laboratory quality control
TRANSCRIPT
Laboratory Quality Control
USE AND NEED OF QUALITY CONTROL IN A CLINICAL LABORATORY
OBJECTIVES1. Students will be able to define Quality Control
and Quality Assurance , so that when ask would recall with 100% accuracy.
2. Student will be able understand the importance of controls and be able to differentiate among them, so that when ask would be able to state the importance of each and identify the difference from other controls with 100% accuracy.
3. Students will be able to establish the difference between random errors and systematic errors, so the when ask would state so accurately.
OBJECTIVES4. Students will be able to understand the
concept of trouble shooting , so that when given a scenario would correctly state the use and purpose of trouble shooting.
Definitions Quality Control - QC refers to the
measures that must be included during each assay run to verify that the test is working properly.
Quality Assurance - QA is defined as the overall program that ensures that the final results reported by the laboratory are correct.
Definitions Quality Assessment - quality
assessment (also known as proficiency testing) is a means to determine the quality of the results generated by the laboratory.
Quality Assessment may be external or internal.
Variables that affect the quality of results
The educational background and training of the laboratory personnel
The condition of the specimensThe controls used in the test runsReagentsEquipmentThe interpretation of the resultsThe transcription of resultsThe reporting of results
Internal controls Verify the internal control function for each
sample processed. If an internal control fails, the sample must be
repeated. Examples of failure causes are
Improper sample addition Faulty device
External controls Kit (manufacturer prepared) controls (all tests):
Must be performed: With each new lot or lot change. With each new shipment. When temperatures exceed the manufacturer
recommended storage conditions. With each new operator prior to performing
testing on patient specimens With each processor/equipment daily. Controls run on every testing day
Store and use according to manufacturer recommendations.
Data is recorded in the QC log book located in the laboratory.
In-house controls Must be performed:
With each new lot or lot change. With each new shipment. When temperatures exceed the manufacturer
recommended storage conditions. With each new operator prior to performing
testing on patient specimens. Refer to procedures regarding preparation of in-
house controls and their storage. Data is recorded in the QC log book and the
containers for that lot or shipment are labeled ‘Ready to Use’.
Numeric value QC
When the method is put into service, you also establish your own means and standard deviations (SDs) for control materials.
A validated chart is generated using 20 or more measurements. Once the chart is plotted the mean and the standard deviations (±1s, ±2s and ±3s) are calculated using excel program.
Numeric value QC (cont.)
If it becomes evident that the mean needs to be adjusted due to shift change, lot change or reagent change, a QC Chart Change Form should be completed and filed in the appropriate QC binder.
When there are changes in the source or lot numbers of control materials, it is important to carefully establish the means and SDs for the new materials while the old materials are still in use.
Numeric value QC (cont.) The control limits are supposed to describe
the variation expected when performance is stable, i.e., when there are no problems occurring. Therefore, it is advisable to eliminate all control values from all runs that have been rejected, even if some of those control values are within 2 SD of the mean.
Errors in measurementTrue value - this is an ideal concept which
cannot be achieved.
Accepted true value - the value approximating the true value, the difference between the two values is negligible.
Error - the discrepancy between the result of a measurement and the true (or accepted true value).
Sources of error
Input data required - such as calibration values and values of physical constants.
Inherent characteristics of the quantity being measured
Instruments used - accuracy, repeatability.
Observer fallibility - reading errors, blunders, equipment selection, analysis and computation errors.
Environment - any external influences affecting the measurement.
Theory assumed - validity of mathematical methods and approximations.
Random Error An error which varies in an unpredictable manner,
in magnitude and sign, when a large number of measurements of the same quantity are made under effectively identical conditions.
Random errors create a characteristic spread of results for any test method and cannot be accounted for by applying corrections. Random errors are difficult to eliminate but repetition reduces the influences of random errors.
Random Errors
x
x x
x x
True x x x x
Value x x x
x x x
x
x
x
Systematic Error An error which, in the course of a number of
measurements of the same value of a given quantity, remains constant when measurements are made under the same conditions, or varies according to a definite law when conditions change.
Systematic errors create a characteristic bias in the test results and can be accounted for by applying a correction.
Systematic errors may be induced by factors such as variations in incubation temperature, change in the reagent batch or modifications in testing method.
Systematic Errors
x
x x x x x x x
True x
Value
Levey-Jennings Chart Include the name of the test and the name of the
control material in a prominent place so that this information is quickly and easily discerned when viewing the chart.
