There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the word: diagnosis'
Add an apostrophe s ('s) after the existing s at the end of the word: diagnosis's
Examples:
The test should support the diagnosis' confirmation.
The test should support the diagnosis's confirmation.
The singular possessive form is diagnosis's.
The singular possessive form of "test" is "test's".
The singular possessive pronouns are mine, yours, his, hers, and its. The singular possessive adjectives are my, your, his, her, and its.
The singular possessive is ant's.The plural possessive is ants'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form is classmate's.
The singular possessive of dish is dish's
Taco's is the singular possessive.
diagnosesThe plural form of diagnosis is diagnoses. It is pronounced dye-egg-no-seas.
The singular possessive is biker's; the plural possessive is bikers'.
The singular possessive is fish's.
Grave's is the singular possessive.