For example, creatinine is among the many
substances that show a change in plasma concentra-
tion during pregnancy. Beginning at about 10 weeks
of gestation and continuing until term, renal blood
flow increases with an accompanying increase in
glomerular filtration rate. Consequently the clear-
ance rate of creatinine increases and the plasma
concentration of creatinine declines.
The plasma concentrations and urinary excretion
rates of the sex hormones and their metabolites
change dramatically as a result of pregnancy
(Kandeel and Swerdloff 1994). The plasma concen-
tration of progesterone increases 10-fold between
early and late pregnancy due to the placental produc-
tion of progesterone following the corpus luteum-
placental shift in progesterone production at 7 to 9
weeks of gestation. Many other hormones produced
by the placenta enter the maternal circulation; human
chorionic gonadotropin is one that is especially
important in the laboratory evaluation of pregnancy.
It is detectable in maternal plasma within a week of
conception, increases in concentration rapidly,
reaching a peak at 9 to 13 weeks of gestation, then
declines quickly to a plateau that is maintained
throughout the remainder of the pregnancy. The
prompt appearance of this placental hormone in the
maternal plasma and urine make it an ideal early
marker of pregnancy.
RACE
A race is a local geographic population distin-
guished as a more or less distinct group by geneti-
cally transmitted physical characteristics. The
distinguishing characteristics most often thought of
when referring to the human races are skin color and
facial features but other characteristics—in
particular, ones that affect laboratory study
values—may also differ among the races.
There are generally thought to be between six
and ten major geographic races, the exact number
depending upon the criteria used by the taxonomist
to make the classification. For instance, American
Indians arose from migrations of Asiatics into the
Americas so the two populations can be considered
one. However, thousands of years have passed since
the migrations and the geographically isolated
American Indians have developed genetic differences
from their forebears; thus, the populations can be
considered distinct. Table 6.2 lists nine major races
and their historic geographic locales. In addition to
the major races there are numerous local races and
microraces. These terms refer to distinctive but
small populations of peoples such as the Capoid
local race of Southern Africa which is comprised of
two microraces, the San and the Khoikhoin.
Although originally settled by American Indians,
most of the residents of North America are now of
European and African extraction with appreciable
but smaller numbers of persons of Asiatic and
Hispanic (a European-American Indian admixture)
extraction. Because the majority of the residents are
of European racial stock, most reference frequency
distributions have been derived from clinical popula-
tions consisting predominantly or entirely of
individuals of the European race. Only recently
have efforts begun to determine the appropriateness
of these distributions when applied to individuals of
other races. These efforts have not yet resulted in
the appearance of any widely employed race-based
reference ranges. The only analyte with race-based
reference frequency distributions at the Johns
Hopkins Hospital is red cell glucose-6-phosphate
dehydrogenase. None of the analytes measured in
the clinical laboratories at the Massachusetts General
Hospital have race-based reference frequency distri-
butions. It is almost certain, however, that some
analytes will eventually be recognized to require the
use of race-based reference frequency distributions
Biologic Variability
6-5
Table 6.2 Major geographic races of man
African
Sub-Saharan Africa
American Indian
North and South America except
western Alaska and the Aleutian Islands
Asiatic
Central, East, and Southeast Asia,
western Alaska and the Aleutian Islands
Australian
Australia and Tasmania
European
Europe, North Africa, and the Middle East
Indian
Indian subcontinent
Melanesian
New Guinea
Micronesian
Yap, Pohnpei, and Guam
Polynesian
Islands from Easter Island to the
Hawaiian Islands to New Zealand