The measurement unit can be included in the label or included in the label for the y-axis.
Other information included in the cover sheet accompanying the charts are the lot number of the control material, the current mean and standard deviation, and the time period covered by the chart.
The horizontal or x-axis represents number of run. The vertical or y-axis represents the observed control
value and is set to accommodate the lowest and highest results expected.
On the y-axis, locate the values that correspond to the mean and draw a horizontal line. The mean is calculated by adding all the measurements for a particular control and dividing that sum by the number of points. Excel can be used to determine this by highlighting the values and calculating the mean.
Westgard rules Westgard rules are commonly used to
analyze data in Shewhart control charts. Westgard rules are used to define
specific performance limits for a particular assay and can be use to detect both random and systematic errors.
There are six commonly used Westgard rules
The violation of warning rules should trigger a review of test procedures, reagent performance and equipment calibration.
The violation of mandatory rules should result in the rejection of the results obtained with patients’ serum samples in that assay.
Shewhart Control ChartsA Shewhart Control Chart depend on the use of IQC specimens and is developed in the following manner:-
Put up the IQC specimen for at least 20 or more assay runs and record down the cut-off value or antibody titre (whichever is applicable).
Calculate the mean and standard deviations (s.d.)
Make a plot with the assay run on the x-axis, and cut-off or antibody titre on the y axis.
Draw the following lines across the y-axis: mean, -3, -2, -2, 1, 2, and 3 s.d.
Plot the cut-off obtained for the IQC specimen for subsequent assay runs
Major events such as changes in the batch no. of the kit and instruments used should be recorded on the chart.
Shewhart Chart
0102030405060708090100
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+3 sd
-3 sd
+2 sd
-2 sd
-1 sd
+1 sd
VZV IgG ELISA: Target Value = 49 U/ml
Antibody
Units
Target value
Assay Run
Warning rules Warning 12SD : It is violated if the IQC value
exceeds the mean by 2SD.
Warning 22SD : It detects systematic errors and is violated when two consecutive IQC values exceed the mean on the same side of the mean by 2SD.
Warning 41SD : It is violated if four consecutive IQC values exceed the same limit (mean 1SD) and this may indicate the need to perform instrument maintenance or reagent calibration. Systematic error.
Mandatory rules Mandatory 13SD : It is violated when the IQC value
exceeds the mean by 3SD. The assay run is regarded as out of control. Random error.
Mandatory R4SD : It is only applied when the IQC is tested in duplicate. This rule is violated when the difference in SD between the duplicates exceeds 4SD. Random error.
Mandatory 10x : This rule is violated when the last 10 consecutive IQC values are on the same side of the mean or target value.
Westgard Rules: 1 3SD
0102030405060708090100
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+3 sd
-3 sd
+2 sd
-2 sd
-1 sd
+1 sd
VZV IgG ELISA: Target Value = 49 U/ml
Antibody
Units
Target value
Assay Run
Westgard Rules: 10X
0102030405060708090100
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+3 sd
-3 sd
+2 sd
-2 sd
-1 sd
+1 sd
VZV IgG ELISA: Target Value = 49 U/ml
Antibody
Units
Target value
Assay Run
Monitoring FrequencyWeekly Each day an assay is performed, the day’s
results are plotted on the appropriate weekly chart and checked by the analyst after entry.
The supervisor and/or quality assurance manager or their designee check the weekly chart at the end of the week.
Monthly Values from the month are compiled and
reviewed by the supervisor and/or quality assurance manager or their designee at the end of the month.
Follow-up action in the event of a violation
There are three options as to the action to be taken in the event of a violation of a Westgard rule:
Accept the test run in its entirety - this usually applies when only a warning rule is violated.
Reject the whole test run - this applies only when a mandatory rule is violated.
Enlarge the greyzone and thus re-test range for that particular assay run
Trouble shooting Out-of-Control runs. Systematic errors may be caused by factors
such as a change in reagent lot, improperly prepared reagents, deterioration of reagents, inadequate storage of reagents, change in sample reagent volumes due to pipettor maladjustments or misalignment, change in temperature of incubators and reaction blocks, deterioration of photometric light source, and change in procedure from one operator to another.
Trouble shooting Out-of-Control runs.Random errors may be caused by factors
such as inadequately mixed reagents, unstable temperature and incubation, unstable electrical supply, and individual operator variation in pipetting, timing, etc.
