Introduction
Our understanding of blood clotting is intimately
tied to the history of civilization. With the advent
of writing 5000 years ago, it could be argued that
the first symbols used for blood, bleeding, or clot-
ting represented the first published coagulation
pathway. The ancient peoples of the world always
held blood in utmost mystical esteem. Through
the ages, this esteem has been transmitted to mod-
ern times in the many expressions that use
“blood,” such as “blood is thicker than water,”
“blood of our fathers,” and others.
Mysticism aside, the study of blood clotting
and the development of laboratory tests for blood
clotting abnormalities are historically inseparable.
The workhorse tests of the modern coagulation
laboratory, the prothrombin time (PT) and the acti-
vated partial thromboplastin time (aPTT), are the
basis for the published extrinsic and intrinsic
coagulation pathways, even though it is now
known that these pathways do not accurately
reflect the function of blood clotting in a living
organism. In this chapter, and ultimately this text-
book, the many authors hope to present a clear
explanation of coagulation testing and its impor-
tant place in the medical armamentarium for diag-
nosing and treating disease.
Constituents of the
Hemostatic System
With the evolution of vertebrates and their pres-
surized circulatory system, there had to arise some
method to seal the system if injured—hence the
hemostatic system. Interestingly, there is nothing
quite comparable to the vertebrate hemostatic sys-
tem in invertebrate species. In all vertebrates stud-
ied, the basic constituents of the hemostatic sys-
tem appear to be conserved.
Figure 1-1 illustrates the three major con-
stituents of the hemostatic pathways and how
they are interrelated. Each element of the hemosta-
tic system occupies a site at the vertex of an equi-
lateral triangle. This representation implies that
each system constituent interacts with and influ-
ences all other constituents. In the normal resting
state, these interactions