The best thing to do to get rid of crabgrass fully is to tear your lawn completely up and then reseed the lawn with new grass seed and then apply Crabgrass prevention chemicals to the lawn while the new lawn is growing and apply the Crabgrass preventer often to keep the Crabgrass from coming back.
You can use Crabgrass killer but do know that some desirable grass could be killed by some Crabgrass killer such as Bluegrass, Fescue and Ryegrass.