Diagnosis is the process that identifies the disease that affects our patient. Diagnosis implies a nosography, i.e. a classification of human diseases. Human diseases are in principle a finite number, and may be grouped in families or categories, even though with some approximation and imprecision. The actual instances of diseases are more numerous than diseases themselves, because the same disease may present different characteristics in different patients due to their genetic constitution, the effect of environmental factors, and the concomitance with other diseases; moreover some randomness is probably present in the determinism of diseases.

      Modern nosography begins in the XVIII century with the development of morbid anatomy (reference text: Morgagni's De sedibus et causis morborum per anatomen indagatis 1761), but the greatest impulse comes from discoveries made one century later: Pasteur's and Koch's microbiology (1860-1900; Koch: one germ - one disease); Garrod's studies of inherited genetic diseases (reference text: Inborn errors of metabolism 1909); Funk's interpretation of avitaminoses (1912). Rational therapy followed shortly (e.g. Ehrlich's Salvarsan in 1906; Pasteur's artificial vaccines; etc.). Obviously, modern nosography required much more than the physical examination of the patient, which had been practiced since immemorial times: it required laboratory analyses carried out by means of chemical and microbiological methods, as well as the microscope. It is important to stress that nosography is not a static discipline: some diseases may disappear, and new diseases may emerge.
The discovery of nosography: the magical half century (1860-1910)
discovery of infectious diseases and medical microbiologyfirst discoveries from 1860 on by Pasteur and Koch (but hypotheses since G. Fracastoro, XVII century)
discovery of inherited diseases and medical geneticbegins with the studies by A. Garrod (1905 on)
identification of avitaminoses and nutritional diseasesbegins with the identification of the cause of beri-beri by Christiaan Eijkman (1897); the concept of vitamin was formalized by Casimir Funk (1912)
discovery of allergic and autoimmune diseasesbegins with the studies by Paul Ehrlich on the horror autotoxicus (1901 on)

      The most widely accepted nosography is the International Classification of Diseases (ICD) that was adopted by the WHO in 1994 and has undergone several revisions. The current version is the eleventh revision (ICD-11) which lists approximately 2,000 diseases. The ICD-11 is freely available on the WHO website at the link:

      For psychiatric diseases the reference nosographic classification is that of the Diagnostic and Statistical Manual of Mental Disorders (DSM) edited by the American Psichiatrc Association, and currently running its fifth revision (DSM-5). You can find a presentation of the DSM at this link. However, we shall not make use of this nosography because psychiatric diseases usually do not cause laboratory detectable alterations, with the possible exception of some types of dementia (e.g. paralytic dementia, a late neurological manifestation of chronic syphilis).

      A list of the categories of diseases listed in the ICD-11 is as follows:
01 Certain infectious or parasitic diseases
02 Neoplasms
03 Diseases of the blood or blood-forming organs
04 Diseases of the immune system
05 Endocrine, nutritional or metabolic diseases
06 Mental, behavioural or neurodevelopmental disorders
07 Sleep-wake disorders
08 Diseases of the nervous system
09 Diseases of the visual system
10 Diseases of the ear or mastoid process
11 Diseases of the circulatory system
12 Diseases of the respiratory system
13 Diseases of the digestive system
14 Diseases of the skin
15 Diseases of the musculoskeletal system or connective tissue
16 Diseases of the genitourinary system
17 Conditions related to sexual health
18 Pregnancy, childbirth or the puerperium
19 Certain conditions originating in the perinatal period
20 Developmental anomalies
21 Symptoms, signs or clinical findings, not elsewhere classified
22 Injury, poisoning or certain other consequences of external causes
23 External causes of morbidity or mortality
      More or less, the order of the chapters of all textbooks of medicine reflects this classification. We remark that the classification reflects an apparently illogical order: some categories are etiological (infectious diseases, cat. 01) or pathogenetic (neoplasms, cat. 02), but in the majority they are based on the affected organ or system; categories 19, 20 and 21 reflect specific functional conditions or events. This disorder is not accidental and reflects the inherent complexity of its subject matter. However, it is obvious that in some cases the same disease may fit two or more categories. For example viral hepatitis is an infectious disease listed in cat. 01, but is also a disease of the liver (cat. 13), consistency is maintained because the code of the disease is one and the same, even though it fits two categories. This is evident if one explores the links on the web version of the ICD-11, and the student is invited to do so, in order to familiarize with this important nosographic instrument.
      In the systematic part of this course we shall follow the essential lines of the ICD-11, giving relevance to those diseases for which the laboratory analyses are more crucial to the diagnosis. Indeed there are diseases which may cause little and aspecific alterations in the laboratory parameters, and major alterations evident using other diagnostic instruments and techniques, e.g. medical imaging. Thus, the course does not directly reflect the social and clinical relevance of the diseases, but rather their peculiarities as can be revealed by laboratory analyses. Diseases that cause no characteristic alterations of laboratory parameters will not be dealt with in this course, but are duly considered in other courses: e.g. psychiatric disease, essential hypertension, etc. Some pathological conditions that shall receive great attention in this course are not present as such in the ICD-11: this occurs because these conditions are not diseases, but syndromes (groups of symptoms) that may occur in several diseases: for example we shall discuss at length the acidoses and alkaloses (alterations of the blood pH), but these are events that may occur in different diseases and do not constitute diagnoses; rather, within each type of acidosis or alkalosis we may find several different diseases.

      The concept of diagnosis according to William Osler and Archibald Garrod. William Osler (1849-1919) was probably the last among the great theorists of classical medicine. In agreement with many previous physicians he considered diagnosis as a classification: the act of assigning a patient to a category, for which therapy is known and prognosis can be inferred at least in a probabilistic sense. He echoes Laennec: "without diagnosis there is no rational therapy", Koch: "one germ - one disease", Virchow, and all the greatest physicians of the past. Archibald Garrod (1857-1936) succeeded to Osler at Oxford, and studied the inherited defects of metabolism. He was a pioneer of the concept that diagnosis should explain why a given disease affects a given patient in a given form and with a given degree of severity. We can call Osler's concept as a general diagnosis and Garrod's as a personalized diagnosis. Garrod's concept does not superseed Osler's; rather the former complements the latter: you can see Osler's diagnosis as the first step in the understanding of the patient's condition and Garrod's as the second step; and obviously you cannot do the second step if you have not done the first. Notice also that Garrod's diagnosis is not dissimilar from Osler's, but applies to same statistical concepts to a higher degree of precision. You will in any case need group studies, but groups which are homogeneous for Osler (i.e. made of patients having the same general diagnosis) will not be homogeneous for Garrod (because the aptients are genetically heterogeneous). In order to have a group homogeneous according to Garrod's view, you need patients that not only share the same general diagnosis, but also the same allelic variants of the genes which are in some way linked to the disease.