On 2008/11/29, at 6:19 AM, Martin Vlietstra wrote:
As a diabetic I watch my sugar level. In the UK it is measured in
mol/L.
However, when I was working in Germany, it was measured in mg/dL. I
can see
two advantages of mg/dl:
1) There are never any decimal points - levels are typically 60 to
105.
2) Knowing that an adult male typically has 5 litres of blood in his
system,
it is easy to calculate how much sugar I shoudl have in my bloodstream
without having to know the molecular formula for sugar.
Dear Martin,
Let me consider your first point. Some of what you say here appears to
be simply an artefact of the conversion process. This is probably best
explained with an example.
Here is some data that I quote from the Mayo Clinic (at http://www.mayoclinic.com/health/blood-sugar/SA00102
) and I have interspersed some remarks.
Fasting blood sugar test
What is a fasting blood sugar (glucose) test?
Measures the amount of sugar (glucose) in your blood after you fast
for at least eight hours or overnight.
What's normal?
A normal range is 70 to 100 milligrams of glucose per deciliter of
blood (mg/dL) or 3.9 to 5.6 millimoles per liter (mmol/L).
Note the precision of the initial figures, 70 and 100, as these have
an implied accuracy to one significant figure.
What level suggests prediabetes?
A level of 100 to 125 mg/dL (5.6 to 6.9 mmol/L) indicates impaired
fasting glucose — commonly known as prediabetes. To confirm the
diagnosis, your doctor may repeat the fasting blood sugar test
several days to a week later.
Now the figures have been rounded to the nearest 25. Presumably, we
could consider these figures as +/- 12.5 and this would make the
second, higher, figure of 6.9 somewhere between 6.25 and 7.64.
Let's say 7 without a fractional component to match the significant
figures in the original data. The lower figure using rational and
reasonable rounding now becomes 6 without a decimal.
What level suggests type 1 or type 2 diabetes?
A level of 126 mg/dL (7 mmol/L) or higher is consistent with either
type 1 or type 2 diabetes. To confirm the diagnosis, your doctor may
repeat the fasting blood sugar test on another day. If your test
results are 126 mg/dL (7 mmol/L) or higher twice in a row after at
least eight hours of fasting, you may have diabetes. Further testing
may be needed to determine whether you have type 1 diabetes or type
2 diabetes.
Noter how the Mayo Clinic writer has now reversed the accuracy by
assuming that the value 7 mmol/L is accurate and precise and that the
former value of 125 mg/dL in the previous paragraph has now
mysteriously become 126 mg/dL without a word of explanation.
This mystery is simply the result of a poor understanding of accuracy
and precision that lead to bad rounding practices.
At another loaction (http://www.medicinenet.com/script/main/art.asp?articlekey=17384
) you will find this table where they didn't bother to specify any
units at all:
For most people, good blood sugar levels are:
On waking up (before breakfast) 80 to 120
Before meals 80 to 120
2 hours after meals 160 or less
At bedtime 100 to 140
I will assume that they refer to mg/dL and that 18 is a reasonable
conversion factor to go from mg/dL to mmol/L based on the molar mass
for glucose as 180.15588 g/mol (12.0107 g/mol)(6) + (1.00794 g/mol)
(12) + (15.9994 g/mol)(6) = 180.15588 g/mol. If I use the implied
accuracy of these figures based on the number of significant figures,
and I use appropriate rounding then these figures become:
For most people, good blood sugar levels are:
On waking up (before breakfast) 4 to 7 mmol/L
Before meals 4 to 7 mmol/L
2 hours after meals 10 mmol/L or less
At bedtime 6 to 8 mmol/L
These figures now look, to me, to be at least as easy as the German
figures that you quoted.
With respect to your second point, your figure of 5 kilograms of blood
in a normal male looks a little low to me. Working on a ratio that
blood composes about 1/11 (one eleventh) of a human body, then 5
kilograms of blood is only enough for a man who has a body mass of 55
kilograms.
Cheers,
Pat Naughtin
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
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