Transcript Slide 1

Internal Quality Control (QC) for Medical
Laboratories: An introduction
Dr. Otto Panagiotakis and Dr. Alexander Haliassos
ESEAP – Greek Proficiency Testing Scheme
for Clinical Laboratories
http://www.eseap.gr
[email protected]
• Analytical Quality Control •
The most important tool for ensuring the quality
of laboratory results through the identification and
reduction of errors
It includes two parallel mechanisms:
• Internal (intra-laboratory) quality control
• External quality control, or External quality assessment
(EQA) or Proficiency Testing (PT)
What means a value of total cholesterol 245mg/dL
reported for the analysis of an QC control?
Correct result: There is not a
value, but a range from
repeated measurements
We need a tool to compare the
reported with the expected
235
245
255
265
240
250
260
result.
It is the control chart Levey-Jennings
Control charts Levey-Jennings:
+3s
+2s
+1s
mean
-1s
-2s
-3s
Central horizontal line: expected mean value
Dotted horizontal lines: control limits (mean ± nSD)
Control charts Levey-Jennings:
Their design is based on the assumption that:
• The values resulting from previous measurements are
subject to random variation
• This variation follows a uniform (normal) distribution
How we draw the control charts?
• We select a parameter (p.ex. Cholesterol)
• we measure this parameter in a control material for
20 days using the method (assay) and the instrument
(analyzer) that we evaluate
• from those values we calculate the mean and the SD
p.ex. mean = 200 mg/dL and SD = 4 mg/dL
• subsequently, the control limits at the level of 2s, 3s
mean ± 2s: 200 ± 2(4)=200 ± 8  from 192 to 208
mean ± 3s: 200 ± 3(4)=200 ± 12  from 188 to 212
Cholesterol (mg/dL), Lot No: xxx, January 2015
216
x+3s
212
x+2s
208
mean
200
192
x-2s
188
184
x-3s
1 2
3
4
5
6 7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Interpretation of control charts (1)
• The control charts reflect the distribution resulting from the
initial measurements of the control materials
• In the charts we draw the values of each measurement
• Evaluation: depending on their position in the chart
1) If the analytical procedure is correct, the new measurements
will have the same distribution with the originals:
• Extremely rare (0,3%) a value> mean ± 3s
• Unlikely (5%) a value> mean ± 2s
• Very likely (32%) a value> mean ± 1s (limits without a value)
Control Rules
Rules to decide whether a series of measurments is
under control or out of control
Control rules
control limits
12s
mean ± 2s
13s
mean ± 3s
Single rule methods
Multiple rules methods
Interpretation of control charts (2)
2) If the analytical procedure has a problem, it increases
the probability of a value exceeding the control limits
This can happen :
• Either with the appearance of a constant error (bias)
(shifting of the mean of the distribution to or values)
• or by increasing the random error
(enlargement of the distribution)
Situation out of control / Unacceptable results
Situation
under control
(normal)
•
Systematic
error (bias)
Distribution
enlargement
The problem of erroneous rejects
The QC methods are detection systems
The detection systems have some characteristics:
The frequency of true warnings, true alarms
The frequency of erroneous warnings, false alarms
In the QC methods they are called respectively :
• Error detection probability (Ρ)
• Probability of false rejection (Ρο)
The ideal on a single rule QC method would be:
Ρ=1,0 (100%) and Ρο=0 (0%)
However, for each control rule: Ρ < 1 and Ρο > 0
A realistic goal is : Ρ=0,90 and Ρο=0,05
Rule 12s: high Ρ and high Ρο
For Ν=1 Ρο=5%
For Ν=2 Ρο=9%
For Ν=3 Ρο=14%
• The single rule QC method using the 12s rule
should only be used with Ν=1.
• If Ν=2, almost a false rejection in 10
• This is not a problem of the analytical procedure.
• This is an intrinsic problem of the QC method
and related to the selected control threshold.
