LABORATORY QUALITY
CONTROL
Course Code RIT 2.2 Revision C
Definitions:
• Quality Control:– the process of detecting errors
• Quality Assurance:– the systems or procedures in place to
avoid errors occurring
… to ensure the reliability of the test
results to give the best patient care !
Unreliable Performance ?
• Potential consequences include:– patient misdiagnosis
– delays in treatment
– increased costs
• avoidable retests cost US 200million USD per year
• Even a small calibration bias can effect
treatment rates:
– 1% +ve bias in cholesterol result
 5% increase in patients exceeding the treatment cut-off
– 3% +ve bias
 15% increase in patient treatment.
Error Classification..
• Pre-analytical:– errors before the sample reaches the laboratory
• Analytical:– errors during the analysis of the sample
• Post-analytical:– errors occurring after the analysis
Pre - Analytical Errors..
• Improper preparation of the patient:– patient fasting
• glucose test
– stress and anxiety
• urinary protein
Pre - Analytical Errors..
• Improper preparation of the patient
• Improper collection of the blood sample:– sample haemolysis
• LDH, potassium or inorganic phosphate
– insufficient sample volume
• unable to carry out all requested tests
– collection timing
• 24 hour urine
Pre - Analytical Errors..
• Improper preparation of the patient
• Improper collection of the blood sample
• Incorrect specimen container:– serum or plasma
– fluoride tubes for glucose
• to inhibit glycolysis
– EDTA unsuitable anti-coagulant for calcium
Pre - Analytical Errors..
•
•
•
•
Improper preparation of the patient
Improper collection of the blood sample
Incorrect specimen container
Incorrect specimen storage:– sample left overnight at room temperature
• falsely elevated K, Pi and red cell enzymes
– delay in sample delivery
• falsely lowered levels of unstable analytes
Other Factors..
• The sex of the patient
– male or female
• The age of the patient
– new born / juvenile / adult / geriatric
• Dietary effects
– low carbohydrate / fat
– high protein / fat
• When the sample was taken
– early morning urine collection pregnancy testing
• Patient posture
– urinary protein in bed-ridden patients
Other Factors..
• Effects of exercise
– creatine kinase / CRP
• Medical history
– heart disease / diabetes / existing medication
• Pregnancy
– hormonal effects
• Effects of drugs and alcohol
– liver enzymes / dehydration
Analytical Errors..
• The sample:
– labelling
• barcoding / aliquoting
– preparation
• centrifugation / aspiration
– storage temperature
• short –term refrigeration
• medium term freezing at –20oC
• long term freezing at -80oC
– correct test selection
• Laboratory Information Management System (LIMS)
Analytical Errors..
• The sample:
• Glassware / pipettes / balances:
–
–
–
–
used incorrectly
contaminated
poorly calibrated
reuse of pipette tips
Analytical Errors..
• The sample:
• Glassware / pipettes / balances:
• Reagents / calibrators / controls:
– poor quality
– inappropriate storage
• correct temperature
• badly maintained fridges or freezers
– stability
• shelf-life / working reagent
– incorrect preparation
Analytical Errors..
•
•
•
•
The sample:
Glassware / pipettes / balances:
Reagents / calibrators / controls:
The application:
– incorrect analytical procedures
– poorly optimised instrument settings
Analytical Errors..
•
•
•
•
•
The sample:
Glassware / pipettes / balances:
Reagents / calibrators / controls:
The application:
The instrument:
– operational limitations
• temperature control/read times/mixing/carry-over
– lack of maintenance
• worn tubing / optics / cuvettes / probes
Other Factors..
• Calculation errors:
– incorrect factor / wrong calibration values
• Transcription errors:
• Dilutions errors:
– incorrect dilution or dilution factor used
• Lack of training:
• The human factor:
– tiredness / carelessness / stress
Post - Analytical Errors..
• The prompt and correct delivery of the
correct report on the correct patient to
the correct Doctor.
• How the Clinician interprets the data to
the full benefit of the patient.
Accuracy ?
How correct
your result is.
Precision ?
The
reproducibility
of your results.
Accurate and Precise..
Imprecise but Accurate !
Precise but Inaccurate !
Specificity ?
• The ability of a method to measure solely
the component of interest.
• A lack of specificity will affect accuracy
– falsely elevated values
• hormones and drugs
– falsely low values
• BCP method with bovine albumin
Sensitivity ?
• The ability to detect small quantities
of a measured component.
– will affect both precision and accuracy at the
bottom end of the assay range.
Normal Distribution..
Frequency
Mean value (x)
Measured value
Values fall randomly about a mean value.
Precision ?
• How disperse the values are.
• Quantified by measuring the Standard
Deviation (SD) of the set of results.
Standard Deviation (SD)..
SD =
(
 (xi - x)
2
)
n -1
The lower the SD the better the
Precision.
Example:
Mean result (x) = 100 mmol/L
Standard deviation (SD) = 1.0 mmol/L
Number of results (n) = 100
Mean +/- 1SD..
Frequency
-1SD
x
+1SD
68%
99 100 101
Values fall randomly about a mean value.
Mean +/- 2SD..
Frequency
-2SD
x
+2SD
95%
98
100
102
Values fall randomly about a mean value.
Which is more Precise ?
Potassium SD = 0.1 mmol/L
Sodium
SD = 2.0 mmol/L
Coefficient of Variation..