Relate causes to recent changes.
Systematic errors are most often related to reagent or calibration problems. A sudden shift is usually due to a recent event such as replacement of reagent, introduction of a new reagent lot number, or a recent calibration. When a shift is identified, the operator should inspect the reagent, calibration, and maintenance records for clues to resolving the problem.
Relate causes to recent changes.
A systematic trend can be more difficult to resolve than a shift, simply because the problem is occurring over a longer period of time.
Review QC records, including documentation of function checks. Trends can be the result of a slowly deteriorating reagent, a change in instrument temperature, or a deteriorating filter or lamp.
Use a systematic logical trouble-shooting approach in isolating the cause, making only one change at a time and documenting each action taken.
Relate causes to recent changes Problems resulting in increased random error are much
more difficult to identify and resolve, mostly due to the nature of the error, which cannot be predicted or quantified as can systematic error.
Random errors are more likely due to improperly mixed reagents, pipette tips not fitting properly. A clog in the pipettor, imprecise pipettor, the power supply, and even power fluctuations. Many of the sources of random error can be observed by physical inspection of the analytical method during operation. Careful inspection of reagents and the sampling/reagent pick-up and dispensing activities will often identify the cause of the problem.
If nothing is observed during the inspection process, consult trouble-shooting guides and manufacturer recommendations.
Corrective Action After the cause of the problem has been
identified, it must be corrected and the solution verified by retesting all of the controls.
Once in-control, patient samples from the out-of-control run should be repeated as necessary. The out-of-control event must be documented using the Occurrence Management Form.
Key operators can often recognize the most common problems with a given system and are more skilled at problem resolution than the infrequent operator. The knowledge of these key operators should be tapped to identify logical trouble-shooting approaches which can be used by all operators.
Non-numeric value QC These results are either a pass or fail for a
particular method. If the run fails, further investigation into the
cause is necessary. Complete an Occurrence Management Form and document the corrective action taken to resolve the problem.
Kit controls are documented in the QC log book located in the laboratory.
In-house controls are recorded on the reagent/kit log and the lot is labeled ‘Ready to Use.’
QC Recording What
A complete description of the process, i.e., the method procedure. Initial evaluation data to document method performance. Daily information about routine operation:
The analyst Observed control values Decisions made about the control value Identification of any problems Documentation of any corrective action Evidence of supervisory review.
All individual data values Summary statistics – monthly and cumulative QC problems and decisions Trouble-shooting activities
Corrective actions and follow-ups Proficiency testing results Preventive maintenance – scheduled and unscheduled
QC Recording Reason for maintenance, what was done, by whom. Frequency and length of downtime Signs of instrument deterioration Calibration, calibration verification – scheduled and
unscheduled. Lot numbers and expiration dates, what was
changed, why, by whom. Reagent changes – planned and unplanned. Instrument function checks (temperatures,
alignments, gating, etc.).
QC Recording How
The format for recording information must permit convenient timely review by being clear, complete, and well organized.
The record must be current and contain all the relevant information.
Hard copies of all QC logs and/or charts are in the laboratory binders.
QC records, reagent changes, calibrations, etc., are retained at least two years, and all maintenance records must be kept for the lifetime of the instrument plus two years.
QC Recording Who
The supervisor, lead analyst, or QA Manager is responsible for the organization of the initial method evaluation study, but other analysts may participate in the studies and be involved in collecting the method performance data. A formal report of the method evaluation can be found in the Validation binder.
Everyone who operates a method has the responsibility for recording any changes they make to the testing process, control results they obtain during operation, decisions on reporting or not reporting patient test results, and corrective actions made.
The supervisor, their designee, or the QC specialist is responsible for reviewing the daily QC, reviewing proficiency testing results, following up on problems, reviewing the monthly QC statistics, updating the cumulative QC statistics, identifying chronic and long-term problems with the system, and recommending additional studies, changes or improvements. These individuals may also be responsible for reviewing method evaluation studies, selecting appropriate QC procedures, and periodically reassessing or validating the QC design.
QC Recording When
Review QC records daily, weekly and monthly as well as periodic quality audits.
These studies will aid trouble shooting and help correlate daily performance data with information from method evaluation experiments, proficiency testing surveys, and patient test results.
Findings should lead to better preventive maintenance, improved training of analysts, and redesign of troublesome steps in the testing process.
HOME WORKDefine the terms trend and shift as it relates
to QC giving atleast two examples.
END