Plural of Diagnosis

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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).

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The Standard Rules for Forming the Plurals

The table below shows the standard rules for forming the plurals of nouns in English.
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.
Another Interactive Exercise
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See Also

Unusual pluralsPlural forming table Quirks with forming plurals Forming the plurals of abbreviations Forming the plurals of compound nouns