basic control rules

7/29/2019 BASIC CONTROL RULES

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BASIC QUALITY CONTROL

STATISTICS

Geromil J. Lara, RMT, MSMT


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MEAN

The mean (or average) is the laboratorysbest estimate of the analytes true value for

a specific level of control.x=xi/n

Where:

xi= each data point

n= the number of data points in the set


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Glucose QC Values

using Level 1 as control material:

Day 1 4.5

Day 2 4.7

Day 3 4.6Day 4 4.4

Day 5 4.6

_______

22.8 / 5 runs = 4.56 Mean


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Standard Deviation

Standard deviation quantifies how closenumerical values are in relation to eachother.

Low standard deviation indicates that all ofthe data points are very close to the samevalue (the mean),

High standard deviation indicates that thedata are spread out over a large range ofvalues.

Also used to set up limits

SD = (Xn-Mean)2n-1
http://en.wikipedia.org/wiki/Meanhttp://en.wikipedia.org/wiki/Mean


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STANDARD DEVIATION

FORMULA: (MEAN) +/- (SD) (1) FOR 1SD

LOWER LIMIT 4.56 - (0.1)(1) = 4.56 - .1 = 4.46

HIGHER LIMIT 4.56 + (0.1)(1) = 4.56 +.1 = 4.66


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Formula :(Mean) +/- (SD) (2) for 2SD

Lower Limit 4.56 - (0.1) (2) =4.56 - .2 = 4.36

Higher Limit 4.56 + (0.1) (2) = 4.56 +.2 = 4.76


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Formula :(Mean) +/- (SD) (3) for 3SD

Lower Limit 4.56 - (0.1) (3) = 4.56 - .3= 4.26

Higher Limit 4.56 + (0.1) (3) = 4.56 +.3 = 4.86


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LEVEY JENNINGS CHART

4.56

4.76

4.66

4.86

4.46

4.36

4.261 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


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Coefficient of Variation

The Coefficient of Variation (CV) is ameasure ofvariability.

A method or instruments CV isexpressed as a percent and iscalculated as:

CV (%)= (Standard Deviation / Mean) (100)

Example: 0.1/ 4.56 (100) = 2.19%


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Importance of CV

Coefficient of Variation is useful forcomparison of precision for twodifferent methods or instrument.

It can also be used as a part of theInternal Quality Control systemwhen performing patients precisiontesting.

The lower the CV, the better


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SDI ( Standard Deviation

Index)

A statistic that measures your labsbias relative to your consensusgroup. An estimate of reliability.

Example : __100 -98__= 1

2


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0 - Ideal score; Identical to your peergroup

(+/-)1 to 1.5

acceptable to marginal performance;may need to investigate test system bias

(+/-)1.5 t0 2.0marginal performance;

may need to perform corrective action.


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CVR ( Coefficient of Variation

Ratio)

A statistic that compares your labsprecision to that of other labs in a

consensus group.

Ex. 10 = 2

5


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Less than 1

Acceptable performanceIndicates that precision is better than peergroup

1 to 1.5Acceptable to marginal performanceMay need to investigates test system

imprecision

1.5 to 2.0Marginal performance

May need to perform corrective action


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Using Levey-Jennings Chart to

Evaluate Run Quality


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Shift

is defined asabruptchanges inthe control mean.

Shifts in QC data represent asudden and dramatic positive ornegative change in test system

performance


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Example of Shift

Mean

2SD

1SD

3 SD

-1SD

-2SD

-3SD1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


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Shifts maybe caused by:

Sudden failure or change in the lightsource

Change in reagent formulation

Change of reagent lot

Major instrument maintenance

Sudden change in incubation temperature(enzymes only)

Change in room temp. or humidity Failure in sampling system & reagent

dispense system

Inaccurate calibration/recalibration


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Trend

indicates a gradual loss ofreliability in the test

system.

Trends are usually subtle.


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Example of Trend

Mean

2SD

1SD

3 SD

-1SD

-2SD

-3SD1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


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Trend may be caused by:

Deterioration of the instrument light source

Gradual accumulation of debris insample/reagent tubing

Gradual accumulation of debris onelectrode surfaces

Aging of reagents

Gradual deterioration of control materials

Gradual deterioration of incubator chambertemp.

Gradual deterioration of light filter integrity

Gradual deterioration of calibration


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Trends and Shifts areindicative of a problem

and these should beinvestigated and

corrected!


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WESTGARD RULES


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ERROR DETECTION

Systematic Errora trend or shift away from the laboratorymean

Random Errorany random deviation away from thelaboratory mean


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This is a warning rule that is violated when asingle control observation is outside the 2slimits.This rule merely warns that random error orsystematic error may be present in the testsystem.


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Violation indicates

systematic error

Two consecutive controls

exceed the mean by +/- 2SD


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This indicatessystematic error 4 consecutive controls on one side of the

mean exceeds +/-1SD


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This indicates systematic error 10 consecutive controls on one side of

the mean


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Sources of

Systematic Error Improper alignment of sample

or reagent pipettes resulting tochange in reagent and calibratorvolume.

Drift or shift in incubatorchamber temperature,deterioration of photometric

light source

Change of Calibration /reagentlot


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Sources of

Systematic Error Deterioration of

reagent/calibrator/controlproduct while in use, storageor shipment

Incorrect handling of controlproduct

Inadequate storage of reagentor calibrators/control products

Change in test operator


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This rule identifies random error

Any QC result outside 3s violates this rule.


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If there is at least a 4s difference betweencontrol values within a single run, the rule isviolate forrandom error.


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Sources of

Random Error Power Supply

Double pipetting of control

sample

Misplacement of Control samplewithin the run

Random air bubbles in reagent orsample pipette system


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Sources of

Random Error Incorrect reconstitution of the

control product

Inappropriate Storage ofcontrol

Inadequately mixed reagents Individual operator variation in

pipetting , timing , etc.


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BIAS/SDI RESULTS

ACCURACY /RELIABILITY ARE AFFECTED

Mean

2SD

1SD

3 SD

-1SD

-2SD

-3SD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


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PRECISION/REPEATABILITY/CV/

CVI ARE AFFECTED

Mean

2SD

1SD

3 SD

-1SD

-2SD

-3SD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


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IDEAL CONTROL RUNNING

Mean

2SD

1SD

3 SD

-1SD

-2SD

-3SD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


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QC- THE OUT OF CONTROL PROBLEM

Bad Habit #1 Repeat the controls

Bad Habit # 2 Try a new control

Good Habit #1 Inspect the control chart orRules violated to determine the type of error

Good Habit #2 Relate the type of error topotentialCausesGood Habit # 3 Verify the solution and documentthe remedy

Develop Good Habits- Solve Problems


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If you think its hard

it will be

basic control rules

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