PMCH and obstructive sleep apnea syndrome: In conclusion, our study found that the plasma tRF-21-U0EZY9X1B level of 4 to 6-year-old OSAHS children was down regulated, which was closely related to Hb, MCH, TG and CK-MB, and could be used as a potential biomarker for the diagnosis of OSAHS in children.