Sunday, 6 December 2009

Whole Blood or Plasma?

Mary, Mary quite contrary
How does your meter read?
With millimoles ,
Or milligrams
And clinical accuracy?

Mary Mary quite contrary
What does you meter mean?
Glucose in blood whole?
Or from the plasma sole
It's average your A1c?
With Apologies to Northerner: imitation is the sincerest form of flattery! (I don’t think it scans as well as your poems)

Any PWD on the internet soon realises that different parts of the world use different measurements for blood glucose readings. In the US, some parts of Europe and the Middle East they use milligrams per decilitre. In the UK , some parts of Europe and many places that are English speaking (Canada, Australia etc) they use millilmole per litre. To be awkward in France they use grams per litre.
It’s easy to change between the two, there are many convertors on the web but all you have to do to convert mmol to mg/dl is to multiply by 18, if you want to convert mg/dl to mmol/l you divide by 18. After a while you become bilingual.

But there is another difference that isn’t so obvious. When we measure our glucose we use whole blood from a capilliary. When a laboratory measures blood they measure the levels in the plasma.
For many years all the blood glucose meters reported the glucose level as in whole blood but this was not the same as a laboratory measurement would be. A laboratory plasma measurement would be about 12% higher than a whole blood one. In recent years some manufacturers have included a calculation (done automatically) that works out what the plasma reading would be and displays that as its reading.

Imagine you have 2 meters, both perfectly accurate (of that later) one whole blood, one plasma calibrated. You do a test with same spot of blood.
If it were plasma calibrated and it read 72mg/dl, the whole blood meter would read 64.3mg/dl.
So what constitutes a hypo depends upon what meter you are using

When people in different countries are writing about their levels, they may not be using meters calibrated in the same way.It is important to know what type of meter you have because some blood glucose targets are written to reflect whole blood readings whilst others are written for plasma, if you are reading targets written for whole blood, they may be too low for people using plasma calibrated meters.

In the US all modern meters give plasma readings but in Europe some give plasma and some whole blood, and it’s not always easy to find out which does what. If you want to find out you may have to search.The place I found mine was not with the meter instructions but in very tiny print in the leaflet that comes with the testing strips.

In the UK, the manufacturers have been changing over the years and now according to DUK they all use plasma except for for those made by of of the largest manufacturers Roche ie Accu chek meters. DUK says that ‘ Roche is in the process of adjusting their meters to give results as plasma values, which they hope will be completed before the end of 2009.’ So if you have an Accu chek in the UK read the strips leaflet carefully.

I think that France may well be in the exactly same postion as the UK but I haven’t been able to find out about all makers. My old Meter, an Accu chek go used whole blood measurements, this is no longer made and I don’t know what the newer ones use. (in France they market the Nano and the Nano-performa). The newer Lifescan meters are plasma calibrated (one touch ultra meters). I couldn’t find a definitive answer for the Abbott meters (Optimum plus and Freestyle Papillon). In the UK all Abbott meters are plasma calibrated and I found this statement on the Swiss site ‘ont déjà programmé leurs systèmes de mesure de glycémie pour indiquer des valeurs plasmatiques comme celles relevées sur les lecteurs de laboratoire.’

This chart shows equivalents in mg/dl and mmol for both whole blood and plasma calibrated meters.

Of course even when you know what the meter reads, it’s not necessarily very accurate. They are allowed to be up to 20% out,. There is a convertor on the Lifescan website which demonstrates this clearly.So with that reading (4mmol) you could either be quite hypo and need some glucose quickly , just at a ‘safe’ level or have a very normal blood glucose reading!
And if you find that your HbA1c doesn’t really reflect what your meter has been telling you, perhaps this built in inaccuracy is a possible reason.

This post isn't really applicable to the UK anymore as all new meters are plasma calibrated