Dr. Meliyanthi Gunatillaka Consultant Chemical Pathologist National Hospital of Srilanka Colombo...

Dr. Meliyanthi GunatillakaConsultant Chemical Pathologist

National Hospital of SrilankaColombo

Application of Westgard multi rules in medical

laboratories

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Components of Quality Assurance

Internal Quality Control (IQC)Steps taken by all health care professionals in their day to day activities to ensure generation of reliable laboratory results. (Prospective activity)

Pre-analytical phase

Analytical phase

Post-analytical phase

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Components of Quality Assurance cont…

External Quality Assessment (EQA)

Organized Inter-laboratory comparison

Tool to assess IQC to improve performance Performed by an independent agency Retrospective and periodic

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Control of analytical phase• Validated analytical methods

• Calibration of analytical procedure using traceability established calibration material

• Quality control material in the assay Monitor the performance by Levy Jennings charts Application of Westgard multi rules

• Equipment & Reagents

•

• Measurement Procedure

• Is used to determine the value of a quantity

• This estimate contains a measurement error, which is the difference between the obtained value and the true value of the measurand.

• Measurement error has 2 components: Random error & systematic error

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Measurement Accuracy

Is the extent of the agreement between the result of a measurement and the true value.

( True value of an analyte is obtained by using a reference method and CRM with stated uncertainties and traceability.)( Practically not possible)

(Approximations are obtained using QC material)

Accuracy encompasses the concepts of precision and trueness

( Earlier it was only the trueness)April 21, 2023 6

• ACCURACY

• PRECISION Is the extent of agreement between independent results of repeated measurements

• Imprecision :Expressed as SD or Coefficient ofVariation CV mmol/L or CV %

• TRUNESS Is the extent of agreement between the mean value of repetitions and the true value of the measured

• Expressed as a % of difference between the mean value of multiple repetitions and the true value.

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Random error

Is an unpredictable analytical variation which influence each measurement differently in either a positive or negative direction and to a different extent in magnitude.

•Possible causes of imprecision are

• Wrong pipetting technique

• Variable reaction timing and temperature

of measurement procedures

• Instrument instability

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Systematic error (bias)Constant systematic bias denotes a constant difference between the true value and the observed value regardless of the concentration level.

Proportional systematic bias denotes a difference between the true value and the observed value, which changes proportionally as the concentration level changes.

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Possible causes of systematic errors

Errors in the assigned value to the calibrator

Deterioration of calibration material

Incorrect sample or reagent volume pipetted

Incorrect reaction timing or temperature

Incorrect instrument setting (wavelength)

Calculation errors

Presence of interferents in samples(affect the

individual samples)

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• Calculations

• Mean

If the distribution of results is a normal Gaussian curve the mean is the total score of all the measurements divided by the number of measurements.

If not, remove the outliers (values with > 3SD) and recalculate the mean.

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• Standard deviation

A descriptor of the extent of dispersion of a population of test values (set of measurement data)

_

SD = √(X – X )2 ÷ (N – 1 )

Coefficient of variation

Relates the SD to the actual measurement so that measurements at different levels can be compared.

CV = SD CV = SD x 100

mean mean

Selection of Quality control material

Control materials should be used as they were patients samples and each and every one of the steps and stages that make up measurement procedure should be followed

Essential characteristics

Homogenicity: No significant differences between the parts that make up a lot of the control material

(dispensing errors, variability in lyophylization due to residual water and reconstitution errors)

( Check the procedure for reconstitution of QC)April 21, 2023 14

• Stability

The component that is analysed must be stable over a

sufficient period of time. Once recontituted the aliquoted vials should be stored in appropriate temperatures according the manufacturers instructions. Labels : date of reconstituition

No freezing and thawing cycles.

( ALP increases if stored at 2- 80 C)

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• Value of the control quantity

• Value should be close to the clinical decision level or cut off levels

• If various cut off values exist, a number of control material can be used with different concentrations

• Physiological level : upper limit of reference range

• Pathological ranges : low, medium & high

• Commutability

The QC material must respond to the changes or alterations in the measurement procedure in the same way as patients samples.

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• Types of QC material

Defined matrix material : non biological or systhetic matrix in the form of buffers, stabilizing agents

Biological matrix: Derived from biological fluids, blood,plasma,serum & urine

Matrix varies from lot to lot

Human origin : especially for immunoassays

Animal origin: cheaper and less risk of infections

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• Quality Control materials with assigned values• Manufacturers assigned value

Method & instrument specific value

Details of the certified reference material used

( with UM & stated references)

Range and standard deviation should be given

Laboratory assigned value

The mean value obtained following minimum of 20 repititions should fall within the 2SD of the method & instrument specific assigned value of the manufacturer ( preferably within the UM)

If not : peer review world wide values : local agent

iif

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• Perform the glucose test under optimum conditions for minimum 20 times • Example 01- Obtained Readings• 1-120, 2-120, 3-120, 4-120, 5-119, 6-120, 7-120, 8-121, 9-120,10 -120, 11-120, 12 -120, 13-120, 14-120, 15-119, 16- 120, 17-120, 18- 121, 19-120,20 -120,

• Mean = 120.0 • Standard Deviation = 0.45 • +1 s = 120.0 + 0.45 = 120.45• - 1s = 120.o - 0.45 = 119.55

• +2s = 120.0 + 2x 0.45 = 120.9• - 2s = 120.0 - 2x0.45 = 119.1

• +3s = 120.0 + 3x 0.45 = 121.35• -3s = 120.0 - 3x 0.45 = 118.65

How do we find the mean glucose value of the in-house QC sample?

