Whenever a set of lab data, radiology or other studies are looked at, they must be looked at in the context of the clinical picture of the patient.
I have often seen residents present a history and physical examination and then go off at a tangent because of the unusual findings of the lab data or the CT findings. The history and physical exam seem to be left behind and unimportant. Why ? In these instances, the history and physical examination have usually not been done adequately as in some peoples opinion, it cannot provide quantitative data that modern medical tests can [I would disagree!] The patient's history is a story of events and intangible to some doctors who need reference ranges and absolute numbers to make a decision e.g. CRP. The physical exam which involves tapping, feeling, probing and listening no longer seems to be as convincing as the BNP of 300. As a result, the history and physical are not being taken seriously enough and left behind in favour of labs, xrays and serum rhubarb.
Moreover, based on CT findings, sometimes residents provide an encyclopedic list of causes, for example, the 10 causes of 'ground glass' shadowing, which has no relevance to the history at all. They compartmentalize their thinking instead of integrating it with the history and physical.
In order to work out a differential diagnosis the doctor needs to keep the thorough history and physical examination in mind as the primary point about which the various tests can then be ordered and interpreted. With a good history and physical, the differential diagnosis can sometimes be whittled down to a few main causes quite quickly thereby avoiding expensive, needless and time wasting tests.
In order to develop better clinical and diagnostic thinking, junior doctors need guidance to understand what they hear in the history and what they see or have missed from the physical exam. Frequently, reviewing the patient on a senior round will reveal the true cause by the words that come from the lips of the patient. What the patient says can change the diagnosis in a second despite the CRP, BNP and LDH. The relevance of the tests is based on what is considered is wrong with the patient. This can only be determined by asking the patient what is wrong with them! It comes back to us asking the right questions ....
Remember, Common things are Common. Common things present Commonly. Hence, when a patient has preceding viral upper respiratory symptoms and then develops bibasal crackles, it is far more likely to be a primary viral or secondary bacterial pneumonia than a drug induced pneumonitis.
Never make a diagnosis purely from a radiographic image without the attached history and physical exam. The features on a CT might be accounted for by many causes and only a history can indicate which cause it is likely to be.
Speaking to, and examining the patient is free. It is immediate and gives much valuable information that cannot be derived from a CT scan or MRI.
Using a textbook to look up the various causes of XYZ disease is encouraged but if the listed cause does not fit the major features of the patient's presenting illness, then it is less likely to be the underlying disease (although not exclusively!)
Therefore, history, physical, lab studies and radiology need to be used to complement eachother and not used in isolation otherwise the patient's true condition gets lost in the confusion. Remember, lab studies and radiology should not be used to make the diagnosis. They should be used to confirm what you are already thinking and to support or refute your differential diagnosis.