Difference Between Diagnosis and Prognosis

Although we hear the terms diagnosis and prognosis more often in medicine they are not restricted to that field alone. Diagnosis refers to identifying the nature or cause of a certain phenomenon and prognosis refers to the future of a condition. This article attempts to explain the meaning of prognosis and diagnosis and the context in which they are used, highlighting the differences between both terms.


Diagnosis can be defined as identifying the nature or cause of a certain phenomenon. It determines the relationship between the cause and the effect. In medicine, doctors arrive at a diagnosis by carefully reviewing the history, examination findings, and investigation results. Clinical interview involves making up lists and narrowing them down. For example, if a patient presents with knee joint pain, doctor may think of trauma, arthritis, or referred pain. After a careful history taking and examination, the doctors eliminate the less likely causes off the list. At this stage, the doctor has a small list of probable diagnoses.  This is called differential diagnosis. Investigations are chosen to arrive at a diagnosis or to confirm clinical suspicions.

Computer technicians use various models to arrive at a diagnosis of a technical problem. Ex: Bayesian network, Hickam’s dictum and Sutton’s law.  There are psychological and technological methods of problem solving used by professionals, to arrive at a diagnosis.


Prognosis refers to the future of a condition. It explains the likelihood of a condition to get resolved. In medicine, prognosis can be good or bad. Prognosis is not an objective measurement but a subjective comment based on previous cases. Good prognosis means that the patient is very likely to recover, and the threat to life is less. Bad prognosis means that the chances of survival are bad. Prognosis doesn’t provide any idea of duration.  In cancers, the patient can suffer for a long time or can die the next day. In either case, the prognosis is bad. Small wounds, common cold has an excellent prognosis. In medicine, a clear diagnosis is necessary to provide a prognosis. In difficult to diagnose sinister cases, doctors struggle to answer the hard questions such as “how long does he has?”

What is the difference between Diagnosis and Prognosis?

• Diagnosis explains the reason for a symptom.

• Prognosis explains how likely it is to go away.