A diagnosis is the summary conclusion from signs and symptoms that a patient presents, plus the results of physical examination and laboratory investigation. ‘Clinical diagnosis is, at whatever stage of the diagnosis-oriented process of fact-finding, the corresponding  perception of a person’s current (or past) state of health. In these terms, diagnosis of a particular illness is a perception of its presence/absence first and foremost: and if clearly perceived to be present,  more detailed diagnosis is a perception of particulars of that case of the illness (at the time).’(1) Diagnoses can stay on a symptom level (e.g. fatigue) or reach the highest level where the patho-anatomical/physiological cause(s) and treatment are implicitly included (e.g. Hashimoto’s hypo-thyroidism). The International Classification of Primary Care (ICPC) is the WONCA acknowledged and worldwide used summary of practically all diagnoses prevalent in primary care.(2-5)  Diagnoses are mostly needed to make treatment or management choices in clinical practice.

ICPC was originally based on the ICHPPC-2-defined (International Classification of Health Problems in Primary Care) but allows for many more symptom diagnoses. While it used to be obvious that medical doctors do not start treating without a diagnosis, it becomes increasingly popular to either use treatment as a diagnostic test, or to steer treatment decisions on the results of (individual) signs, symptoms and test results (so-called test-treatment combinations).

A diagnosis with a disease probabiltiy of 100% is very rare.(6) This is only possible with a perfect reference test (gold standard).  Even pathological classification and MRI imaging are not infallible. Thus a diagnosis is never 100% sure. This 100% disease probablitiy is not necessary when using thresholds.(7, 8)

1.  Miettinen OS, Flegel KM. Elementary concepts of medicine: I. Medicine: challenges with its concepts. Journal of evaluation in clinical practice. 2003;9(3):307-9.

2.  International Classification of Primary Care, Second edition (ICPC-2) 2012 [updated 2012; cited 2012 11/1/2012]. Available from: www.who.int/classifications/icd/adaptations/icpc2/en/.

3.  Bentsen BG. International classification of primary care. ScandJPrimHealth Care. 1986;4(1):43-50.

4.  Bentzen N, Bridges-Webb C. An international glossary for general/family practice. FamPract. 1995;12(3):267.

5. Brage S, Bentsen BG, Bjerkedal T, Nygard JF, Tellnes G. ICPC as a standard classification in Norway. FamPract. 1996;13(4):391-6.

6. Knottnerus AM, J. Assessments of the accuracy of diagnostic tests: the cross-sectional study. In: Knottenerus AB, F., editor. The evidence base of clinical diagnosis: Theory and methods of diagnostic research. Oxford: Wiley-Blackwell; 2008.

7. Pauker SG, Kassirer JP. The threshold approach to clinical decision making. NEnglJMed. 1980;302(20):1109-17.

8. Pauker SG, Kassirer JP. Therapeutic decision making: a cost-benefit analysis. NEnglJMed. 1975;293(5):229-34.