It is often challenging to decide whether clinical data fit into one problem or several problems. If there is relatively long list of symptoms and signs, and an equally long list of potential explanations, one approach is to tease out separate clusters of observations and analyze one cluster at a time. Several clinical characteristics may help.
Patient age: The patient’s age may help; younger adults are more likely to have a single disease, whereas older adults tend to have multiple diseases.
Timing of symptoms: The timing of symptoms is often useful. For example, an episode of pharyngitis 6 weeks ago is probably unrelated to the fever, chills, pleuritic chest pain, and cough that prompted an office visit today. To use timing effectively, you need to know the natural history of various diseases and conditions.
Involvement of different body systems: Involvement of the different body systems may help group clinical data. If symptoms and signs occur in a single system, one disease may explain them. Problems in different, apparently unrelated, systems often require more than one explanation. Again, knowledge of disease patterns is necessary.
Multisystem conditions: With experience, you will become increasingly adept at recognizing multi system conditions and building plausible explanations that link manifestations that are seemingly unrelated. To explain cough, hemoptysis, and weight loss in a 60-year-old plumber who has smoked cigarettes for 40 years, you would rank lung cancer high in your differential diagnosis. You might support your diagnosis with your observation of the patient’s cyanotic nailbeds. With experience and continued reading, you will recognize that his other symptoms and signs fall under the same diagnosis. Dysphagia would reflect extension of the cancer to the esophagus, pupillary asymmetry would suggest pressure on the cervical sympathetic chain, and jaundice could result from metastases to the liver. Related risk factors should be explored promptly.
Key questions: You can also ask a series of key questions that may steer your thinking in one direction and allow you to temporarily ignore the others. For example, you may ask what produces and relieves the patient’s chest pain. If the answer is exercise and rest, you can focus on the cardiovascular and musculoskeletal systems and set the gastrointestinal (GI) system aside. If the pain is more epigastric, burning, and occurs only after meals, you can logically focus on the GI tract. A series of discriminating questions helps you analyze the clinical data and reach logical explanations.