QC % Westgard Flashcards
Mean, median, and mode
Mean= average
Median = the middle value in a body of media
Mode – value most frequently occurring
Standard Deviation
1SD = 68%
2SD = 95%
3SD = 99.73%
Reference Range is 95% of healthy population
Confidence Interval
95% Confidence interval
2SD on either side of mean
Accounts for unavoidable error caused by sampling variability and imprecision.
%CV
SD X 100/Mean
What is a more precise CV?
lower number
CV Ratio
CVR = Lab CV/Peer Group CV
Precision
F test
analysis of variance
Used to determine differences of precision between two different methods for the same analyte
Accuracy
Paired T-test
Used for method correlation- looking at the means and reference intervals of two different methods for the same analyte (patient population)
Determination of control range
Assay aliquot of control serum with regular batch assays for 15-25 days
Treat controls the same as specimen
Mean and SD = produce a Levey-Jennings Chart
Sources of variance or error
Sampling factors
-Time of day
-Patients position
-Physical activity
-Fasting/ not fasting
-Time between draw and measurement
Procedural factors
-Aging of chemicals/reagents
-Experience of technologist
-Variations in standards, reagents, environment, methods or apparatus
-Changes in method, instruments or personnel
Levey-Jennings Charts
-Evaluate unacceptable runs
-Maintain stability of analytical measurement system
-Detect changes in the system
-Plot Mean, 1 SD, 2 SD, 3 SD
-2 SD = warning
-3 SD = action needs to be taken
-Each analyte has its own chart
-When procedural changes are made they should be noted on chart
—–lot changes, new reagents, new instrument, instrument maintenance
Shift
A sudden and sustained change in one direction in control sample values
Malfunction of instrument
Trend
A gradual change in the control sample results
A systematic drift = control value moves progressively in one direction fro the mean for at least 3 days
Suggests a progressive problem with test system or control samples.
Example ?
Dispersion
Random error or lack of precision increases
Indicate instability problems
Westgard rules
System of rules that define specific performance limits for a particular assay
Can detect both systematic and random errors
Designed to improve the power of QC methods uses ± 3SD limits to detect trends or shifts
1 2s
1 point is outside 2 SD = Warning
1 3s
1 point is outside 3SD
2 2S
two consecutive points are outside 2SD on same side of center line– Within a material and across runs.
R 4s
Range of two points is greater than 4 SD–Within a material and across runs.
4 1s
4 consecutive points exceed 1SD on the same side of the mean
10x
10 consecutive points are above or below the mean.
Either 10 in a row within a material or 5 consecutive points across 2 runs (2 levels of control)
Out of Control Procedures
Review the procedures used
Search for recent events
-New reagent
-New lot (control, reagent , calibrator)
-Environmental conditions
-Maintenance
Prepare new controls
-Typically the first method done
Calibrate the instrument
Prepare new reagents
Follow manufactures troubleshooting guide
Contact instrument manufacturer
Calibration
refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. Instrument calibration is intended to eliminate or reduce bias in an instrument’s readings over a range for all continuous values.
How to Calibrate
Selection of reference standards with known values to cover the range of interest.
Measure the reference standards with the instrument to be calibrated.
Plot relationship between the measured and known values of the reference standards (usually a least-squares fit to the data) called a calibration curve
When to Calibrate
Before major critical measurements
After an “event”
-Maintenance
-New parts
-New instrument, reagent, control etc
-New lot
When observations or control appear questionable
-When controls are out, or when you observe a problem
Per requirement of manufacturer or procedure
*Calibration may not solve the problem.
QC Problem: Drift (Trend)
Check outdates on materials
Calibrate assay
If issue is not resolved replace reagent and calibrate again.
QC Problem: Shift
Look at the QC for the last 1-2 months to determine when the shift took place
What happened on that date?
-New reagent lot?
-New calibrator lot?
-Was the instrument repaired or had preventative maintenance performed around that date?
To confirm that the QC shifted calibrate a new well-mixed reagent pack with fresh calibrator.
If the shift continues= suspect an instrument malfunction, call service
QC Problem: Random Error
Repeat control
If the repeat control is out of range, calibrate
If multiple assays have QC flyers suspect a QC problem (if they are run on the same control) or an instrument issue.
QC Problem: Poor Precision
Replace reagent, mix well and calibrate
If problem persists, put a fresh reagent on the opposite instrument and calibrate. If time permits run QC 5X as a patient on both instruments.
QC Problem: Shift with Poor Precision
Has the assay been calibrated lately? If not calibrate on a reagent pack that is at least half full, preferably a new pack.
If the QC does not improve after calibration, or the assay has been calibrated recently, compare the QC values to a second instrument if possible.
If the QC recovery is the same on both instruments, suspect a shift due to reagent lot or calibrator lot change
-If the QC recovery improves on the opposite instrument suspect an instrument issue and call to set up a service call