CV =
SD
x 100%
Mean (x)
A %CV takes into consideration the
magnitude of the overall result.
Example:
Potassium %CV = (0.1 / 5.0) x 100%
= 2.0%
Sodium %CV = (2.0 / 140) x 100%
= 1.4%
Sodium has the better CV and in this
example is performing better than
potassium.
Interpretation..
10  40
41  50
51  70
71  100
101  120
unacceptable performance
need for improvement
acceptable
good
excellent
TS Calculations
V = (Result - Mean for Comparison) x 100
Mean for Comparison
The mean for comparison could be either:
– the all method mean
– your method mean
– your instrument mean
TS Calculations
TS = Log10 (3.16 x TCV)
x 100
V
TCV is Target Coefficient of Variation
TS Calculations
TS = Log10 (3.16 x TCV)
x 100
V
3.16 is selected as a constant because:
– the log10 of 3.16 is 0.5
– so if V = TCV, then the target score will be 50
TS =
log10 3.16 x TCV x 100
V
=
log10 3.16 x 3.7
3.7
=
log10 (3.16) x 100
=
50
x 100
How can Analytical
Quality be
Controlled ?
• Internal Quality Control (IQC).
– daily monitoring of quality control sera
• External Quality Assessment (EQA).
– comparing of performance to other laboratories.
Internal Quality Control..
• Daily monitoring
– precision
– accuracy
• Quality control sera
– results within control limits indicates
that analytical system is running
satisfactorily
What is Acceptable ?
A sodium control has a target value
of 140 mmol/L
139 mmol/L
120 mmol/L
140 mmol/L
160 mmol/L
141 mmol/L
180 mmol/L
What is Acceptable ?
• A range of acceptable values is established
• Sodium Control:- 137  143mmol/L.
What are the Options ?
• Unassayed serum:
– the cheaper option !
• but the laboratory must establish its own ranges
– cannot be used to assess accuracy !
• no externally assigned target values
• Assayed serum:
– with predetermined targets and ranges
• established by the manufacturer.
Unassayed Serum..
• Analysed extensively by the laboratory.
– a minimum of 20 sets of data generated
– a mean +/- 2SD range established
• 95% of results acceptable
– some laboratories may adopt tighter ranges
Assayed Serum..
• Targets and ranges generated by the
manufacturer:
– abc utilises RIQAS
• database of 5,000 laboratories
• method / instrument / temperature specific values
Levey Jennings Chart
+2SD
143
+1SD
141.5
X
X
X
X
Mean
X
X
X
X
140
X
X
X
X
X
X
-1SD
X
-2SD
X
X
138.5
137
Levey Jennings Chart
+2SD
143
+1SD
141.5
X
Mean
X
X
X
X
X
X
X
-1SD
-2SD
X
X
X
X
X
X
140
X
X
X
138.5
137
Levey Jennings Chart
+2SD
143
+1SD
X
X
X
X
Mean
X
X
X
X
-1SD
-2SD
X
X
X
X
X
141.5
X
X
140
X
X
138.5
137
Levey Jennings Chart
+2SD
143
+1SD
X
X
X
X
X
Mean
X
X
X
X
X
X
X
X
X
X
141.5
X
X
140
-1SD
138.5
-2SD
137
Levey Jennings Chart
+2SD
143
X
X
X
X
+1SD
141.5
X
X
X
Mean
140
X
X
-1SD
-2SD
X
X
138.5
X
X
X
X
X
X
137
Levey Jennings Chart
+2SD
143
X
X
X
+1SD
141.5
X
X
X
Mean
X
X
X
X
X
X
140
X
-1SD
X
-2SD
X
X
X
138.5
137
Levey Jennings Chart
+2SD
143
+1SD
X
X
X
X
141.5
X
X
X
X
X
Mean
140
X
X
-1SD
-2SD
X
X
X
X
X
X
138.5
137
Westgard Rules..
• Decision criteria is dependent on the
precision of the method or analyser
– the less precise the method the more
difficult the decision.
• Westgard provides multiple QC rules:– defines acceptability
• minimises false rejections
• maintains high error detection
Westgard Flowchart..
Control data
No
1 point
In control – report data
outside 2 SD
Yes
1 point
outside 3 SD
No
No
2 consecutive
No
Difference between
4 consecutive control
No
values outside
2 controls within
the same 2 SD
a run
of the mean and
exceeds 4 SD
further than
values on one side
1 SD from the mean
Yes
10 consecutive
No
values
on one side of
the mean
Yes
Yes
Yes
Out of control – reject analytical run
Yes
External Quality
Assessment..
.. the main objective of EQA is not to
bring about day to day consistency
but to establish inter-laboratory
comparability
EQA Options..
• International / National / Regional
• International schemes provide:– a larger database of results
– a wider range of analytical methods
– a global representation of diagnostic
manufacturers
• Compulsory or Voluntary
A Typical EQA Scheme..
• Participants receive unknown samples.
– these are analysed ‘blind’
– the results returned to scheme
organiser
– they are statistically analysed
– to generate a comparative report
– report sent to participant
RIQAS
•
abc
International Quality Assessment Scheme
– launched in 1988
– 5000 participants
• Management tool
– to assess, review and improve performance
RIQAS..
• Annual subscription
– two six monthly cycles
• Weekly samples
– one vial reconstituted per week
– tested blind as if a patient sample
• Results reported back to abc
– statistically analysed
• Weekly Report generated