Is it possible to estimate the concentration or amount of cellular mRNA using RNA-Seq data?

Question

[Zahra]

Hi all Is it possible to estimate the concentration or amounts of cellular mRNA using RNA-Seq data?

Answer 1

[Istvan Albert]

RNA-Seq can estimate the relative abundance of different transcripts in a sample, it is not a direct measurement of mRNA concentration or amounts.

[Azade]

Why not? Would you mind explaining it more? With the RNA-Seq data, we can estimate how a gene behaves in terms of expression. So why can't we use the total counts to measure the total amount of cellular microRNAs?