internal quality control (qc) for medical laboratories: an introduction dr. otto panagiotakis and...
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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)
Correct result: There is not
a value, but a range from
repeated measurements
We need a tool to compare
the reported with the
expected result.
What means a value of total cholesterol
245mg/dL reported for the analysis of an QC
control?
235 245 255240 250 260
265
It is the control chart Levey-Jennings
Central horizontal line: expected mean value
Dotted horizontal lines: control limits (mean
± nSD)
Control charts Levey-Jennings:
+2s
mean
+1s
-1s
-2s
+3s
-3s
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
Control charts Levey-Jennings:
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 2015216
212
208
200
192
188
1841 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
x-3s
x+3s
x-2s
x+2s
mean
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
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
Interpretation of control charts (2)
•
Situation under control (normal)
Systematic error (bias)
Distribution enlargement
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
The problem of erroneous rejects
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.
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
Rule 13s: low Ρο and low Ρ
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)
216
212
208
200
192
188
1841 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
x-3s
x+3s
x-2s
x+2s
x+1s
x-1s
204
196
Cholesterol (mg/dL), Lot No: xxx, January 2015
The 6 control rules according to Westgard
12S
13S
22S
R4S
41S
10x
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
Not a rejection but warning for a potential
problem Further control is required based on the
other criteria
Rule 12S
One value (measurement) > 2s limit
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
Applied in a series for each of the two sera
Sensitive to random errors
Rule 13S
One value (measurement) > 3s limit
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
Rule 22S2 consequent values > the same limit of 2s
In the last 2 series for each
serum
In the same series for both
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
In the last 2 series for each
serum
In the same series for both
Sensitive to systematic errors
Rule 22S2 consequent values > the same limit of 2s
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
In the last 2 series for each
serum
In the same series for both
Rule R4S
The difference in value of the two sera> 4s
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
In the last 2 series for each
serum
In the same series for both
Sensitive to systematic error
Rule R4S
The difference in value of the two sera> 4s
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
In the last 4 series for each
serum
In the last 2 series for both
Rule 41S
4 consecutive values > the same limit of 1s
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
1 2 3 4 5 6 7 8 9 10
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
Sensitive to systematic errors
Rule 41S
4 consecutive values > the same limit of 1s
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
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
1 2 3 4 5 6 7 8 9 10
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
+3s +2s +1smean - 1s - 2s - 3s
1 2 3 4 5 6 7 8 9 10
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
Sensitive to systematic errors
Rule 10Χ
10 consecutive values at the same side of the mean
values
12s
Situation out of control - Series Rejected
Situation under control - Series Accepted
Flowchart according to Westgard
13s 22s R4s 41s 10x
yes
yes yesyesyesyes
no no
no
no
nono
control
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.