CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 235
Table 10-1 Quality Control Terminology and Definitions
Terms generic to all quality control programs in a clinical laboratory setting are as follows:
Term
Definition
Accuracy
Accuracy is how close a measurement is to a value verifi ed by other analytical methods.
Analytical error An analytical error occurs during testing. For example, in an autoanalyzer, a hundred samples may be
relatively accurate, and one sample may be extremely abnormal; often the cause is never determined.
Average for a
laboratory test
Coefficient of
variation (CV)
Control specimen
Normal frequency
curves
Population norms
Postanalytical error
Preanalytical error
Proficiency programs
Quality control charts
Quality control
programs
Random error
Also known as a mean; chiefl y associated with control specimens; determined by the sum of all the
determined values divided by the total number of values.
The coefficient of variation (CV) is a calculation that shows variability between measurements for the same
laboratory test performed more than once. It is normal to have some variability due to variables such as changes
in electrical voltage, age of the spectrophotometer bulb, and standard errors in pipetting. ESSENTIAL TO
REMEMBER: CV is always measured as a percentage!
Contains known concentration of constituents being measured; may be assayed or unassayed; used to
measure precision (reproducibility). Originally these results were displayed on Levey-Jennings charts but are
now stored in computerized documents that are fi led for review as necessary.
Established for each constituent measured, including calculated ones.
Gaussian curve—graph (sometimes called a bell curve) that plots the distribution of determined values
around the mean value of the control specimen (also called normal frequency curve).
Levey-Jennings chart most common: provides the days of the month, along with 1 and 2 SDs delineated on the chart.
Laboratory test values for an entire ethnic or geographic group.
Moving averages over a period of time (months to years) should equal zero. Each day a quality control value
is either positive or negative, and should “balance out” over a signifi cant period of time. Shifts and trends
would adversely affect the moving averages due to multiple and sequential positive or negative values.
The normal range is determined by repeat testing and the history of the values determined. The normal or
reference range aids in identifying the abnormal or diseased patient. A large number (100 to 300) of healthy
individuals is tested, and the mean, median, and mode of each type of procedure is calculated, along with
the SD (1 and 2 SD).
A postanalytical error occurs following the completion of testing. It most often involves handling and
transmittal of the report.
A preanalytical error occurs before testing is completed, and may include patient misidentification, improper
sample collection, or improper standardization of instrumentation.
External programs are sent to laboratories that, as a part of being licensed or accredited, are required to
participate in a proficiency testing program. While controls evaluate precision, meaning reproducibility,
proficiency testing (PT) programs assess accuracy. Test results from groups of laboratories throughout the
country that employ similar procedures with similar equipment and supplies are compared. These results are
usually also furnished to state laboratory licensure offi ces for action if results are not within an established
range.
Contain constituents in both normal levels and abnormal levels—sometimes three-level controls; may be
assayed and unassayed; suffi cient stock on hand to handle the annual historical workload; inconvenience; run
at intervals; time or specimen numbers.
Internal and external programs both may be used to effi ciently cover the 24-hour schedule. As an example,
a commercial (externally purchased) set of controls may be used in the morning, and an internal specimen
from a patient may be used a number of hours later to determine if any malfunction with an instrument has
occurred.
A random error whose cause cannot be defi nitively identifi ed is usually environmental or operator generated.
(Continues)
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