I've noticed that "Diagnostic" can be used as a plural! Such as in some windows warning messages. What's the subtle difference between Diagnostic and Diagnosis? How do we use each of them in a correct way?

  • 1
    The exact same question was asked previously but closed for having no research (!) There's another question on singular v plural. And several others on the word "diagnostic/diagnostics" if you maybe search ELL SE.
    – Stuart F
    Aug 4, 2023 at 8:38
  • Compare the plural ending with the adjective to create a noun: haptic, haptics; bionic, bionics; dialectic, dialectics; kinetic, kinetics; genetic, genetics; linguistic, linguistics; therapeutic,therapeutics.
    – TimR
    Aug 6, 2023 at 11:50

1 Answer 1


Yes, diagnostics is often used. M-W defines diagnostic as:

the art or practice of diagnosis —often used in plural

Cambridge has this definition for diagnostics, which a term also used in IT:

methods or systems for discovering the cause of a problem, illness, etc.

  • Requests for off-site testing and diagnostics are frequently ignored or refused without explanation.

The most thorough explanation is given by Collins:

[usually pl., with sing. v.]

  • the art, science, or method of diagnosis, esp. medical diagnosis

  • a distinguishing sign or symptom; characteristic

  • Computing
    a routine for testing a piece of hardware or for locating an error in a program

The difference between diagnostics and diagnosis in medicine is that:

Diagnostics provide a cluster of objective measures directed toward identifying the cause of a disease... Diagnosis, by contrast, rests on a patient’s history and symptoms and observed physical and laboratory findings.(NLibMedicine)

More concretely,

Diagnosis is defined as the process of recognising an injury, condition, or disease from the signs and symptoms the individual is displaying. Diagnostic tests are used to ascertain a diagnosis. Blood tests, health history, physical assessments, and other procedures may be used to help make a diagnosis. Diagnostics include using testing and other procedures to arrive at a diagnosis for a particular disease or illness. (Talking Health Tech)

So while the diagnosis is the actual identifying of the problem or disease, diagnostics are the methods used to make that identification, or the art of doing it (the field).

Here is a definition of IT diagnostics by Easy Tech Junkie:

Computer diagnostics are software programs designed to scan the hard disk and other computer hardware components of a computer system, and identify any issues that may be hampering the overall performance of that system.

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