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Cholesterol/LP-LPA2 Unit of Measurement Help?

I’ve recently ran some advanced lipids, one I’m interested in particular is my Lp-PLA2 but my result is in the following measurement U/L.

Everything online in terms of research or studies it is measured in the two following measurements -

“There are two types of Lp-PLA2 tests:

The Lp-PLA2 test measures the amount of the enzyme in the blood, reported in units of nanograms per mL (ng/mL).

The Lp-PLA2 activity test measures the activity of the enzyme, reported in units of nmol/min/mL. While these units look complex, they simply describe the amount of substance that Lp-PLA2 converts each minute.”

How do I make sense of my level please?

PL PLA2 - 520 u/L

Range -

< 560 u/L - Low Risk

560 - 619 - Intermediate Risk

620 - 634 - Borderline Risk

> or equal to 635 u/L - High Risk

Definitions of Enzyme Units

Enzymology would be less complicated if everyone used the same unit definition. A standard unit definition is given below:

1 unit (U) is the amount of enzyme that catalyses the reaction of 1 umol of substrate per minute (definition A).

In most R&D settings, 1 umol of substrate is actually quite a lot of material and other definitions may be preferred to avoid expressing quantities in fractions of units. The following non-standard definition is commonly used:

1 unit (U) is the amount of enzyme that catalyses the reaction of 1 nmol of substrate per minute (definition B).

Note that the change in definition has a profound effect on the stated number of units i.e. 1 unit of enzyme according to definition A would equate to 1000 units according to definition B!

You may also see enzyme units expressed as milli-unit (or mU) which simply means a thousandth of a unit, irrespective of how the unit has been defined.

Clearly the actual amount of an enzyme in a tube is not altered simply by changing the unit definition, but care is required when comparing the activities of samples from different suppliers. As long as the unit definitions are provided you can transform the stated number of units into nmol per min, which is unambiguous and allows valid comparisons to be made.

For clarity in your own work you may prefer to use ‘nmol per min’ (or ‘umol per min’), though if constant repetition is required then clearly the much shorter term ‘unit’ has its attractions.

For example, similar to the activity test you had performed (U equivalent to nmol/min):

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Thank you for the info!

So would that mean that U/L is equivalent to nmol/min/ml?

Or is that different as the example paper is in U/ml and ML is smaller than a Litre?

Sorry for my very basic question

U/L would be equivalent to nmol/min/L if they are using the nonstandard definition of 1 U = 1 nmol/min.

1000 mL is 1 L. So the activity measured with this commercial assay you used seems about ~1000x of the typical units I’ve seen and shared with you here on the order of 0-300 U/mL if non-standard definition is being used.

If standard definition is being used, then 1 U = 1 umol (micromol) / min.

It that case 1 U / L = 1 nmol / min / mL :-). It’s all just a fun units conversion issue so that’s why the definition of U should be provided on your test result to avoid confusion. :slight_smile:

It appears you are in good company. Congrats!

Do you know the lab or test kit used for your assay?

Thank you very much for your help, it’s greatly appreciated.

Yes I used Medichecks.com.

I find it strange they have used a different measurement in comparison to the other tests.

This is the actual result I got - Lab report

I know on their viewer it is in their low risk range but to me it seems a bit too high on their scale, which makes me think I have excess plaque activity, this is why I wanted to compare it to other units of measurement to gauge an accurate view.


Looks like I found the test or similar to what you had done.

Method comparison was performed against a competitor activity test. Passing/Bablok regression analysis between methods resulted in widely divergent results which may be explained by different standardizations. On average, competitor results are decreased by the factor 2.8. Using the obtained factor, values can be easily converted.

Yes, they are using the standard definition of U = umol / min. Different tests can have different response based on the enzyme kit (see parity plot above). So no worries, you would fall most likely in ~ < 200 nmol/min/mL range on the other assay I shared from Labcorp. That’s why it’s so important to share ref ranges along with the absolute score for your particular test. Well done!

Thank you very much for taking the time to find this for me, it’s put my mind at ease and simplified the data.

Thank you!!

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