Plural of “diagnosis”

The plural of diagnosis is diagnoses.

Why

"Diagnosis" comes from Greek "diagnosis," literally "a distinguishing" or "discernment" (built from "dia-," through or apart, and "gignoskein," to know), and it follows the same Greek -is/-es plural pattern as "crisis/crises" and "analysis/analyses": the plural takes a full extra syllable, "dy-ag-NO-seez."

Medical writing distinguishes carefully between a "diagnosis" (the identification of a specific condition in one patient) and "diagnoses" (either multiple diagnoses for one patient with several conditions, or diagnoses made across a group of patients), a distinction that matters enormously in clinical documentation even though the grammar itself is straightforward once the -is/-es pattern is known. Insurance and billing systems have added a further practical layer to the word's everyday use: standardized diagnosis codes (such as those in the ICD system) assign a specific numeric identifier to each individual diagnosis, meaning a single patient's file might list several distinct diagnoses side by side purely for administrative and billing purposes, independent of how many separate medical conditions are actually being actively treated at once.

Usage note

Because "diagnosis" appears constantly in everyday health journalism and personal medical conversation, not just in clinical writing, the plural "diagnoses" is genuinely common outside specialist registers, unlike some of its more narrowly academic Greek cousins that most readers rarely see pluralized at all.

"Diagnose," the verb form, is fully regular in every tense (diagnoses, diagnosed, diagnosing) and shares no irregularity with the noun's plural — note that the third-person singular verb "diagnoses" ("the doctor diagnoses the illness") is spelled identically to the noun's plural "diagnoses," a coincidental overlap resolved entirely by sentence context rather than by any difference in spelling. Patient advocacy writing increasingly emphasizes the difference between receiving "a diagnosis" and living with a chronic condition day to day, a distinction of meaning and emphasis that exists entirely separately from, but is sometimes discussed alongside, the singular/plural grammar of the word itself. Telemedicine has made preliminary diagnoses more accessible, though many providers still require an in-person follow-up before a final diagnosis is confirmed.

Common real mistakes

Mispronouncing the plural with the singular's vowel ending is a frequent spoken error, and in writing, some learners produce "diagnosises" by overapplying the general -s rule before absorbing the whole family pattern shared with "crisis," "thesis," and "analysis." A further mistake some writers make is treating "diagnostic," the adjective derived from "diagnosis," as if it carried the same irregular plural behavior, when in fact it's a fully regular adjective that never inflects for number at all, unlike the countable noun it's built from.

The wider pattern

"Diagnosis/diagnoses" belongs to the same Greek -is/-es family as "crisis/crises," "thesis/theses," and "analysis/analyses," a group whose shared, still fully productive pattern is reinforced daily for anyone working in medicine, science, or academic writing.

Examples

• The doctor delivered the diagnosis after reviewing the test results.
• Several diagnoses were considered before the condition was confirmed.
• Her initial diagnosis was later revised after further testing.
• The clinic tracks diagnoses across thousands of patients each year.
• A second opinion led to a completely different diagnosis.
• Two separate diagnoses complicated the treatment plan significantly.
• The specialist's diagnosis matched what the family had suspected.
• Doctors compared diagnoses from three different hospitals.
• An early diagnosis often improves the long-term outcome.
• The study analyzed diagnoses recorded over a ten-year period.
• The insurance claim listed three separate diagnoses from the same visit.
• A misdiagnosis early on delayed her access to the correct treatment.
• The clinic tracks how quickly diagnoses lead to effective treatment plans.
• A rare genetic disorder took years before receiving an accurate diagnosis.
• The pathology report confirmed the diagnosis within forty-eight hours.
• Two independent diagnoses agreed almost exactly on the underlying cause.
• The specialist explained each diagnosis carefully to the anxious family.