diagnosticand

Diagnosticand is a term used in discussions of diagnostic reasoning to denote the entity whose condition is being assessed. It refers to the object of diagnosis—the patient, specimen, device, or problem—whose state is to be determined. The diagnosticand is distinguished from the diagnostician and from the diagnostic tools used to reach a conclusion.

In medicine, the diagnosticand is typically the patient, whose symptoms, history, and test results are interpreted to establish a diagnosis. In engineering, software, or environmental sciences, the diagnosticand can be a faulty component, a system, or an observed event that requires fault localization or root‑cause analysis.

Diagnosis proceeds by gathering data about the diagnosticand, formulating hypotheses about its possible states, and testing these hypotheses against evidence. Probabilistic reasoning, differential diagnosis, and diagnostic testing are common methods for refining the likelihood of each state of the diagnosticand.

Because the term is not widely used in everyday practice, some writers prefer simpler labels such as patient, case, or object of diagnosis. When used, clear definitions of the diagnosticand and the scope of the inquiry help prevent ambiguity in interdisciplinary contexts.

See also: diagnosis, differential diagnosis, diagnostic reasoning, diagnostic test, patient.