Generally, a delay or failure to diagnose a disease is actionable, if it has resulted in injury or disease progression above and beyond that which would have resulted from a timely diagnosis. This situation may be difficult to prove. For example, a patient may allege that a doctor failed to timely diagnose a certain cancer, resulting in "metastasis" (spread of the cancer to other organs or tissues). But experts may testify that "micrometastasis" (spreading of the disease at the cellular level) may occur as much as ten years before a first tumor has been diagnosed, and cancerous cells may have already traveled in the bloodstream and lodged elsewhere, eventually to grow into new tumors. Therefore, it may be difficult in some cases to establish that a patient has suffered a worse prognosis because of the failure or delay in diagnosis.

If a patient is treated for a disease or condition that he or she does not have, the treatment or medication itself may cause harm to the patient. This is in addition to the harm caused by the true condition continuing untreated.

Most doctors are trained to think and act by establishing a "differential diagnosis." Doing so calls for a doctor to list, in descending order of probability, his or her impressions or "differing" diagnoses of possible causes for a patient's presenting symptoms. The key question in assessing a misdiagnosis for malpractice is to ask what diagnoses a reasonably prudent doctor, under similar circumstances, would have considered as potential causes for the patient's symptoms. If a doctor failed to consider the patient's true diagnosis on his/her differential diagnosis list, or listed it but failed to rule it out with additional tests or criteria, then the doctor is most likely negligent.

Failure to Treat and Erroneous Treatment

The most common way in which doctors are negligent by failing to treat a medical condition is when they "dismiss" the presenting symptoms as temporary, minor, or otherwise not worthy of treatment. This situation may result in an exacerbation of the underlying condition or injury, causing further harm or injury. For example only, an undiagnosed splinter or chip in a broken bone may result in the lodging of a piece of bone in soft tissue or internal bleeding caused by the sharp edge of the splintered bone.

Erroneous treatment is most likely to occur as a result of a misdiagnosis. However, a doctor who has correctly diagnosed a disease or condition may nonetheless fail to properly treat it. Other times, negligence is the result of a doctor attempting a "novel" treatment that fails, when in fact a more conventional treatment would have been successful.