If your child was well when tested, and the test is negative, the child does not need to be retested and does not have G6PD deficiency. This usually starts 1-3 days after the drugs or toxins are taken. Common drugs which may cause the problem are Septra or Bactrim, two names for a drug often prescribed for ear infections, and aspirin. Sometimes a reaction is mild and not even noticed. The treatment is to stop the drug and support the child with i.v. fluids, sometimes blood transfusions, and other common treatments if indicated.
Children usually recover just fine.
Very rarely, a child may develop a hemolytic reaction to a common infection. However, if your child with G6PD deficiency has a cold, fever and low energy, probably he just has a virus and not a hemolytic reaction. A hemolytic reaction can be diagnosed reliably with common blood tests. If there is no hemolysis on the tests, the child does not have it. You can have it done with the first screening blood work, or later if the lab does not get enough blood.