Lp-PLA2 Activity Unit Converter
Lp-PLA2 Activity Unit Converter
Convert lipoprotein-associated phospholipase A2 activity between nmol/min/mL, U/L, nkat/L and µkat/L — and see why there is no conversion to the ng/mL of the old mass assay, which is no longer on the market.
Lp-PLA2 activity converter
Activity, nmol/min/mL ⇄ U/LLp-PLA2 activity 225 nmol/min/mL — exactly Labcorp’s threshold
The conversion, from the definition of the enzyme unit
1 nmol/min/mL = 1 nmol/min per 0.001 L = 1,000 nmol/min/L = 1 U/L
nkat/L = nmol/min/mL × 16.6667
µkat/L = nmol/min/mL × 0.0166667
- nmol/min/mL = U/L
- exactly, and this is the only real content of the conversion. The unit looks unfamiliar because the per-millilitre denominator is unusual, but cancelling it against the micromole in the enzyme unit leaves a factor of one
- 1 kat = 1 mol/s
- the SI unit of catalytic activity. One U/L is one micromole per minute per litre, which is 1/60 of a micromole per second per litre, hence 0.0166667 µkat/L and 16.6667 nkat/L
- no ng/mL on this ladder
- activity and mass are different quantities. An enzyme’s activity per molecule is not fixed — it depends on which lipoprotein carries it and on conditions — so there is no factor between nmol/min/mL and the ng/mL of the old mass assay. The two correlate loosely and cannot be interconverted
- no molar mass needed
- this record declares no molecular weight, because a catalytic rate is not an amount of substance. The factors above come from the definitions of the unit and the katal and from nothing else
Worked example
Lp-PLA2 activity 225 nmol/min/mL — exactly Labcorp's threshold
225 nmol/min/mL = 225 U/L — the same quantity, factor exactly 1
= 3,750 nkat/L = 3.75 µkat/L
At Labcorp's threshold, which it reports as the start of increased risk; the FDA clearance for the PLAC activity assay uses the same 225 nmol/min/mL
There is no ng/mL equivalent to give. The mass assay reported a concentration, not a rate, and no factor relates them
The same activity in every unit in use
| nmol/min/mL | U/L | nkat/L | µkat/L |
|---|---|---|---|
| 100 | 100 | 1,667 | 1.67 |
| 150 | 150 | 2,500 | 2.50 |
| 200 | 200 | 3,333 | 3.33 |
| 225 | 225 | 3,750 | 3.75 |
| 300 | 300 | 5,000 | 5.00 |
| 400 | 400 | 6,667 | 6.67 |
Activity assay against the withdrawn mass assay
| PLAC activity | PLAC mass (ELISA) | |
|---|---|---|
| Reports | nmol/min/mL (= U/L) | ng/mL |
| Measures | catalytic rate | enzyme concentration |
| Threshold used | 225 nmol/min/mL | no longer applicable |
| Availability | FDA-cleared, in use | not marketed; diaDexus ceased trading in 2016 |
| Pre-analytical behaviour | stable | up to 50% higher after days at 4 °C or −20 °C |
An activity, not a concentration
Lipoprotein-associated phospholipase A2 is an enzyme carried mostly on LDL that hydrolyses oxidised phospholipids in the arterial wall, generating lysophosphatidylcholine and oxidised free fatty acids. It is expressed by macrophages in the atherosclerotic plaque, and the interest in measuring it has always been that it might report on vascular inflammation specifically, rather than on systemic inflammation the way CRP does.
What is measured now is activity: how fast the enzyme in a sample turns over a substrate, reported as nanomoles of substrate consumed per minute per millilitre of plasma. That is a rate, and it is the reason this converter has no molecular weight and no mass units. An amount of enzyme and a rate of catalysis are different quantities, related by a turnover number that is not constant, so there is no factor that converts a nmol/min/mL into the ng/mL that the older mass immunoassay reported. Readers who find a ng/mL threshold in a paper from the 2000s cannot apply it to a current result.
The mass assay is also simply gone. It was withdrawn after its manufacturer ceased trading in 2016, and it had a pre-analytical problem that the activity assay does not: samples refrigerated or frozen for a few days before testing read up to 50% higher than the same samples tested fresh. The surviving FDA-cleared test is the activity assay, with a single decision threshold of 225 nmol/min/mL.
What the number means is narrower than the test’s reputation. It is cleared as an aid to predicting coronary heart disease in people with no prior cardiovascular event, with the group at or above the threshold carrying roughly twice the absolute five-year risk. It is not a causal target: darapladib inhibited the enzyme markedly in two large outcome trials and reduced neither coronary events nor mortality. So a raised Lp-PLA2 is a risk marker that may move a borderline decision, and lowering it is not itself a goal of treatment. Reference intervals are method- and laboratory-dependent, and the interval printed on your own report takes precedence over any figure here.
Frequently asked questions
Is nmol/min/mL the same as U/L for Lp-PLA2?
Yes, exactly. One enzyme unit is one micromole of substrate per minute, so 1 U/L is 1 µmol/min/L, which is 1,000 nmol/min/L, which is 1 nmol/min/mL. The factor is one, and a result of 225 nmol/min/mL is 225 U/L.
How do I convert Lp-PLA2 activity to ng/mL?
You cannot, and that is the honest answer rather than an evasion. The ng/mL figure came from a mass immunoassay measuring how much enzyme protein was present; nmol/min/mL measures how fast it works. The ratio between them is a turnover number that varies with the carrying lipoprotein and the sample, so no fixed factor exists. The mass assay is no longer marketed.
What Lp-PLA2 activity is considered high?
Labcorp reports 225 nmol/min/mL and above as increased risk and below that as reduced risk, in adults with no prior cardiovascular event; the same figure is the decision threshold in the FDA clearance for the PLAC activity assay. Reference intervals are method- and laboratory-dependent, and the interval printed on your own report takes precedence over any figure here.
Does lowering Lp-PLA2 reduce cardiovascular events?
The evidence says no. Darapladib, a direct Lp-PLA2 inhibitor, lowered activity substantially and failed to reduce events in two large outcome trials, in stable coronary disease and after acute coronary syndrome. So the enzyme looks like a marker of plaque inflammation rather than a driver of it, and the result is used to refine a risk estimate rather than as a treatment target.
How does it differ from hs-CRP?
Both are inflammatory markers used to refine cardiovascular risk, but Lp-PLA2 is largely vascular and much less affected by acute illness, obesity and intercurrent infection, which move hs-CRP a great deal. The practical consequence is that Lp-PLA2 needs fewer repeat samples. See the hs-CRP unit converter for the contrast.
Related calculators
References
- US Food and Drug Administration. 510(k) K141575 — PLAC Test for Lp-PLA2 Activity, diaDexus Inc. Results in nmol/min/mL; decision threshold 225 nmol/min/mL. Accessed October 2026.
- Labcorp. Test 123283 — Lipoprotein-associated phospholipase A2 activity. Reduced risk below 225 nmol/min/mL, increased risk at or above 225 nmol/min/mL. Accessed October 2026.
- Academy of Diagnostics & Laboratory Medicine (AACC). Ask the Expert: Testing for Lipoprotein-Associated Phospholipase A2. Clinical Laboratory News, July 2016 — the mass assay’s storage artefact and diaDexus’s bankruptcy.
- Assay package inserts and the issuing laboratory’s own report — reference intervals are method-dependent and the local interval governs.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/