Principles of Diagnosis


Art and Science of the Clinical Diagnosis of  Disease

The Common Vein

Copyright 2009


The diagnostic process requires a variety of skills and tools which need to be used prudently and thoughtfully to define the essence of the patient’s problem and to arrive at a diagnosis so that the patient can be treated. The first of these tools is the clinical tool and revolves around the human interaction of the patient and the physician.  The second set of tools includes all the mechanical probes that allow investigation of the biology of the body.

The usual order of things is that the patient presents with a problem to a physician and the clinician divides the process into history taking and examination.  During history taking, the essence of the problem is defined by a series of questions relating to the symptom and each question is directed and refined to elucidate the problem.  A general set of questions usually follows regarding other systems of the body. This part of the diagnosis is referred to as the clinical history.

Once the clinician has a sense of the problem, examination of the patient has two parts, one with the specific focus of the presenting problem and the second a more general approach relating to the general wellbeing of the patient. This part of the diagnosis is called the clinical examination.

The next step in the diagnostic workup moves to the mechanical probes of the diagnostic work up, and the process also consists of two parts;  the focused approach and the general approach.  The general tests are called routine tests and include a series of blood tests, a urine test and perhaps an EKG.  More specific tests are chosen by the clinician depending on the direction of thinking, and the provosional diagnosis at that point.

Often, at this time a specific imaging test is ordereed, and the interpreting physician, commonly a radiologist, will review the images with a focused approach and a general approach as well.

Often the diagnosis by this time is reached, but sometimes the diagnosis requires further testing, or time to evaluate the persistence or nature of the problem and the diagnosis.

We will first discuss the principles that guide us to evaluate the patient ,whether in the realm of the clinical history and examination, or whether it is in the realm of interpretation of tests.

Universal Principles


At the frst level of diagnosis detection is necessary.  Detection is the recognition of an abnormality.  The clinician has to be able to identify that which is abnormal.  This aspect is essential and the process cannot move forward unless this event takes place.  No matter how smart, or how much book knowledge a person has, without detection of the abnormality the diagnostic process cannot proceed.  This concept is pertinent at the level of history taking, clinical examinatiion all through every diagnostic tool we have in our medical bag of tricks.


Once detected, perception of the abnormality is necessary, which implies that the abnormality has to be understood in context.  A little black mole on the skin can be detected by all, but perception in context and understanding is in the realm of an experienced eye.  The perception of the structure will often require a second look from a different angle, or with a different tool in order to understannd it.  Palpation percussion and auscultation are used in clinical examination, two views on a CXR, or 3D reconstruction of a CTscan, infer the second look from a different angle approach.


A decision is usually unconciously made at this point as to whether the abnormality is a mere curiosity in which case no more time will be spent on it, or whether it is worth analysing.  If the latter is true, its characteristics are further analysed.  This analysis is part of the discipline of perception.  The experienced eye, has through training analysed the black mole on the skin inumerable times, so that for the practiced eye perception becomes intuitive and often a “geshtalt”.


After analysing the abnormality, we start to comprehend the information , so although we may not have reached a diagnosis, some form of understanding is in place.  This also is part of perception and and analysis but it is the end of that process, heralding a change of gear to the next stage of combining and integrating a single element with other pieces of information.


Knowledge, memory, and experience now come into play as we integrate what we have seen and comprehended with what we have learned and what we know.  The penny either drops or it does not.  A diagnosis made is not a diagnosis in isolation since by the nature of disease there is a chain reaction to all events.  Thus for example if the diagnosis of acute cholecystitis is made by finding a positive Murphy’s sign, then it behoves the clinician to further determine if a complication such as perforation may have taken place.

Judgement and Prudence

One step beyond integration is the decision as to whether the findings have relevance to the patient and the current symptoms, whether it is an associated finding, implying it is important but not of immediate relevance, or whether it is an incidental finding in which case it is a mere curiosity.  The review of prior medical history is essential before we make our final judgement.  Finding an enlarged liver may have been previously diagnosed as an insignificant Riedels lobe in the past, and therefore this finding taken in this context is different from a new finding of hepatomegaly.  Finding a black mo;le which we think is suspicious may have been present on an old photograph with exct same features 6 years ago.


The impression is what we report as our official evaluation of the clinical presentation, and it is this part of the initial evaluation that crystallizes the patients evaluation at this point.  This may represent a final diagnosis and require no further diagnostic workup, or the diagnosis may still require laboratory investigation and imaging.

The Tools

There are traditional clinical tools whichin the realm of clinical medicine  include palpation, auscultation percussion augmented by a stethoscope, otoscope, opthalmoscope, thermometer, beaumonomoter, hammer and tuning fork.   beyond the bedside, the chemistry lab, the blood lab, culture labs, EKG, echo lab, radiology department, subspecialty diagnostic tools are all designed to enable diagnosis of diease.


