CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 249
for comparing two different tests, rather than two different methodologies. The
precision for the two tests might be different, and the test with the greatest
degree of precision would most likely be chosen. The t-test and the F-test are the
two most common methods for comparing precision (reproducibility) for two
methods and two different procedures (Table 10-3).
Predictive Value
Predictive analysis is important in that it attempts to match laboratory test values
with the actual incidence of disease in a given population. False positive
results in a population with a low level of a certain disease state would incorrectly
increase the percentage of persons deemed to have the disease.
Sensitivity and specificity are important in this area. These concepts are discussed
later in this chapter in the section on clinical and statistical significance.
Briefly, a test that is too sensitive would yield an increased number of false positives
where extremely low levels of a certain metabolite may be present, and a
test that is not specific may measure entities unrelated to a disease process. If
a condition has a low prevalence (number of cases of a specific disease present in
a given population) and the test methodology is not 100% specific, false-positive
results will be higher than the predictive value. The predictive value refers to a
test result that might be substantially elevated or found at a lower level than
would be expected for certain disease states. Procedures should be highly specific
to avoid large numbers of false positives, because some tests will yield a positive
result in the presence of chemicals found in certain foods and medications. False
positives may cause undue mental stress and treatment for a disease that is not
present.
On the other hand, if a test is not specific enough, false positive results may
arise due to other conditions. An example of this is the common test for syphilis.
It is a serological test that may provide false positives for a number of illnesses
not related to syphilis. But screening tests for large populations must be sensitive
enough to identify those who may have the disease, and further definitive testing
and physical examinations could then be performed only on those who might
have the ailment. Most laboratory procedures will include information regarding
Table 10-3 Types of Comparisons for Laboratory Procedures by
Analysis of Results
t-Test The t-test measures reproducibility and is also called the paired t-test. This process
compares the accuracy of two methods because it tests the difference between the
mean value for each testing method. Remember that there are numerous methods for
performing any laboratory test. The reference or current method is considered to refl ect
true value. A null hypothesis results when there is no difference in the two methods.
F-test The F-test measures method accuracy (accuracy of test results is determined by
performing proficiency testing). The F-test compares the precision or reproducibility for
two procedures. The SD of the method showing the larger variance divided by the SD of
method showing smaller variance = % difference.
NOTE: Method refers to how a test is performed. Procedure refers to a different test, e.g., a total protein and a calcium.
Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.