smoothing function for the calculation of the mean
values (Gardner 1985, Strike 1996). Smoothing
leads to a reduction in the variability in the estimates
of the means that would otherwise arise from day-to-
day variation in the makeup of the clinical popula-
tion from which the test samples come. In addition,
if the test sample mean is recalculated for each
successive result rather than for blocks of results,
method trueness can be continuously monitored,
although continuous monitoring may not offer any
better performance than block-wise monitoring
(Smith and Kroft 1997).
External quality control
External quality control refers to the procedures
for quality control that involve the participation of
two or more laboratories. Proficiency testing is an
external quality control program mandated by a
regulatory body for the purpose of determining
laboratory quality. Most commonly, programs of
external quality control are conducted by profes-
sional societies or manufacturers of control
materials. Control material is provided to participat-
ing laboratories where control samples are assayed
on a regular schedule, usually in conjunction with
the internal control samples. All of the participating
laboratories receive control material from the same
production lot. The control sample results are trans-
mitted to the program sponsors who analyze the data
and issue reports that describe the result distributions
among the participating laboratories and indicate the
location of the individual laboratory results within
that distribution. In this way, external quality
control serves as an adjunct to internal quality
control by providing an additional mechanism for
monitoring the long-term trueness of a method.
There are a number of other important aims
served by external quality control. These aims
include to provide a measure of the "state of the art"
for the measurement of an analyte; to obtain consen-
sus values for control material when neither defini-
tive nor reference methods exist for the measurement
of an analyte; and, importantly, to investigate the
sources of inter-laboratory variability in the
measurement of an analyte (Büttner
et al.
1983b).
By analyzing subgroup results from a large number
of laboratories, it is possible to compare the
performance of different methods and to evaluate the
extent to which inter-laboratory measurement
variability is explained by laboratory variables such
as laboratory size and laboratory workload.
METHOD PRACTICABILITY
Practicability refers to those properties of a
method that relate to practical aspects of its imple-
mentation. These include speed, cost, technical skill
requirements, dependability, and safety (Büttner
et
al.
1980a). These are clearly important concerns in
the decision to employ a particular method.
The speed of a method is determined by the time
needed for method and sample preparation, the time
spent assaying the sample, and the time required for
calculation of the results. The time needed for
method preparation is at its longest if specimens are
received infrequently and one-at-a-time, for the
method must then be set up anew for each specimen.
Method preparation is at its shortest if samples are
received and run while the method is maintained in a
fully operational state. There is sometimes a trade-
off between the speed of a method and its quality.
This may mean that two methods need to be set up
in the same laboratory, a slower method of high
quality that is used for routine work and a rapid
method of lower quality that is used when circum-
stances demand a quick turnaround, provided, of
course, that lower quality results can be tolerated
clinically in exchange for rapidity in obtaining the
results.
The cost of a method includes not only the
expense of the reagents and materials used in sample
preparation and assay, but also capital and operation
costs, such as maintenance and repair costs for the
instrument on which the method is implemented,
labor costs, which will vary depending upon the
technical expertise required of the staff who run the
method, and overhead costs. Besides its affect upon
method cost, the technical skill requirements of a
method, which determines who on the staff can run
the method, determines when it can be run—only
when the appropriate staff members are scheduled to
be at work.
The dependability of a method quantifies the rate
or frequency with which the method succeeds in
producing valid results. Dependability is not wholly
intrinsic to the method but can vary from location to
location due to differences in laboratory conditions
and staff quality. This is particularly relevant when
the method is to be used outside of the central clini-
cal laboratory, in a setting closer to the patient.
Such point-of-care testing may take place in a satel-
lite laboratory, on a hospital ward, in a physician’s
office, or in the home of the patient. In these
Laboratory Methods
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