We have isolated total lipids from HepG2 cells using chloroform:methanol (2:1, v/v). I recommend you to repeat the extraction using chloroform:methanol twice. For total cholesterol estimation, both Zak's method or commercially available kits can be used.

For further details, see the following discussion.

https://www.researchgate.net/post/What_is_the_procedure_for_extraction_and_estimation_of_total_lipids_from_cultured_animal_cells

Thanks for the reply @Arunaksharan. As I didn't find any such reference where Zak's method is used for cell cholesterol estimation, can you please inform what would be the amount of test sample that can be used for further analysis like serum?

For our study, we have collected the sample from a T-75 cm2 flask and lysed to a total volume of 200 uL. which was sufficient for the analysis.