Combined with a retrospective review and comprehensive analysis of several related reports, we concluded the following: (1) mNGS may become an important method for the detection of pathogens for DAH and/or HLH; (2) the clinical manifestations of DAH in children may not be typical, but only acute hypoxic respiratory failure and Hb decreased significantly without bleeding elsewhere, a combination of chest imaging can be considered to assist the diagnosis; and (3) when MP infection is complicated with cytopenia, whether MP is refractory or not, MP complicated with HLH should be considered. This evidence concerns the gene GSTM1 and hemophagocytic syndrome.