TABLE 36-2
Common Tests of Hemostatic Function
Test Name
Plasma Procoagulants
Prothrombin Tim e (PT)
Activated Partial
Throm boplastin Tim e
(aPTT)
Description
Functional evaluation o f
the extrinsic and final
com m on pathways. Time
for clot formation is
determined after addition
o f thromboplastin and
Ca2+ to a plasm a sample.
Functional evaluation o f
the intrinsic and final
com m on pathways.
A ctivation o f the contact
phase is by addition o f a
surface reagent to a
plasm a sam ple.
Tim e for clot form ation is
determined after Ca2+ and
phospholipid are added to
the incubation mixture.
Component Measured
Sensitivity
Factor V II,
Factor X,
Factor V, Prothrombin and
Fibrinogen
(afibrinogenemia,
dysfibrinogenem ias)
Factor V III, Factor IX ,
Factor Va inactivation
An abnormally short
aPTT suggests
resistance to activated
Protein C.
Common Use
M onitoring oral
anticoagulant therapy
Preoperative screening
Liver dysfunction (effects
on synthesis and
secretion o f vitam in In-
dependent factors)
Preoperative screening
D iagnosis o f bleeding
disorders
Heparin m onitoring
(poor), but w idely used
during surgery involving
extracorporeal circulation
A ctivation o f Factor XI to
XIa in the first stage is
sensitive to Factor XII
Interferences/Complications
Particularly dependent on the
the thromboplastin (tissue
factor and phospholipids)
Vitamin K -deficiency arising
from antibiotic therapy
decreases y-carboxy-
glutam ate formation
U se for oral anticoagulant
m onitoring requires careful
“standardization” by the INR
method.
Cannot com pare results from
different laboratories
M ost com m on cause o f
prolongation is heparin
Standardization is difficult.
Cannot compare results
from different laboratories.
Can be affected by platelet
fragments (microparticles)
arising from poor sam ple
acquisition and handling
technique.
Preincubation in the
absence o f Ca2+
produces Factor XIa.
(
continued)
