Whether the doctor lived up to the standard of care will likely require an expert opinion. One of the issues the expert will examine is the defendant doctor's differential diagnosis method. When trying to diagnose a patient, a doctor makes a list of diagnoses in order of probability and tests them by asking the patient questions, making further observations of the patient, or ordering tests. The goal is to rule out diagnoses until there is only one diagnosis remaining. However, in many instances, a doctor learns more information that requires him or her to supplement the list with other potential diagnoses.
There is a statute of limitations (or time limit in which you can file a lawsuit) for medical malpractice cases. This limit varies from state to state, but in general it is about two years from when the injury occurred. To ensure you file a claim before the statute of limitations is up, you should reach out to a medical malpractice attorney as soon after you realize doctor error occurred.