Diagnosis is the process of identifying a disease, condition, or injury from clinical information, including reported symptoms, observed signs, medical history, examination findings, and test results. The term also denotes the resulting identification of a health problem. In medicine, diagnosis provides a basis for decisions about further investigation and care; it is not simply the result of a laboratory test or imaging examination. (cancer.gov)
The diagnostic process
Diagnosis usually develops through repeated information gathering and interpretation rather than a fixed sequence. A medical history establishes the nature and timing of the problem, previous illnesses, medications, family history, and relevant exposures. A physical examination adds observable findings. Tests and consultations may supply additional evidence. These activities can occur in different orders and be repeated as the clinical situation changes. (ncbi.nlm.nih.gov)
Clinicians develop a working diagnosis, which may be a single explanation or a differential diagnosis: a set of plausible alternatives. Further evidence can strengthen, weaken, or replace these possibilities. Diagnostic verification examines whether the leading explanation fits the findings and the patient’s circumstances. (ncbi.nlm.nih.gov)
Diagnosis is collaborative. Patients contribute their experience of illness, while clinicians, laboratory professionals, imaging specialists, and other team members contribute different kinds of evidence. Communication—including explanation of uncertainty and changes in the working diagnosis—is part of the process. Treatment may begin before certainty is achieved, and subsequent clinical developments can inform reassessment. (ncbi.nlm.nih.gov)
Diagnostic evidence and methods
A laboratory test examines blood, urine, other body fluids, or tissue. Results may identify an organism or substance, measure its concentration, or provide broader information about bodily function. Some tests address a specific condition, whereas others identify abnormalities requiring further investigation. Laboratory findings are interpreted alongside history, examination, and other evidence. (medlineplus.gov)
Imaging provides another form of diagnostic information. Computed tomography uses X-rays to produce cross-sectional images that can reveal injuries and structural abnormalities. Magnetic resonance imaging produces detailed anatomical images using magnetic fields and radiofrequency signals, without the ionizing radiation used in CT. Different imaging methods provide different information and have distinct limitations. (nibib.nih.gov)
In pathology, examination of sampled tissue can establish the nature of an abnormality. A biopsy removes tissue for microscopic examination and, where appropriate, additional tests. For most suspected cancers, biopsy is needed to establish the diagnosis with certainty. The resulting pathology report may also provide information relevant to treatment selection. (cancer.gov)
Test interpretation and uncertainty
Diagnostic tests are not perfectly accurate. Sensitivity and specificity describe two aspects of their performance. Sensitivity is the proportion of people with a condition who receive a positive result; specificity is the proportion without it who receive a negative result. A false negative misses a condition that is present, while a false positive indicates a condition that is absent. Specimen quality, test limitations, and operator or equipment errors can affect performance. (reach.cdc.gov)
These measures describe conditional probabilities, not certainty about an individual diagnosis. For example, 95% sensitivity means that approximately 95% of people who have the condition test positive—not that every positive result carries a 95% probability of disease. Interpretation still depends on the clinical situation and other available evidence. (reach.cdc.gov)
Numerical laboratory results often appear beside a reference range derived from a reference population. A value outside that range does not necessarily establish disease, and a value within it does not always exclude disease. Laboratories may use different methods and ranges, so results require contextual interpretation rather than classification as simply “normal” or “abnormal.” (medlineplus.gov)
Screening and overdiagnosis
Screening seeks evidence of disease before symptoms appear. In cancer screening, an abnormal result generally requires diagnostic follow-up; it is not itself proof of cancer. The same type of examination may therefore serve different purposes depending on whether it is used for screening or investigation of an existing problem. (cancer.gov)
Overdiagnosis is distinct from a false-positive result. In cancer screening, it means identifying a cancer that would not have caused symptoms or harm during the person’s lifetime. The abnormality is genuinely present, but its detection may lead to treatment that would otherwise have been unnecessary. Earlier detection alone therefore does not establish that a screening programme improves health outcomes. (cancer.gov)
Documentation and classification
Diagnostic information also supports activities beyond individual care. The International Classification of Diseases, maintained by the World Health Organization, provides standardized categories for recording and comparing diseases and other health problems. Coded clinical information supports public health reporting, epidemiological analysis, service planning, reimbursement, and health-services research. Classification enables consistent aggregation of information, while clinical diagnosis remains the process of establishing what explains a particular patient’s health problem. (who.int)