When a doctor fails to correctly diagnose or treat a disease, there may be grounds for a medical malpractice lawsuit. One of the most important factors in determining if the case is "actionable" (legitimate grounds for a lawsuit) is if injury results. If no losses or damages occur as a result of the erroneous diagnosis or treatment, the patient has not been harmed are there are no grounds for the lawsuit.

Determining the negligence of a doctor is based upon the standard of care. That is, did the doctor act in accordance with the expected behavior of any doctor in the given situation? If a doctor failed to even consider the actual cause of the patient’s symptoms or dismissed it without sufficient testing, that behavior may qualify as negligence. However, a misdiagnosis arising from proper and reasonable action and care on the part of the doctor is most likely not medical malpractice.

Actionable injury can result from a failed diagnosis in two ways, which are not mutually exclusive. They are direct and indirect harm. When a doctor incorrectly diagnoses a patient’s ailment, and the prescribed treatment (for example, prescription drug) causes direct harm to the patient, that patient likely has grounds for a medical malpractice lawsuit. If a doctor correctly diagnoses a disease but elects to use an unproven method of treatment that proves harmful and/or unsuccessful, where a conventional treatment would likely have worked, this is also probably negligence. That is, a correct diagnosis followed by erroneous treatment, while not a common occurrence, is still malpractice.

Indirect harm is when the failure to correctly diagnose or the ignoring of symptoms results in a spreading or progression of the disease that could have been halted or at least delayed had the ailment been treated in a proper and timely manner. Establishing that timely treatment would result in a superior prognosis is sometimes difficult to prove because a disease can spread considerably before its symptoms become apparent.