IS
RETESTING REALLY WORTHFULL IN CLINICAL LABORATORY?
Since the early 1970s, laboratory medicine specialists have
used computer technology and automation to identify and confirm critical
laboratory values.(1,2) The historic practice in clinical laboratories has been
to automatically repeat laboratory values that are above or below a critical
threshold or that trigger other automated “repeat rules” such as a delta check.
These practices were established when laboratory instruments were far less
reliable than today,(3) yet they persist in many laboratories (including ours).
In fact, recent studies show that analytic issues account for only 8% to 15% of
clinical laboratory– related errors, with preanalytic and postanalytic errors representing
85% to 92% of all errors.(4,5) Contemporary laboratory instruments use numerous
safeguards in their hardware and software to improve the accuracy and
reliability of results.
A recent summary of data from a College of American Pathologists
(CAP) Q-Probes survey suggests 61% of laboratories still repeat testing for
critical chemistry values.(6) The survey also suggests that laboratory test
repeat practices have the potential to delay reporting by 10 to 14 minutes and
waste resources without significantly preventing analytic errors.6 Recent
studies have shown that the practice of repeating tests with critical
laboratory values or other results that trigger automated repeating may not be
necessary with today’s clinical laboratory automated analyzers.(3,7) A small
study examined a total of 580 repeated tests for potassium, glucose, platelet
count, or activated partial thromboplastin time and found that the repeated
value was within the acceptable difference for 95.3% of the critical value
tests repeated and 97.6% of all repeated tests.(3) Another study examined 5
different hematology/coagulation tests and 500 consecutive critical results and
the repeated test values for each of the 5 tests. By using their internal
definition of acceptable error, Toll et al7 found that 0% to 2.2% of the
repeated values for these tests were outside of their acceptable criteria. They
concluded that repeated testing for critical values did not offer an advantage or
provide additional benefit in hematology and coagulation settings.(7) Neither
of these studies examined the time delays in reporting the critical values that
repeated testing invariably causes.
The CAP recently published the results of a survey of 86
laboratories, each of which reviewed 40 critical test results from 4 tests at
their institutions.(8) The study found that 61% of laboratories always repeated
critical results and that the median delay in reporting as a result of repeated
testing was 10 to 14 minutes in most laboratories and 17 to 21 minutes in 10%
of the laboratories.(6,8) Based on the findings of these studies and the
Q-Probes survey, we examined the results of our repeated testing. For more than
20 years, we have repeated all tests in an automated manner when the initial
values exceed the AMR of the test (high or low), the result is a critical value,
the result fails a delta check, or the value exceeds a preset “review” limit.
All of the instruments used are directly interfaced to our laboratory information
system, and results are autoverified when all rules are satisfied. When one of
the aforementioned flags occurs, results are not autoverified and are
automatically repeated by the instrument. For initial values above or below the
AMR of the method, the test is repeated on dilution (high values) or the sample
is examined for being a short sample (below AMR) and repeated using a special
“short sample” cup if necessary. For initial values within the AMR, the
technologist reviews the repeated result, and if the result is in agreement, the
initial value is verified manually by the technologist. If the difference
between the initial and repeated results exceeds the 2 SD range of the quality
control sample closest in concentration to the test sample or is greater than
10% (whichever is greater), the test is performed a third time, and the average
of the 2 results that agree is reported.
Based on experience and the reports discussed, it is hypothesized
that the vast majority of repeated values from clinical chemistry laboratory
would agree with the initial value and that there may be only limited benefit
in continuing such frequent repeat analyses. It is also hypothesized that repeated
analysis for critical values is delaying notification of caregivers about these
critical results. The definition of what constitutes a significant difference between
repeated values is variable. Allowable error can be defined by biologic
variability,(9,10) subjective opinion of what is clinically significant,
clinician survey, or regulatory requirements. CAP/CLIA criteria for allowable
error should be chosen because they are understood by all and are the criteria
by which proficiency testing is judged in the United States. This decision can
be questioned because there is not a clear rationale for some of these
criteria, which may lead to some of the differences we observed in the number
and frequency of errors we identified for different tests. For example, the
CAP/CLIA criterion for sodium is ± 4 mmol/L (~ ± 2.8%), whereas the criterion
for calcium is ± 1.0 mg/dL (~ ± 10.0%) It could be argued that the former is
clinically insignificant, whereas the latter is clinically significant. It is
also possible that the differences in the magnitude of what is considered allowable
error led to our finding of considerably more errors for repeated sodium
testing within the AMR than for calcium.
It is interesting that when the data for these 2 tests is examined
closely, this is not the case. There is only 1 additional calcium repeated
result of the 3,079 results within the AMR in which the difference is between
0.5 and 1.0 mg/ dL. In contrast, for the 31 errors within the AMR for sodium, 19
exhibited a difference between the initial and repeated tests that exceeded 10
mmol/L. Results below the AMR (linear low) may be due to “short sampling” or
other preanalytic or analytic error. Results above the AMR are repeated on
dilution to obtain a final result, and the absolute or percentage differences
from the repeated results are most frequently greater than the CAP/ CLIA-derived
allowable error. Clearly, when initial results are outside the AMR, repeated
testing will continue to be necessary.
The results of several studies for repeated testing when automated
chemistry testing; the repeated testing is unnecessary and delays the reporting
of results, which is a particularly important problem for critical values. Findings
of these studies also suggest that for some tests such as sodium and pO2,
repeat testing may be necessary to detect some large errors in the initial result.
The reasons for these errors are unclear at this time and may require
prospective evaluation. Finally, while it may not be necessary to repeat the
analysis for samples that trigger a delta check flag, it will still be
necessary for technologists to check the identity and feasibility of the result
for the previous sample. Delta checks provide a means for identifying mislabeled
samples, sample integrity problems as a result of preanalytic problems, and
random analytic errors. While all these studies strongly suggests that random
analytic errors are rare, it does not address the first 2 causes of a delta
check flag, and these will need to be investigated by the laboratory. The
delays observed in reporting critical values that result in repeated testing
were similar to those described by survey participants in the Q-Probes study.(6,8)
It is not surprising that the delays for blood gases were shorter than the
delays for other chemistry tests because the analytic time is much shorter. However,
the median delays observed for tests such as potassium and glucose are far
greater than the actual analytic time. This is most likely due to a
technologist taking time to review results, determine if a third test is
necessary, and making a decision about manual verification of the final result.
Weaknesses of these studies are that the data are from a single laboratory and
only several types of automated analyzers. The error rates may vary depending
on instrumentation, quality assurance practices, or other variables of
individual laboratories. For example, because using multiple instruments performing
the same test (eg, 7 Roche Modular P units), calibrations is not accepted if
the quality control samples are outside of 1 SD, which may minimize error
rates.
Conclusion
Finally, the number of repeated tests observed for the
immunoassay and therapeutic drug monitoring categories may be too small to make
firm conclusions. Nevertheless, these results suggest that repeat testing for
many automated chemistry tests, including critical values, can be stopped and
should also serve as a catalyst for other laboratories to examine the value of their
repeat testing practices. Doing so can improve patient care by delivering
critical values more rapidly and could potentially save 2% to 3% of reagent
costs for many tests.
References
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