A KHINE DIVISION OF CHEMICAL PATHOLOGY NHLS TYGERBERG STELLENBOSCH UNIVERSITY Laboratory Management workshop 36 June 2019 MEASUREMENT UNCERTAINTY is a parameter associated with the result of a measurement that defines the range of the values that could reasonably be attributed to the m ID: 928602
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Slide1
MEASUREMENT UNCERTAINTY
Dr A
A
KHINE
DIVISION OF CHEMICAL PATHOLOGY
NHLS TYGERBERG
STELLENBOSCH UNIVERSITY
Laboratory Management workshop 3-6 June 2019
Slide2MEASUREMENT UNCERTAINTY
is a parameter, associated with the result of a measurement… that defines the range of the values that could reasonably be attributed to the measured quantity (UKAS)
Non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used.
Slide3MU is requirement in ISO standards and accreditation
ISO 15189:2012:
“The laboratory shall determine measurement uncertainty for each measurement procedure in the examination phases used to report measured quantity values on patients’ samples. The laboratory shall define the performance requirements for the measurement uncertainty of each measurement procedure and regularly review estimates of measurement uncertainty.”
Slide4WHAT COULD CONTRIBUTE TO MU?
Slide5Slide6How to estimate MU- TQM approach- not practical
Slide7Time Scale
Steps of the evaluation of the measurement uncertainty
7
BMICH
Slide8Approaches in estimating MU
12-Nov-2015
BMICH
8
Definition of the Measurand
Single laboratory
Inter laboratory
Component by Component Evaluation
ISO GUM
Single lab
Validation
Within-lab reproducility & Bias
NordTest
Interlaboratoy Validation
Reproducibility and Bias
ISO 5725
ISO TS 21748
Proficiency Testing (PT)
Between Lab Variability ISO Guide 43
ISO 13528
Slide9ISO 15189 gives some guidance
The relevant uncertainty components are those associated with the actual measurement process, commencing with the presentation of the sample to the measurement procedure and ending with the output of the measured value.
Measurement uncertainties may be calculated using quantity values obtained by the measurement of quality control materials under
intermediate precision conditions that include as many routine changes as reasonably possible in the standard operation of a measurement procedure, e.g., changes of reagent and calibrator batches, different operators, scheduled instrument maintenance
12-Nov-2015
BMICH
9
Slide10Based on assumption approach- uses imprecision from long-term IQC
See NHLS QA UOM sheet
Hierarchical judgement of performance- desirable, minimum and maximum analytical goals- from
CVi
or
CVw
from BV database (Westgard), then CEC endorsed CV limits
When the analyte is not in the BV list, search from other sources such as RCPA,
drgrhodes
Optimum: CVA = < 0.25
x
CVI
Desirable: CVA = < 0.50
x
CVI
Minimum: CVA
= < 0.75
x
CVI
Optimum: BA = < 0.125 (CVI
2
+ VG
2
)1/2
Desirable: BA = < 0.250 (CVI
2
+ CVG
2
)1/2
Minimum: BA
= < 0.375 (CVI
2
+ CVG
2
)1/2
Slide11Why do we estimate MU?
Reference change values (RCVs)
Determines whether the difference between two results is negligible due to uncertainty or significant due to a genuine change in the condition of the patient.
RCV = 2.77 x √(CV
A2
+ CV
I2
)
Compare
the MU and RCV to make medical decisions (but your performance must be acceptable first before you use
the MU in this way)
Look at Performance Index (
CVa
/
CVi
or w)
if <0.5 – performance is acceptable
If < 0.75
– still okay
if > 0,75 – not acceptable (in this case do not report the MU,
in fact so not release results. Stop testing and troubleshoot – check if you have trim out the IQC data when you print the 6 monthly data
Slide12Should we report it on the patient’s result?
Different opinion
What does NHLS say?
What does ISO say?What are peers doing?When should we report MU and when we should not?