We highly recommend using Lexogen's SPLIT RNA Extraction Kit (Cat. No. 008, Protocol for Small RNA Fraction or Total RNA Isolation), which enables the direct extraction of small RNA (down to 17 nt).
When isolating total RNA for input, ensure the protocol can recover small RNAs (including miRNAs).
Many commercial products are using silica-based purification technology that may exclude small RNAs. Please consult the manufacturer's specifications.
For biofluid samples such as plasma, serum, urine, or exosomes, isolation of total RNA is preferred, and there is no need to fractionate for small RNAs.