Grammar Monster
Grammar Monster

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 donkey
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.