The principles that guide us in any diagnostic  procedure are universal.  They include the aim of the procedure, indication, contraindication, advantages, disadvantages, method (patient preparation, equipment, technique) potential complications, and results.

The Art of Diagnosis

Beyond the discussion above is the ill defined art of diagnosis and below we describe the clues and inspirations from the greatest detective of them all –

Holmes,…. Sherlock Holmes

From a review of all the volumes of the stories of Arthur Conan Doyle a few prudent and inspirational quotes have been extracted.

“Not invisible but unnoticed Watson.You did not know where to look so you missed all that was important.”

 “What was vital was overlaid and hidden by what was irrelevant.  Of all the facts which were presented to us we had to pick just those which we deemed to be essential and then piece them together in their order so as to reconstruct this very remarkable chain of events.” ( The Adventure of the Naval Treaty)

 “Crime is common – logic is rare. Therefore it is upon the logic rather than upon the crime that you should dwell.”  (The Adventure of the Coppper Beaches)

 “It is of the highest importance in the art of detection to be able to recognise out of a number of facts which are incidental and which vital, otherwise your energy and attention must be dissipated  instead of being concentrated. ”  (The Adventure of the Musgrave Ritual)

 “We approached the case, you remember with an absolutely blank mind, which is always an advantage.  We had formed no theories.  We were simply there to observe and to draw inferences from our observations.”  (The Adventure of the Cardboard Box)

“Elementary,” said he, “it is one of those instances where the reasoner can produce an effect which seems remarkable to his neighbour, because the latter has missed the one little point which is the basis of the deduction.”  (The Adventure of the Crooked Man)

“There is nothing in which deduction is so unneccesary as in religion.” said he.  “It can be built up as an exact science by the reasoner.  Our highest assurance of the goodness of providence seems to me to rest in the flowers. All other things, our powers, our desires, our food are really necessary for our existance in the first instance. But this rose is an extra.  Its smell and its color are an embellishment of life, not a condition of it. It is only goodness which gives extras, and so I say again that we have much to hope from the flowers.”  (The Adventure of the Naval Treaty)

“The world is full of obvious things which nobody by any chance ever observes.”  (The Hound of the Baskervilles)

“It may be that you are not yourself luminous but you are a conductor of light.  Some people without possessing genius have a remarkable power of stimulating it.”  (The Hound of the Baskervilles)

“I knew that seclusion and solitude were very necessary for my friend in those hours of intense mental concentration during which he weighed every particle of evidence, constucted alternative theories, balanced one against the other, and made up his mind as to which points were essential and which immaterial.”  (The Hound of the Baskervilles)

“It is the scientific use of the imagination, but we have always some material basis on which to start our speculations.” (The Hound of the Baskervilles)

The Process in Medical Diagnosis

There is a traditional and well oiled process in the medical profession starting from “seeing” the doctor through the examination, and on to the use of a variety of tests in the pursuit of the diagnosis.  The diagnostic process is only in place so that an ailment can be appropriately treated .

The Clinical Presentation

The History

Main Complaint is defined by asking the patient what is bothering them.  The complaint is usually a result of an irritation eg pain, or cough, or as a result of loss of function eg shortness of breath or muscle weakness for example.  Sometimes the symptom is more general such as weight loss, malaise, tiredness, or fever.  In this instance it is the patient who is detecting an aberrance by an uncomfortavble sensation.

Once the symptom is accurately defined by the clinician, then analysis of the symptom by specific questions elucidates the problem.  Questions relating to duration, onset, character, aggravating and relieving factors help characterize the main complaint.

Associated symptoms such as fever, weight loss and malaise help give context to the symptom.  Often symptoms are fleeting and the distinction between a fleeting symtom with no relevance to the overall health of the patient, and a more serious symptom relates to these questions.  For example a patient with abdominal pain and weight loss is taken very seriously and requires further work up, while a patient with abdominal pain relieved by passing gas and no other symptoms requires no further workup.

 The Examination

Methods of examining the patient also require a focused examination and a general examination.   The former has become more commonly applied for many reasons, but mostly because of limitations in time.  Important clues to the diagnosis are often missed because of the abbreviated clinical examination.

 A spatially logical method of examination is necessary in order that the examination is efficient and meticulous.

By the time the clinical examination is complete the clinician should be able to crystallise a problem list and approach each problem with a “next step” plan which may either require further diagnostic tests or a treatment plan.

Lab tests

There are a set of general “routine” tests that and some specific tests.  Routine blood tests SMA 12 (albumin alkaline phospphatase, total bilirubin, BUN, calcium, cholesterol, creatinine, glucose, phosphorus, SGOT, total protein and uric acid.) Others include the SMA 6 or the SMA 20.  The specific tests may include cardiac enzymes, liver function tests or blood gases.

Routine urine test includes testing for specific gravity, pH, sugar, ketones, red cells, white cells and billirubin.


Chest X-ray used to be part of routine testing, but most of medical imaging now falls into the realm of a targetted study, and only used once the preliminary diagnosis warrants the test.