Quality Control for Quantitative Tests Flashcards
measure the quantity of a particular substance in a sample
Quantitative Tests
quality control for quantitative tests is designed to assure that patient results are
accurate
reliable
material that contains the substance being analyzed
Control
used to validate reliability of the test system
Control
A substance with a specific concentration
Calibrators
A substance similar to patients’ samples that has an established concentration.
Controls
A calibrator can become a control
False
Used in laboratory conditions
Controls
Characteristics of Control Materials
◼ appropriate for the diagnostic sample
◼ values cover medical decision points
◼ similar to test sample (matrix)
◼ available in large quantity; ideally enough for one year
◼ can store in small aliquots
Types of Control Materials
◼ may be frozen, freeze-dried, or chemically preserved
◼ requires very accurate reconstitution if this step is necessary
Sources of Controls Materials
◼ commercially prepared
◼ made “in house”
◼ obtained from another laboratory, usually central or reference laboratory
Target value predetermined
Verify and use
Assayed Materials
Target value not predetermined
Full assay required before using
Unassayed Materials
In-house pooled sera
Full assay, validation
In-house Materials
Choosing Control Materials
◼ values cover medical decision points
◼ similar to the test sample
◼ controls are usually available in high, normal, and low ranges
Preparation and Storage of Control Material
◼ adhere to manufacturer’s
instructions
◼ keep adequate amount
of same lot number
◼ store correctly
Steps in Implementing Quantitative QC
◼ obtain control material
◼ run each control 20
times over 30 days
◼ calculate mean and +/-
1,2,3 Standard Deviations
Normal occurrence when a control is tested repeatedly
Variability
Variability is affected by:
Operator technique (Medtech)
Environmental conditions (Environment)
Performance characteristics of the measurement (Machine)
Measures of Central Tendency
Mean, Median, Mode
the value which occurs with the greatest frequency
Mode
the value at the center or midpoint of the observations
Median
the calculated average of the values
Mean
Not all central values are the same
True
characteristic “bell-shaped” curve
Normal Distribution
used to monitor
the accuracy and the precision of the assay.
Quality Control
The closeness of measurements to the true value
Accuracy
The amount of variation in the measurements
Precision
The difference between the expectation of a test result and an accepted reference value
Bias
Accurate is always precise
True
Precise is not always Accurate
True
For a set of data with a
normal distribution, a
random measurement
will fall within:
+ 1 SD 68.3% of the time
+ 2 SD 95.5% of the time
+ 3 SD 99.7% of the time
principle measure of
variability used in the laboratory
Standard Deviation
SD expressed as a percentage of the mean.
Coefficient of variation
Graphically Representing Control Ranges
Levey-Jennings Chart
results with no pattern-only a cause for rejection if outside 2SDs
random error
not acceptable, correct the source of error
systematic error
control on one side of the mean 6 consecutive days
shift
control moving in one direction–heading toward an “out of control” value
trend
represents a range of values in which the true value is reasonably expected to lie
Measurement Uncertainty
Measurement Uncertainty is estimated at
95% coverage
If QC is out of control
STOP testing
Do not report patient results until problem is solved
