Biological Standardization
Biological products are distinguished from chemical
products by the biological nature, singly or in com-
bination, of the source materials, of the production
and purification procedures, and of the test methods
needed to characterize such products or determine
their potency. Biological products differ from chemi-
cal products in that they are not adequately character-
ized solely by their physical and chemical properties.
Two biologicals may give the same results in chemi-
cal and physical tests but may have different activities
when compared in biological tests. Measurement of
the amount or concentration of any biological prod-
uct thus requires an in vivo or in vitro biological
assay system and a standard preparation or refer-
ence material. A biological standard for a product
is a preparation such that the properties of a given
amount of it do not change over time, and with which
the properties of other samples of the product can be
compared. Standards are essential if measurements
are to be comparable from one assay to another. The
potencies of the standard are customarily defined in
arbitrary “units”.
The use of biological activity as the basis for ana-
lytical, or assay, techniques developed rapidly in the
late nineteenth and early twentieth centuries, with the
development of vaccines and the discovery of vita-
mins and hormones. It was quickly recognized that
biological test systems were variable, and the prin-
ciples of biological standardization and quantitative
approaches to biological test systems were formu-
lated [6, 7, 10]. Central to these principles were the
importance of comparison with a standard and the
need to determine the variation of the biological sys-
tem [2].
In some early uses of biological assays, attempts
were made to define units in terms of the amount
of product required to produce a specified effect.
There are instances in which this continues today
with, for example, attempts to define the amount
of an antiviral agent in terms of the amount that
will protect 50% of a population of cells, and to
calibrate botulinum toxin for therapeutic use in terms
of “mouse” units. However, it is virtually impossible
to standardize a biological system so that whenever
and wherever it is used the relation between the
amount of a product or material and its response
remains constant, and use of a standard thus leads
to improved reproducibility [2, 9].
Any biological standard must fulfill certain con-
ditions. All samples of the standard material must
be uniform, so that the amount of material required
to produce the observed effect or response is known.
International standards are thus prepared in ampoules
in such a way that the contents of any one ampoule
are as nearly as possible identical to the contents of
any other. The standard must be representative of
the substance for which it is to serve as a standard,
but does not necessarily have to be of high purity.
A representative batch of product may be selected
for the in-house standard by a manufacturer. For an
international standard, suitability is usually shown by
extensive characterization in an international collab-
orative study. The standard must be stable so that
its effects do not change over time. Because of the
nature of biological standards, direct tests for stabil-
ity – that is, tests in real time – are not possible;
predictions of stability may be based on the effects
of storage at elevated temperatures on samples of the
standard coupled with assumptions about the predic-
tive nature of these effects. The quantity of standard
available must be sufficient for the purposes for which
it is intended. The World Health Organization has
published guidelines for the preparation of standards
detailing the way in which these conditions may be
met, and setting specific requirements for interna-
tional standards [13].
Biological standards serve a variety of specific
purposes. Any laboratory routinely carrying out bio-
logical assays will include an in-house standard in
these assays, and may also include “quality con-
trol samples” that might also be considered to
be standards (see Quality Control in Laboratory
Medicine). Although such standards provide compa-
rability between assays within a laboratory, they may
differ markedly between laboratories, as has been
shown when a common sample is measured by sev-
eral laboratories in terms of their individual standards
(see, for example, [11]). Biological standards may
be used in the development and validation of assay
systems; for example, failure of an assay system to
respond in a dose-related way to the standard would
automatically invalidate the system for that mate-
rial [12]. Standards are an essential part of the various
quality assessment schemes which are operated for
some types of assay, often by national authorities.
International standards are indispensable not only for