LABORATORY MEASUREMENT
S
The majority of the studies performed in the
clinical laboratory consist of measurements of the
concentrations of blood cells and of chemical
substances in body fluids (Table 2.1). The chemical
substances that are studied include gases,
electrolytes, metabolic intermediates, waste
products, tissue proteins, plasma proteins,
hormones, micronutrients, drugs, and toxins. Most
of what is discussed in this chapter applies most
directly to such concentration measurements.
However, with some modifications, the concepts can
also be applied other kinds of laboratory
measurements.
THE PROCESS OF MEASUREMENT
The process of laboratory measurement proceeds
in four steps: sample preparation, analyte separation,
analytical signal production and detection, and calcu-
lation of results. Sample preparation consists of the
operations that must be performed on a specimen to
yield the test material that constitutes the sample. It
is the sample, or a portion of the sample, that is
introduced into the measurement system and on
which the measurement is carried out. In the deter-
mination of the plasma concentration of a chemical
substance such as creatinine, sample preparation
consists of the separation of the plasma (or serum if
the specimen is clotted blood) from the blood cells
by centrifugation of the specimen.
There may be an analyte separation step that
represents a more-or-less specific isolation of the
analyte of interest from the other chemical
substances in the sample. For instance, if creatinine
is to be measured using the Jaffé reaction, it can be
absorbed to porous aluminum silicate clay or a
cation exchange resin to separate it from other
plasma substances that react with the signal generat-
ing reagent.
Analytical signals are generated and detected in
a wide variety of ways. In automated blood cell
counting, for instance, the signal consists of a
voltage pulse arising from a change in electrical
impedance across an aperture as a cell passes
through the aperture. The signal is detected by a
voltmeter. One way to produce an analytical signal
for the measurement of chemical substances is by the
formation of a chemical species that absorbs light.
In the Jaffé reaction for creatinine, picrate reacts
with creatinine under alkaline conditions to form a
strongly light-absorbing red-orange compound, the
Laboratory Methods
2-1
Table 2.1
Concentrations of selected blood analytes
Blood cells
cells/L 10
13
red cells
10
12
10
11
platelets
reticulocytes
10
10
neutrophils
10
9
lymphocytes
10
8
eosinophils
Chemical substances
mol/L 10
0
10
-1
sodium, chloride
10
-2
potassium, glucose, urea
10
-3
calcium, carbon dioxide
albumin
10
-4
uric acid
10
-5
bilirubin, iron, haptoglobin
fibrinogen
10
-6
cortisol
10
-7
thyroxine
hydrogen ion
10
-8
10
-9
testosterone, factor VIII
cobalamin
10
-10
ferritin
10
-11
parathyroid hormone
10
-12
Chapter 2
LABORATORY METHODS
© 2001 Dennis A. Noe
