Childhood acute rash is mainly caused by human herpesvirus type 6 causing acute febrile rash disease. Blood tests include blood routine, viral antigen, viral antibody, viral nucleic acid and other items. 1. Routine blood test: It is mainly to check the components in the blood, such as the total number of white blood cells, neutrophils, monocytes and so on. The blood leukocyte count is significantly reduced and the lymphocytes are increased up to more than 90%, including atypical lymphocytes, in the case of early childhood acute rash. 2. Viral antigen: immunohistochemistry is the main method used nowadays. Viral antigen detection is suitable for early diagnosis, and the positive antigen results can be used as the basis for confirmation of diagnosis. 3. Viral antibody: ELISA and indirect immunofluorescence method to determine IgG and IgM antibody is the most commonly used and easiest method. positive IgM antibody, high titer of IgG and 4-fold increase of IgG antibody in the recovery period can indicate the existence of HHV-6 infection. IgM antibody is usually produced 5 days after infection, and it can persist for 2-3 weeks, IgG antibody is produced 7 days after infection and peaks 4 weeks later. IgM antibodies are usually produced 5 days after infection and can persist for 2 to 3 weeks. 4. Viral nucleic acid: Nucleic acid hybridization and PCR methods are used to detect viral nucleic acid. If viral nucleic acid is positive in peripheral blood mononuclear cells, it does not necessarily indicate active infection because the virus establishes latent infection in many tissues after primary infection. Quantitative or reverse transcription PCR testing can be helpful in determining if there is an active infection. Emergency rashes in young children should be seen promptly for evaluation at the discretion of a specialist and refinement of relevant investigations.