Some blood tests show changes when a person has dengue fever. The earliest change is a low number of white blood cells in the blood. A low number of platelets can also signal dengue. Special blood tests can look for the dengue virus itself; the virus's nucleic acids; or the antibodies that the immune system makes to fight off the virus. However, these special tests are expensive. Also, in many areas where dengue is common, most doctors and clinics do not have laboratories for blood tests or special machines.