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Converting grams per centimeter squared to mg/cc

Posted on 2010-01-05
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I received a bone density mineralization number and am trying to make sense of the numbers.  The first test showed bone mineral density of 56.1 mg/cc.  The second test showed 1.151 g/cm squared.  How do I equate the two?
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Question by:LEECHIPTURNER

LVL 16

Expert Comment

basically, if there are 56.1mg/cc (milligrams per cubic centimetre - or, 56.1mg/cm^3) that would equate to 5.61 mg/cm squared.

It's just a simple division of 10 (as there are 10 layers of suared centimtres per cubic centimtre). But then you probably knew that any way!

also, 1.151g/cm squared is just what it says - 1.151 grams per centimetre squared (or 1.151g/cm^2). Converting this to cubic centimtres is just multiplying it by 10 ie, 11.51g/cc (or 11.51g/cm^3).

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LVL 15

Expert Comment

No, you cannot compare those values. g and mg are OK, 1g=1000mg. But cubic cm represent volume and square cm represent surface. These are just different entities. You cannot assume one layer is 1 cm thick. What are your tests measure? May be the first one measures mineralization in bone's volumes, and the second test - just on surface?
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LVL 16

Expert Comment

yuk99

my conversions from cc to cm^2 is a generalised explanation.
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LVL 27

Expert Comment

"my conversions from cc to cm^2 is a generalized explanation."
and that cannot in general be done with any reasonable meaning.
as was pointed out cc is volume and cm^2 is surface.
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LVL 16

Expert Comment

aburr....

Yeah.... and cm^2 x cm is cm^3

In other words... an AREA converted to a VOLUME

So, what's the problem?
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LVL 15

Expert Comment

You cannot convert area into volume. Period!
I'd wait what the author says.
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LVL 16

Expert Comment

Of course you can!!

I don't know how you do YOUR geometry but back in my junior school days I learned L x W is Area whereas, L x W x H is Volume.

It's not rocket-science stuff....

BTW, a box whose base has an area of 1m^2 and a height of 1m, has a volume of 1m^3

There you go: AREA x height = VOLUME

Simple!!!...

Why is everyone picking on me?
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LVL 15

Expert Comment

You are calculating volume using area and length. This is not unit conversion. Conversion is when you change units that measure the same thing. Centimeters and inches, liters and cc, etc.
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LVL 26

Expert Comment

> There you go: AREA x height = VOLUME

Although that's correct, it has nothing to do with the question. E.g., there is no value that can be assigned to <height> in the measurement "1.151 g/cm squared".

Tom
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LVL 58

Expert Comment

In physics, a weight per area is a pressure, not a density. However, medical technology is known to invent its own pseudo-scientific measures. In this case, either the height is implied by the method (a standard bone slice) or the weight is a handy representation of something else (e.g. absorption) using some sort of conversion.

I'm guessing that one measure is physical (a true density), while the other is a medical test result showing "some measure of mineral present" over "some unit of surface" (e.g. on an X-ray), and that the two are correlated (the second is used to estimate the first in a non-invasive manner).

(°v°)
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LVL 58

Expert Comment

My guess was correct and is explained better in http://en.wikipedia.org/wiki/Dual_energy_X-ray_absorptiometry

(°v°)
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Author Comment

A very interesting debate...    As to the question being in two areas, I'm a little confused.  I tried submitting in one area and it rejected it and said I would have better luck using two, so I picked another, and it accepted the submission.

Using Harfang's link, I find that BMD is reported in g/cm3.  However, the test result I have says g/cm2.  So I am assuming a typo on their part.    Assuming that we are comparing 56.1 mg/cc to 1.151 g/cm3, how do we make that comparison (conversion)?
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LVL 15

Expert Comment

Considering 1g = 1000 mg, and cc is the same as cm3, you first value will be 0.0561g/cm3.
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LVL 62

Assisted Solution

You can kind of compare - but not convert  - see the page linked to in your other post http://www.experts-exchange.com/Other/Math_Science/Q_25021093.html
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LVL 27

Expert Comment

"So I am assuming a typo on their part."
Assuming a typo in a medical report is very bad practice. I can think of no typo which will make the conversion possible. Here you can get all kinds of quesses. The only definitive answer must come from the people who wrote the report.
--
There is NO way you can convert 56.1 mg/cc to 1.151 g/cm3 by unit conversion alone
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LVL 15

Expert Comment

I believe the author meant convert those values to the same units to compare.
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LVL 58

Accepted Solution

> Using Harfang's link, I find that BMD is reported in g/cm3.

I do not read the page like that at all. On the contrary, it explains at length that BMD uses area and not volume, and that this makes it difficult to correlate it to true densities.

Apparently, there is no conversion table between the numbers, unless the first number is a BMAD, attempting to estimate true density.

(°v°)
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Author Closing Comment

I asked a doctor to explain it, and he could not.  I need to go to a lab and see if they can explain how these two numbers relate.  Thanks for the info!!
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