CLINICAL LAB SCIENEC

CHAPTER 13: HEMATOLOGY AND COAGULATION 329
the clot when the wound has healed. Medication is used to prolong the clotting
time to prevent the body from forming dangerous clots. These changes must
be regularly monitored by the PT (or prothrombin time). The patient is usually
anticoagulated initially with a powerful anticoagulant called heparin, which is
administered by injection; after the desired results are obtained, the medication
is switched to warfarin (Coumadin), which may be taken orally. The laboratory
test for monitoring the patient prior to switching to Coumadin is the APTT (or
activated partial thromboplastin time). The response of the body to the anticoagulant
may change over time, so long-term treatment may require periodic
changes in the medical regimen prescribed.
Automated and semiautomated instruments are available from a number of
sources. These instruments measure and hold the blood plasma containing the
various coagulation factors in a stable environment. The reagents are also kept
at a temperature conducive to clotting through the formation of a fibrin clot.
For performing the most basic of coagulation tests, the APTT and the PT along
with decalcified plasma are required. A citrated specimen, which is preferred,
binds the calcium and prevents the clotting of the whole blood specimen. Plasma
is removed from the whole blood sample via centrifugation. This process is best
accomplished at the body’s temperature of 37ºC, and the automated systems
maintain a constant temperature while the reaction is taking place. Prothrombin
is a plasma protein coagulation factor that is synthesized from vitamin K.
Prothrombin is converted to thrombin by activated factor X (1 of 13 coagulation
factors) in the presence of calcium ions. The laboratory procedure known
as the PT is the most frequently ordered coagulation test in a clinical laboratory.
The procedure measures the amount of time that elapses before clotting of the
plasma occurs upon the addition of thromboplastin and calcium to a sample of
the anticoagulated plasma.
When a medical need exists to prevent clotting by inhibiting the quick coagulation
of the blood, the PT test is used for monitoring the effects of warfarin
(Coumadin). After the patient is stabilized on warfarin, the APTT test is used.
Most automated and semiautomated instruments provide both of these tests
on the same instrument, with the use of different reagents. Extremely accurate
pipetting and measurement of the first vestiges of clotting by light interruption,
a very sensitive process, are used by these instruments. A calculated result using
the actual clotting time divided by the normal control run with the sample(s) is
called the International Normalized Ratio (INR). When different instruments
and reagents are used, both the patient results and the normal control will be
different, for either a longer or shorter period. This will result in the same ratio
when both results change to an equal extent.
Differential White Blood Cell Count
An important component of a complete blood count (CBC) is that of enumerating
the white blood cells (WBCs) and differentiating among the several
types to provide clues as to a patient’s disease condition. A white blood cell
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