The fist step to Measurement Uncertainty evaluation is the definition of the measurand. Simply this is just defining what we are actually measuring, and in doing so taking into account all the differences or specifics of the assay that distinguish it from similar assays we perform. For a full discussion of how and why we define the measurand each time see the measurand section of the main site and also the publication in Pathology in Practise

Things such a reagent composition are extremely important and can have significant impacts on the performance of the assay. A good example in haemostasis is the activated partial thromboplastin time (aPTT).

The aPTT is used as a global assessment of coagulation via the traditional intrinsic pathway of haemostasis. Activation of contact factors in vitro allows us to assess the contribution of factors traditionally represented in this pathway to the coagulation response. For an in depth discussion o the aPTT see practicalhaemostasis.com.

It is common to ask the same questions of every assay when we are assessing the measurand:

What is the purpose of the examination?

Is it an amount/concentration or activity you are determining?

What is the physiological system and source of sample you are testing?

and then there are specific questions that will be able to differentiate similarly defined assays by the questions above concerning:

Method (Manual/automated…)

Assay type

Reagent considerations

By answering all of the above questions an overall definition of the measurand you are measuring is produced.

so……using a simple aPTT as an example

What is the purpose of the examination?

The purpose is to determine the clot formation time as a consequence of intrinsic activation of coagulation

Is it an amount/concentration or activity you are determining?

We are measuring the activity within the plasma and its potential to facilitate clot formation

What is the physiological system and source of sample you are testing?

This is straight forward and will be similar for all haemostasis assays i.e. we are testing blood, specifically plasma

and then there are specific questions that will be able to differentiate similarly defined assays by the questions above concerning:

Method (Manual/automated…)

This should include the analyser used if applicable as different analysers performing the same assay will have different properties and performances associated with it. It’s also not unusual for there to be multiple different analysers used for the same assay in the same institution. Also by defining the platform of analyser used comparability (and transferability) of results is easier.

Reagent considerations

This is probably the most important means of differentiating between aPTT assays. Again, it’s not uncommon to have multiple assays defined in the same way based on the criteria above, but with very different performances due to the reagents used.

aPTT reagents can have many different means of initiating intrinsic activation. The activators can include (but are not limited to):

Silica

Elegiac acid

Kaolin

Celite

Each of the above activators will result in different sensitivities to mildly reduced Factors VIII, IX, XI, XII and in combination with the lipid composition may result in different results in patients with a lupus anticoagulant. For this reason it is important to know what is contained within our aPTT reagent and what impact the formulation will have on results in subgroups of patents we are testing

Other considerations:

Incubation time of the aPTT

Short incubation times can be used for the increased sensitivity to contact factor deficiencies whereas longer incubation times, as high as 10 minutes, can be used to remove the contact factor influence. although this is entirely impractical in a high throughput test such as the aPTT, an awareness of the incubation time in your assay and also the sensitivity that will impart on the detection of specific deficiencies is advisable.

 

so from here we can define our measurand as:

activated Partial Thromboplastin Time (aPTT) – Intrinsic coagulation activation using silica/elagic acid/etc (delete as appropriate) to determine the time to clot of plasma on the automated analyser (insert as appropriate) with an incubation time of (Insert incubation time)

And thats it! For the aPTT anyway

Conclusion and next steps

The discussion of what to do with this definition is perhaps a bit more difficult and will be covered in later posts. Having such a definition on every report would clearly be impractical and confusing for the requesting clinician. Currently all our definitions are simply held in a spreadsheet for reference should it be required.