Dear Anders,

My position is simple. Quantitative and semi-quantitative measurements are
made based on a calibrator. A calibrator is an embodiment of a scale or unit
of measurement. The expression of uncertainty of measurement is a definition
of how stable the scale is i.e. the quality of a measurement result. In
other words, a unit of measurement is not static, but dynamic within a
specified range. This enables comparisons of results of measurements based
on the same scale of measurement or scale of measurement traceable to the
same reference standard. Any significant systematic error should be
corrected as it will invalidate uncertainty of measurement.

A simple analogy is as follows:

Express 40 litres of water in CUPS. A unit of measurement here is a cup. It
is important to define the size of the cup ( this is done through
establishing traceability of a calibrator in laboratory measurements). The
cup size may be 250 ml, 200 ml etc. I will use 250 ml for this discussion.
It is also important to note that in filling the 250 ml cup, it is
impossible to fill the cup to the 250 ml mark exactly. There will be
instances of over- and under-filling the cup. If we assume that the cup is
over- and under-filled by +/-5 ml, then the definition of a cup will be any
volume ranging from 245 - 255 ml. A unit of measurement (a cup in this
instance) will be any volume from 245 - 255 ml of water. It follows that 40
litres ( 40 000 ml) will be expressed as any number of cups randing from 157
- 163 cups.

Uncertainty of measurement is an important measure of quality of the unit of
measurement provided the unit of measurement is traceable to some reference
standard. Uncertainty of measurement is useless if there is no traceability
of the unit of measurement. 

I hope this does not add any confusion.

David      


****************************************************************************
**********************************************

Dr. PD Rambau

Chemical Pathologist

QA/Operational QC Division

Lancet Laboratories (SANAS Accredited)

Tel: +2711-3580800

Fax: +2711-3580967

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Web Address: http://www.lancet.co.za

P Please protect our environment,do you really have to print this e-mail?

 

 

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-----Original Message-----
From: Clinical biochemistry discussion list
[mailto:[log in to unmask]] On Behalf Of Anders Kallner
Sent: Sunday, January 24, 2010 11:11 AM
To: [log in to unmask]
Subject: Dr Peake's uncertatain doctor

Dear Dr Peake,
It is a novel idea to measure and calculate the MU for a doctor and 
measure results when you probably want to discuss the uncertainty of 
measured S-Sodium concentrations at two locations...

The concept of uncertainty differs from the error concept in many ways 
but I believe the explanation I gave in my first response is not too 
bad; the error concept deals with the tails of the distribution 
whereas the uncertainty deals with the core of the distribution. 
Another major difference is that the uncertainty theory suggests that 
any bias that is known, and is important for the use of the result, is 
corrected. All such operations are associated with an uncertainty 
which has to be added to the uncertainty established by repeated 
measurements or other procedures. The uncertainty thus becomes a 
homogenous concept and uncertainties of various moments of a 
measurement, or clinical investigation, can be added using the 
propagation rules.

The scenario you refer to is nothing special. Firstly you need to 
define if you want to leave it to the doctor to decide which 
instrument (s)he wants the sodium concentration measured by. It is 
reasonable that the MU of that instrument is quoted. If (s)he does not 
know - or care - or the measurement can randomly be made in any of 
those locations then another uncertainty must be quoted. This 
uncertainty will be composed of a within instrument and a between 
instrument variation which of course will be larger than that of a 
specific instrument. 

We have advised one method to assess that uncertainty in the Nordic-UK 
paper and software that are available on the ACB homepage. It is 
nothing absolutely new and nothing that is characteristic for the 
uncertainty concept, but it is a simplified way to analyze the 
variance components beginning from an ANOVA (Analysis of variance). 
Compared to the traditional arithmetical approach (e.g. in the CLSI 
EP15) it allows an interactive mode, an unbalanced design and handles 
the situation - rare but real - when the within variation is larger 
than the between series variation.

Just across the street from your place in Adelaide you will find Dr 
Graham White who has done more than anybody else to make sure all 
rules and procedures that are discussed in the inner circles in IUPAC-
IFCC, BIPM and CLSI will be applicable in the laboratories. Just 
across the plains and mountains is Professor Brynn Hibbert at the Dept 
of Analytical Chemistry, Univ of Sydney who plays a pivotal role in 
the introduction and understanding of the concept of uncertainty.

There are presently two important international documents, specially 
written for the medical/clinical laboratories, in the pipeline and 
while waiting for them to be published I refer you to the Austrian 
recent publication:
National Pathology Accreditation Advisory Council (Australia). 
Requirements for the Estimation of Measurement Uncertainty (2007) 
http://www.health.gov.au/internet/main/Publishing.nsf/Content/86A3CE312
C612377CA257283007BC92D/$File/dhaeou.pdf 
Sincerely,
Anders Kallner 
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This is an open discussion list for the academic and clinical community working in clinical biochemistry.
Please note, archived messages are public and can be viewed via the internet. Views expressed are those of the individual and they are responsible for all message content.
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http://www.laboratorymedicine.nhs.uk
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