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LETTERS TO THE EDITOR
A.J.C.P. . J u l y 1984
Analytic Accuracy and
Survey Results
To the Editor:—The paper by Ehrmeyerand associates, "Monthly Interlaboratory pH and Blood Gas Survey: Establishing Accuracy Based on Interlaboratory Performance" (Am J Clin Pathol
1984; 81:224-229) will confuse many
readers because of imprecise and inaccurate use of the word "accuracy."
In quantitative analysis, accuracy has
a specific meaning, namely closeness to
the true value. Comparison of one's performance in an interlaboratory survey to
consensus values established in peer laboratories does not necessarily provide an
estimate of accuracy.
Retrospective analysis of data from
some interlaboratory surveys, notably the
chemistry surveys of the College of Amer-
ican Pathologists, has shown that for some
analytes, the mean value obtained by laboratories using a particular method closely
approximates the true value, estimated by
measurements at the National Bureau of
Standards. When data of this kind are
available, one can discuss "accuracy" as
judged from interlaboratory data.
In the case of blood pH and gas analysis,
however, accuracy always has been an
elusive quantity. It is well known that instruments from different manufacturers
can give different results, and this is confirmed again by the data in the author's
Table 1. However, the data in this paper
gave no information whatsoever about
accuracy simply, because no estimates of
the true pH, pC0 2 and p02 of the specimens are available.
While it may be comforting for a laboratory director to know that results in
his or her lab are the same as results obtained by other labs with the same instrument, this is no assurance that the
results are accurate. Moreover, if IL,
Corning, and Radiometer instruments as
classes all give significantly different estimates of, say, p02 on a particular specimen, then it follows that at least two (and
possibly all three) classes of instrument
are giving inaccurate results.
namely to suggest a means, percentile
ranks, for dealing with the problems of
"accuracy" (in the generic sense as used
below).
Unequivocally, the CAP,4-5 the Wisconsin, and other pH and blood gas interlaboratory surveys are involved in the
"Accuracy Game." Dr. Batsakis,1 referring to the report of a CAP sponsored
Blood Gas/pH Conference, states that
"interlaboratory comparison programs
can serve as an integral part of an overall
quality assurance in pH/blood gas testing." The comparison programs hardly
can be used as a basis for measuring precision, considering that the CAP program
sends a total of 12 specimens annually.
Our survey, sending 36 specimens annually, is hardly adequate either. By implication, the use of the data for quality
assurance is a means (we are not implying
that this reference in any way indicates
the "only" means) to establish the laboratory's accuracy base.
Federal regulations3 speak to accuracy
as follows:
remedial action taken in response to detected
defects.
ROBERT W. BURNETT, P H . D .
Department of Pathology
Hartford Hospital
Hartford, Connecticut 06115
Authors' Reply
To the Editor:—We concur with Burnett's well-reasoned arguments that "accuracy is an elusive quantity" and that
interlaboratory surveys providing comparisons to extensive data bases, even that
of the College of American Pathologists,
are not in themselves a means of establishing accuracy. We feel that the caveats
we used2 with the word "accuracy," i.e.,
"accuracy relative to other laboratories,"
and "when using peer performance data
for accuracy assessment, one must recognize three potential problems: (1) the
lack ofdefined criteria for acceptable performance, (2) interinstrumental biases,
often a function of the material matrix,
and (3) mixed data bases that obscure interinstrumental performance characteristics" reflect our clear intent. Our other
references to the word "accuracy" are
similarly qualified (though we never explicitly define our intended meaning or
usage of the word itself).
Our failure is in the lack of a proper
reference to the intended meaning of the
word; as Dr. Burnett points out, "in
quantitative analysis, accuracy has a specific meaning, namely closeness of the true
value." As an analytic chemist, one author
(RHL) should turn in his balance. Our
oversite is more than unfortunate; it detracts from the intent of the paper—
Because the Procrustian Bed of regulation has forced an unwilling collaboration between interlaboratory survey
programs and the process of "verification
of accuracy," we intend to address the
question of the ability of an interlaboratory survey to meaningfully measure
and validate laboratory accuracy in a
publication in preparation. Since the conclusion of such debate is critical to current
practices of proficiency testing as a means
of establishing "acceptable" performance
for purposes of reimbursement (Medicare,
CLIA, etc.), Burnett's insistence on the
technically correct, not generic, use of the
word accuracy is even more valid. We
appreciate this needed clarification in our
thinking.
RONALD H. LAESSIG, P H . D .
Professor Pathology and Laboratory
Medicine
Director, State Laboratory of Hygiene
SHARON S. EHRMEYER, MS,
Quality controls are imposed and practiced
by the laboratory to provide for and assure reliable test results. (1) There is documentation
of preventive maintenance . . . validation of
methods, (accuracy, linear range . . . ) ; . . . and
MT(ASCP)
Lecturer, Medical Technology Program
University of Wisconsin-Madison
Madison, Wisconsin 53706