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Mean glucose under routine conditions

• Perform the glucose under routine conditions for 30 days• Example 03• 1-120, 2-74, 3-124, 4-120, 5-117, 6-116, 7-122, 8-126, 9-191,10 -126, 11-120, 12 -123, 13-124, 14-120, 15-117, 16- 116, 17-122, 18- 126, 19-130, 20 -

126, 21-64, 22 -123, 23-124, 24-120, 25-117, 26- 116, 27-122, 28- 126, 29-200, 30 -126,

Corrected (After removal of outliers)• Mean 123.2 120.6• SD 24.0 19.5 • ±1s 147.2 – 99.2 140.1-101.1• ±2s 171.2- 75.2 159.6 -81.6• ±3s 195.2 – 51.2 179.1- 62.1•

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QC Charts and rules Shewhart/Levey and Jennings chart

Analyze the QC material by the analytical method to be controlled on at least 20 times under optimal conditions and calculate the mean, standard deviation and CV % (OCV) Remove the outliers

Construct the control chart :

Y axis-control value

X axis-days

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Mean

+2SD

+3SD

-2SD

-3SD

QC Charts and rules cont…

Warning

Warning

Action

Action

Symbolic Models Used in Internal Quality Control

variance

Analogous models…..

L-J Chart

DaysWithin ±2SD – accept > 3SD –reject between 2SD & 3SD - warning

A Trend is when six or more consecutive plots all fall in one direction, specifically upward or downward.

A shift is when six or more consecutive plots all fall on/above or below the mean line.

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Introduce a control specimen daily to the analytical run, plot each value on the chart.

After 20 days calculate the mean, SD and RCV, remove the outliers & recalculate the mean,SD and construct the chart, plot daily control value and follow the rules to accept/reject the run.

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Westgard Multirule Chart and rules

Introduce two control specimens into each analytical run one for each of the two concentrations ( NORMAL & ABNORMAL)

Plot the charts with mean and SD.

12s ‘’ 1’’ Number of control observations 2s Type of error s – Standard deviation

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If both control results are within 2SD from their target the batch is accepted

If at least one control result is more than 2SD from the target, the remaining rules are evaluated in turn, and the batch is rejected if any one rule is satisfied. If none is satisfied the batch is accepted.

The situation should be investigated before the next batch is analyzed

• Automatic chemistry analysers• Normal & abnormal QC : chemistry

Daily & 50th /100th and at the end : day time

night time : at the start• Low,medium & high QC: immunoassays

Daily all 3 levels• Procedure : calibration of the method

QC:all results within 2SD : accept:run the samples

IF one QC is out, check the violations of rules in other levels. Find the cause. ( calibration) Re run the QC : IF out again : new aliquot of QC

All QC levels satisfactory – Run the samples

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variance


Westgard multirules

13s refers to a control rule that is commonly used with a Levey-Jennings chart when the control limits are set as the mean plus 3s and the mean minus 3s. A run is rejected when a single control measurement exceeds the mean plus 3s or the mean minus 3s control limit.


01

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variance


Westgard multirules


12s refers to the control rule that is commonly used with a Levey-Jennings chart when the control limits are set as the mean plus/minus 2s. In the original Westgard multirule QC procedure, this rule is used as a warning rule to trigger careful inspection of the control data by the following rejection rules.

02

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variance


Westgard multirules


22s - reject when 2 consecutive control measurements exceed the same mean plus 2s or the same mean minus 2s control limit.

03

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variance


Westgard multirules


R4s - reject when 1 control measurement in a group exceeds the mean plus 2s and another exceeds the mean minus 2s.

04

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variance


Westgard multirules


05

41s - reject when 4 consecutive control measurements exceed the same mean plus 1s or the same mean minus 1s control limit.

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variance


Westgard multirules


10x - reject when 10 consecutive control measurements fall on one side of the mean.

06

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variance


Westgard multirules



07

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variance


Westgard multirules



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variance

Analogous models….


2of32s - reject when 2 out of 3 control measurements exceed the same mean plus 2s or mean minus 2s control limit;

What are other common multirules?

In situations where 3 different control materials are being analyzed, some other control rules fit better and are easier to apply, such as:

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variance




31s - reject when 3 consecutive control measurements exceed the same mean plus 1s or mean minus 1s control limit.

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variance





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http://www.westgard.com/images/mrf10.gif


variance





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variance




7T - reject when seven control measurements trend in the same direction, i.e., get progressively higher or progressively lower.

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Thank you

Dr. Meliyanthi Gunatillaka Consultant Chemical Pathologist National Hospital of Srilanka Colombo...

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