Page 341 - Essential Haematology

Chapter 24 Platelets, blood coagulation and haemostasis / 327

Plasmin is capable of digesting fi brinogen, A number of simple tests are employed to assess
fi brin, factors V and VIII and many other proteins. the platelet, vessel wall and coagulation components
Cleavage of peptide bonds in fibrin and fi brinogen of haemostasis.

produces a variety of split (degradation) products
(Fig. 24.12 ). Large amounts of the smallest frag-

ments D and E can be detected in the plasma of Blood c ount and b lood fi lm e xamination
patients with disseminated intravascular coagula- As thrombocytopenia is a common cause of abnor-
tion (see p. 355) . mal bleeding, patients with suspected bleeding dis-
orders should initially have a blood count including

Inactivation of p lasmin platelet count and blood film examination. In addi-
tion to establishing the presence of thrombocyto-
Tissue plasminogen activator is inactivated by plas- penia, the cause may be obvious (e.g. acute
minogen activator inhibitor (PAI). Circulating leukaemia).
plasmin is inactivated by potent inhibitors α 2 -
antiplasmin and α 2 - macroglobulin.
Screening t ests of b lood c oagulation
Tests of h aemostatic f unction Screening tests provide an assessment of the extrin-
‘
sic ’ and ‘ intrinsic ’ systems of blood coagulation and
Defective haemostasis with abnormal bleeding may

also the central conversion of fibrinogen to fi brin
result from:
(Table 24.3 ).

1 A vascular disorder; The prothrombin time (PT) measures factors
2 Thrombocytopenia or a disorder of platelet func- VII, X, V, prothrombin and fi brinogen. Tissue

tion; or thromboplastin (a brain extract) or [synthetic]
3 Defective blood coagulation. tissue factor with lipids and calcium is added to

Table 24.3 Screening tests used in the diagnosis of coagulation disorders.
Abnormalities indicated by Most common cause of coagulation
Screening tests prolongation disorder

Thrombin time Defi ciency or abnormality of fi brinogen or DIC
(TT) inhibition of thrombin by heparin or FDPs Heparin therapy
Prothrombin Defi ciency or inhibition of one or more of Liver disease
time (PT) the following coagulation factors: VII, X, V, Warfarin therapy
II, fi brinogen DIC
Activated partial Defi ciency or inhibition of one or more of Haemophilia, Christmas disease
thromboplastin the following coagulation factors: XII, XI, ( + conditions above)
time (APTT or IX (Christmas disease), VIII (haemophilia),
PTTK) X, V, II, fi brinogen
Fibrinogen Fibrinogen defi ciency DIC, liver disease
quantitation

DIC, disseminated intravascular coagulation; FDPs, fi brin degradation products.
NB. Platelet count and the tests of platelet function are also used in screening patients with a bleeding disorder (p. 328).