal activity. required to exclude abnormal platelet function,
A number of chemical, chromogenic and immu- even if the PFA - 100 test is normal.
nological methods are available for quantifi cation of
other proteins such as fibrinogen, VWF, factor Xa

Test of fi brinolysis

and factor VIII. Factor XIII activity can be assessed
by testing for clot solubility in urea. Increased levels of circulating plasminogen activator
may be detected by demonstrating shortened
euglobulin clot lysis times. A number of immuno-
Bleeding t ime
logical methods are available to detect fi brinogen

The bleeding time was a useful test for abnormal or fibrin degradation products (including D - dimers)

platelet function including the diagnosis of VWF in serum. In patients with enhanced fi brinolysis,
defi ciency. The test involved the application of pres- low levels of circulating plasminogen may be

sure to the upper arm with a blood pressure cuff , detected.