15th Meeting of Balkan Clinical Laboratory Federation

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15th Meeting of Balkan Clinical Laboratory
Federation

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• New techniques which diminish the necessary
  number of normal patients results in the
  Average of Normals Method

P. Karkalousos and S. Psarakis Third
Hospital of Social Security Institute, Greece
Department of Statistics, Athens University of
Economics and Business
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The method of Average Of Normals

• The Average Of Normals is an alternative
  quality control which uses only the patients
  normal results.

Advantages

• It has no cost at all
• It detects all kinds of systematic errors
• It is always available for checking
• It replaces the medium control level

Disadvantages

• It needs a special laboratory software
• It does not detect random errors

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• The basic idea of Hoffmann-Waid was that

the daily mean value (AON) of the normal
results must be steady or at least, vary within
certain limits.
The daily mean is calculated by the equation
The control limits of Average Of Normals depend
on
1. the number of daily normal results (N)
2. the standard deviation of the normal patients
(Sp)
3. the mean of the normal patients (µ)
The equation of the limits is
15th Meeting of Balkan Clinical Laboratory
Federation
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The number of daily normal results (N) depends on
1. the control limits
2. the truncation limits (reference values)
3. the ratio between patient and analytical
variation (SDR)
Control limits are steady
Because
Truncation limits are steady
Analytical variation is less than 4
N depends mostly on the variation of the normal
results
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Former studies
The studies have shown that N is
increasing simultaneously with the ratio
SDR Analytical Variation / Biological Variation

• The studies were based on computer simulations
  and to power curves production.

15th Meeting of Balkan Clinical Laboratory
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The relation between N and SDR ratio
N
SDR

• N is increasing simultaneously with SDR

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Federation
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The question is

• How the number N will be decreased?

We approached the problem by two different ways
1. Westgard rules (22s, 41s, 10x, 7x) 2.
Exponentially weighted moving average (EWMA)
Both ways were studied with computer simulations.
15th Meeting of Balkan Clinical Laboratory
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Our software program
15th Meeting of Balkan Clinical Laboratory
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The studied Westgard rules
41s
10x
22s
7x
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Power curves of four different Westgard rules
7x
10x
41s
22s
Probability of error detection
Number of Normals
SDR 3, SE 2
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Federation
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Power curves of combinations of Westgard rules
10x 22s
10x 13s
13s
Probability of error detection
Number of Normals
SDR 3, SE 2
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Exponentially Weighted Moving Average EWMA

• It is a kind of moving average method.

• It detects only systematic errors.

• It detects smaller systematic errors than
  Levey-Jennings.

15th Meeting of Balkan Clinical Laboratory
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The structure of EWMA chart
UCL
zi
µ?
LCL
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Power curve
Probability table
Historical data
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Performance of EWMA chart
SE 3
SE 2
Probability or error detection
SE 1
Number of rows (SDR 3) Each row is 20 normal
patients
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Comparison of AON methods
SDR 4, T 2, ? 0,2, SE 2
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Conclusions
?he westgard criteria 10 , 7 , 41s, 22s decrease
the necessary number of normals (especially the
mean rules).
Multi-rule methods of AON like 10 13s, 10
41s are not so effective.

• EWMA method, when used for the purpose of AON,
  does not diminish the necessary number of normals.

15th Meeting of Balkan Clinical Laboratory
Federation
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15th Meeting of Balkan Clinical Laboratory
Federation
Thank you for your attention