 # Medical Values Conversion Problem

I need to convert medical test results from µg/mL to µmol/L. My brother and I both had the same blood test done 35 years apart and his results are in µg/mL while mine are in µmol/L. I have no idea why the units changed over the years, and there’s no way to directly compare the results between us.

The blood test measures Hexacosanoic Acid C26:0, Docosanoic Acid C22:0, and Tetracosanoic Acid C24:0. I searched the Internet but haven’t found a conversion factor that is based on these very-long-chain fatty acids. Is there an easy way to figure this out? This is not a homework assignment, I swear.

Hexacosanoic acid Molecular weight : 397 g/mol
Docosanoic acid molecular weight : 341 g/mol
tetracosanoic Acid molecular weight : 369 g/mol

It would be good to know the concentration of each. In absence of such data, let’s assume that they are all present in equal proportion, that gives an average molecular weight of about 366 g/mol

So 1 µg/mL = (1000/366) µmol/L

Or So 1 µg/mL = 2.7 µmol/L

A question stated as an assertion from a non-chemist:

ISTM that: If you have three separate tests giving three separate values for the three separate acids, then using their individual molecular weights to make 3 separate conversions would be correct.

OTOH, ISTM that: If you have one test giving a total value of all three acids together, then the 366 average is the way to go in the absence of any further data.

Which situation is the OP in? And is my logic valid chemistry?

There are 3 separate tests giving 3 separate value. What I really need to know is the amount of C26:0 in his blood, and the ratio of C24/C22 and the ratio of C26/C22 if that helps (I probably should have mentioned that earlier).

In that case I (again not a chemist or medical person) would follow @am77494’s math, but substituting the individual numbers (397, 341, 369) into his formula where he used the 366 average.

These ratios can simply be computed from his numbers and separately from your numbers without any unit conversions. Made up example:

His C24 is 12µg/mL and his C22 is 24µg/mL. The ratio is 12/24 = 1/2 = 0.5.
Your C24 is 36µmol/L and your C22 is 72µmol/L. The ratio is 36/72 = 1/2 = 0.5.

You have the same ratios even though the rulers used to measure the values are different.

Agree with LSLGuy on the mol to weight conversion.

I am not sure about the ratio conversion though. Again I am not a chemist or a medical professional but a chemical engineer.

A lot of times, Ratios are done on mol basis and not weight basis because it gives you an idea of how many numbers of species A are there compared to species B.

Excellent point. That’s what I get for commenting past the end of my expertise. Consider my ratio comment withdrawn.

Sounds like maybe the OP needs to have a lot more knowledge to understand how to even interpret the numbers he seeks to determine.

Okay, that all makes sense. Let me run some numbers and tell me if I have it right. Let’s say my C26:0 number is 1.396 µg/mL and the molecular weight of Hexacosanoic acid (C26:0) is 397g/mol. If I plug in that number I get…

1 µg/mL = (1000/397) µmol/L, or 1 µg/mL = 2.52 µmol/L

So if my value is 1.396 µg/mL I simply multiply that by 2.52 to get 3.51 µmol/L

Is that correct?

Yes it is

Thanks, everyone for your help. I was never any good at math or biochemistry…

That converter is for Acetaminophen (e.g. Tylenol) only. The molecular weight of Acetaminophen js 151.163 g/mol while the molecular weight of Hexacosanoic Acid (C26:0) is 396.69 g/mol. It makes a difference.

Damn, you’re right. On my phone I missed that.

Here is a problem:

I have a vaccine formulation consisting of freeze-dried samples of vira A, B, and C. A and B are attenuated strains, and the amounts given are (different!!) multiples of the respective 50% cell-culture infective dose in each case. However, virus C has been inactivated and the datasheet says I have 2.0 ELISA U. How much is 1 ELISA unit, and how do they know what is the appropriate dose for vaccination?