The blood test is most accurate after 24 hours of fever. 1, white blood cells are immune cells, when bacteria invade, the body’s white blood cells will be in an active state, mobilized to actively deal with bacteria, this time to check the blood can be found significantly higher white blood cells, but this mobilization takes time, usually need about 24 hours, it can be fully mobilized, so usually wait until 24 hours after the fever The blood test can only accurately show the germ response in the body.2. Blood tests can help determine the type of infection. Generally, a low or lower peripheral blood white blood cell count and an increased lymphocyte count or percentage suggest a viral infection.3. An increased white blood cell count and neutrophil count or percentage suggest a bacterial infection.