A doctor can diagnose someone based on symptoms that are associated with certain types of medical conditions using observation, clinical testing or a combination of the two. Medical conditions can share common symptoms, so a combination of observation and testing can help the doctor isolate conditions, states OpenClinical.
A doctor can use a variety of tools to associate the symptoms presented in the patient with a certain type of medical condition. For routine conditions such as seasonal influenza, which starts in fall and reaches its highest point in January and February, symptoms can include nausea, fever, vomiting, diarrhea and fatigue, explains the Centers for Disease Control and Prevention. If a patient presents these symptoms in the typical time frame of prevalence of the illness, the doctor can conclude that the patient likely has seasonal influenza, states Flu.gov.
In more complex conditions that require a combination of diagnostic tools, doctors augment their observational diagnosis by using diagnostic testing to confirm that the symptoms presented are, indeed, associated with a certain medical condition or group of possible conditions, explains Merck Manuals. Doctors can opt to choose from a range of testing options, including body fluid analysis, imaging, endoscopy, tools such as electrocardiogram and electroencephalography, biopsy and genetic testing.