Rule 13s: low Ρο and low Ρ
resulting in non detected large errors
As the control limits are extended erroneous rejections
(Po) decrease, but also P decreases
If a rule with high P is selected will have also high Ρο
Single rule QC methods have serious drawbacks
Need to find other QC methods
Multiple rules methods
They do not use a single control rule
but a combination of rules (at least 2)
Advantage: Low Po and simultaneously high P
The most well known Westgard method:
• 6 control rules
• 2 control sera (N=2) resulting to
• 2 Levey-Jennings control charts L-J, one for
each serum (control material)
• Control limits at three levels(±1s, ±2s, ±3s)
Cholesterol (mg/dL), Lot No: xxx, January 2015
216
x+3s
212
x+2s
208
x+1s
204
200
196
x-1s
192
x-2s
188
184
x-3s
1 2
3
4
5
6 7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
The 6 control rules according to Westgard
12S
13S
22S
R4S
41S
10x
Rule 12S
One value (measurement) > 2s limit
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
1
2 3
4
5
6
7
8 9 10
Not a rejection but warning for a potential problem
Further control is required based on the other criteria
Rule 13S
One value (measurement) > 3s limit
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
1
2 3
4
5
6
7
8 9 10
Applied in a series for each of the two sera
Sensitive to random errors
Rule 22S
2 consequent values > the same limit of 2s
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
1
2 3
4
5
6
7
8 9 10
In the last 2 series for each serum
In the same series for both
+3s
+2s
2 consequent values > the same limit of 2s
+1s
mean
+3s
- 1s
+2s
- 2s
+1s
- 3s
mean
1
- 1s
- 2s
+3s
- 3s
+2s
1 2 3 4 5 6 7 8 9 10 +1s
mean
In the last 2 series for each serum- 1s
- 2s
In the same series for both
- 3s
Sensitive to systematic errors
Rule 22S
1
2 3
4
5
6
7
8 9 10
2 3
4
5
6
7
8 9 10
Rule R4S
The difference in value of the two sera> 4s
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
1
2 3
4
5
6
7
8 9 10
In the last 2 series for each serum
In the same series for both
+3s
+2s
The difference in value of the two sera> 4s
+1s
mean
+3s
- 1s
+2s
- 2s
+1s
- 3s
mean
1
- 1s
- 2s
+3s
- 3s
+2s
1 2 3 4 5 6 7 8 9 10 +1s
mean
In the last 2 series for each serum- 1s
- 2s
In the same series for both
- 3s
Sensitive to systematic error
Rule R4S
1
2 3
4
5
6
7
8 9 10
2 3
4
5
6
7
8 9 10
Rule 41S
4 consecutive values > the same limit of 1s
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
1
2 3
4
5
6
7
8 9 10
In the last 4 series for each serum
In the last 2 series for both
+3s
+2s
4 consecutive values > the same limit of 1s
+1s
mean
+3s
- 1s
+2s
- 2s
+1s
- 3s
mean
1
- 1s
- 2s
+3s
- 3s
+2s
1 2 3 4 5 6 7 8 9 10 +1s
mean
In the last 4 series for each serum- 1s
- 2s
In the last 2 series for both
- 3s
Sensitive to systematic errors
Rule 41S
1
2 3
4
5
6
7
8 9 10
2 3
4
5
6
7
8 9 10
Rule 10Χ
10 consecutive values at the same
side of the mean
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
1
2 3
4
5
6
7
8 9 10
In the last 10 series for each serum
In the last 5 series for both
Rule 10Χ
10 consecutive values at the same
side of the mean
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
+3s
+2s
+1s
mean
- 1s
- 2s
- 3s
1
2 3
4
5
6
7
8 9 10
2 3
4
5
6
7
8 9 10
+3s
+2s
1 2 3 4 5 6 7 8 9 10 +1s
mean
In the last 10 series for each serum
- 1s
- 2s
In the last 5 series for both
- 3s
Sensitive to systematic errors
1
Flowchart according to Westgard
values
control
12s
no
Situation under control - Series Accepted
no
yes
13s
yes
no
22s
yes
no
R4s
yes
no
41s
no
yes
Situation out of control - Series Rejected
10x
yes
Differences of Internal and External QC
Internal QC
• performed daily in the laboratory
• uses samples (control materials) of known concentration
• it is always required
External QC
• performed periodically (weekly, monthly …)
• uses samples (control materials) of unknown concentration
• useful in conjunction with the internal QC
External QC does not replace the Internal QC
• the inter-laboratory comparisons are infrequent
• the results are reported deferred (not in real time) and
therefore it is not possible the immediate intervention
with corrective measures
• even if the performance is satisfactory, it assures the
proper functioning of the laboratory only on the day of
the inspection (participation)
These programs do not lead to quality improvement of
the laboratory if it is not performed daily the internal QC.