1/7 This issue come up a lot, here is my take on this: @stsimikas @OxPL_apoB
1- ApoB-100 represents all cholesterol-rich particles (VLDL, IDL, LDL, Lp(a)) and laboratory calculated or measured "LDL-C" is really IDL, LDL, Lp(a). So when you order an apoB you also get VLDL-apoB
1- ApoB-100 represents all cholesterol-rich particles (VLDL, IDL, LDL, Lp(a)) and laboratory calculated or measured "LDL-C" is really IDL, LDL, Lp(a). So when you order an apoB you also get VLDL-apoB
https://twitter.com/ethanjweiss/status/1368201696803921924
2/7 2- if the patient has normal TG, VLDL-C (and remnant cholesterol) is very low, so in this case apoB adds very little. In fact, correlation of apoB with LDL is >0.9, so not worth the extra expense
3/7 3- If TG/VLDL is elevated, the chol content shifts to larger particles, and in this case the correlation of apoB to LDL declines. The real LDL-C is lower and VLDL-C is higher. Thus lab LDL-C is less predictive, but apoB captures all the cholesterol on VLDL so better predictor
4/7 4- Bottom line: apoB most useful in high TGs. I dont measure with normal TGs usually. If you can get it for free, like at LabCorp, then great
5/7 5- the usefulness of apoB is that it can reflect all cholesterol carrying particles. If we started the cholesterol field over from scratch, it would be the measure of choice. But, LDL-C is excellent measure in most patients
6/7 6- For Lp(a), its a different beast. The risk may not come just from cholesterol, but more so from @OxPL_apoB and anti-fibrinolytic effects. See graph I tweeted recently. Correcting for apoB does not modify Lp(a)-risk, whereas it completely explains LDL-C risk. 
7/7- What do do: a-in most cases LDL-C is fine, b-if TG elevated, get apoB also, c-in all cases, you need Lp(a), at least once. ApoB will also be higher relative to LDL in both high TG/Lp(a). This is my practice, there is no "right' answer what to measure. I hope this is helpful.
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