There are various ways of reaching a diagnosis. The saying: "One recognises only what one knows" is of great importance. Knowledge of diseases and pattern recognition are the basis. Recognition of clinical presentations and reaching a diagnosis is the outcome. In the case of infectious diseases, an attempt can be made to detect the pathogenic organisms directly by microscopy (for example thick smear for malaria, Ziehl-Neelsen staining of sputum for pulmonary tuberculosis, faecal specimen for amoebae, bone marrow aspirate for visceral leishmaniasis, etc). Cultures and serological tests are usually difficult or impossible in rural areas. Radiology and ultrasound are mostly of limited availability.
A patient will have certain complaints: symptoms, examples of which are neck pain, cough or loss of strength in the legs. There will also usually be objective signs identified by the physician treating the patient. Examples are neck stiffness, crepitations and hyperreflexia. One and the same disease may take different forms in different people. There is a spectrum of manifestations: there is individual variability (for example immunological resistance) and furthermore the symptoms and signs depend on the stage of the disease. Sometimes the degree of infection (for example worm load) is important. Whether a particular symptom, for example blood in the urine (haematuria), is highly indicative of a specific diagnosis depends on the local frequency (prevalence) of the disorder (for example, bilharziasis is frequent in Africa but not in India). A symptom may be specific to a greater or lesser extent, for example fever can be caused by numerous diseases. Fever is thus fairly aspecific for a certain diagnosis. Muscle spasms triggered by sudden noise are strongly indicative of tetanus. This sign rarely occurs in other diseases and is thus relatively specific for tetanus.
Often, a definite diagnosis is not possible and a probable diagnosis must be established: the disease that is most likely in the differential diagnosis. The differential diagnosis is the list of those diseases that might explain the patient's clinical picture. It is not advisable to make a long list by including all sorts of rare possibilities. By definition, rare diseases are always rare. It is however important to think of a rare disease if it is severe and treatable in the given circumstances.
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