I have personally met several persons who have experienced delayed or missed diagnoses of venomous spider bites here in the Greater Rochester area. While similar conditions, not treated with antibiotics immediately, offer far more serious consequences, the consequences of a missed or delayed diagnosis of venomous spider bite can also have a serious effect on life, health, and quality of life. Especially in cases of tissue necrosis, in which cultures return negative for any serious bacterial infection, testing for the presence of venom ought to be seriously considered. My contact data is available here.
I am not a medical professional. Any such wound should be examined and diagnosed by trained medical personnel. If you suspect that you are experiencing such a medical condition, seek medical treatment immediately.