In the case of toxoplasma infection, antibodies become detectable early in the disease and persist for years. Thus, if you had had infection with this organism, it would be evident on your blood test. A negative result clearly excludes the disease.
The usual practice is to test for IgG (one sort of antibody) first and then if it is found to test for IgM (an antibody indicative of recent infection). If IgM is found, the sample will usually be sent to the toxoplasma reference laboratory where they are also able to use a dye test to look for live toxoplasma.