Plural of Diagnosis

The Quick Answer

The plural of diagnosis is diagnoses.

The Plural of Diagnosis

The plural of diagnosis is diagnoses.

Hippocrates was known to make diagnoses by smelling his patients' sweat.
In a differential diagnosis, a physician rules out the diagnoses until a final determination can be made.

The noun diagnosis has a Greek root, which is the derivation of the plural diagnoses. There is no alternative English plural form. The noun diagnosis adheres to the standard rules for forming the plurals of nouns in English (shown in the table below).

Type	Example of Type	Forming the Plural	Plural
Most Nouns	lamp scythe	add s	lamps scythes
Noun Ending s, sh, ch, x or z	box dress	add es	boxes dresses
Nouns ending [consonant] o	zero tomato	add either s or es (There are no rules for this - you have to know.)	zeros tomatoes
Nouns ending [vowel] o	patio ratio	add s	patios ratios
Nouns ending [consonant] y	story penny	change the y to an i and add es	stories pennies
Nouns ending [vowel] y	donkey chimney	add s	donkeys chimneys
Nouns ending f or fe	dwarf hoof	ves and/or s (There are no rules - you have to know.)	dwarfs hooves or hoofs
Nouns ending is	diagnosis crisis	change the is to es	diagnoses crises
Exceptions	man louse	some nouns undergo a vowel or letters change	men lice
More exceptions	salmon sheep	some nouns do not change at all	salmon sheep
Foreign rulings	fungus medium	some nouns adopt foreign rulings	fungi media

Why Is There Confusion over the Plural of Diagnosis?

Diagnoses is the only way to make the noun diagnosis plural.

Confusion arises because some mistakenly believe that all nouns ending in s should form a plural that adds es to the end of the word. When a noun ends with is, you need to replace the is with an es to form the plural. This is because its plural form derives from Greek.

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