What could cause a sudden rise in LDL cholesterol in young adults who have not changed their lifestyle or started any new medications?

Depending on the degree of the rise in LDLc noted, there are a number of reasons.

Despite the person reporting that they have "not changed their lifestyle," one must consider reduced activity, or either a diet with more saturated fat - due to being less diet conscious, or due to a low carbohydrate or...

My first response to a large change in LDLC would be to repeat in 2-3 weeks (it is not an emergency) because I have seen several patients who have widely fluctuating LDLC levels for no apparent reason. If the repeat lab is more like prior results (before the spike), it would suggest a lab variation ...

I would be evaluating this patient for secondary causes, especially things that would have changed recently. I would review the diet more closely as I've seen patients, especially lean individuals, go on a keto diet resulting in a significant increase in their LDL-C. I would want to otherwise rule o...