The patient removed the tic himself but then came to see his PCP so this would be an initial encounter. You will need an "S" code as the primary diagnosis followed by the W57.XXA. S code: insect bite nonvenomous of (hip, thigh, ear etc) initial encounter. If the physician saw the patient for the tic bite there should be documentation for that examination. If he did not mention the location in his notes, I would ask. If no treatment was given( for rash/infection/swelling etc) other than to recommend patient return in 2 wks for blood work, I would not use the Z20.818.