If you are writing a medical report, an email to a colleague, or even just talking with a doctor, you will need the plural of diagnosis. The correct plural is diagnoses. Many people mistakenly write diagnosises or diagnosi, but those forms are incorrect. This guide explains the rule, shows you how to use the word naturally, and helps you avoid the most frequent errors.
Quick Answer: What Is the Plural of Diagnosis?
The singular is diagnosis. The plural is diagnoses (pronounced dye-ag-NOH-seez). It follows the Greek-to-English pattern where words ending in -sis change to -ses. For example, crisis becomes crises, and thesis becomes theses. Never add an extra -es to make it diagnosises.
Why This Mistake Happens
English learners often treat diagnosis like a regular noun. Regular nouns add -s or -es to form the plural (e.g., doctor → doctors). But diagnosis comes from Greek, and it follows an irregular pattern. The ending -sis changes to -ses. This is the same pattern you see in analysis → analyses and hypothesis → hypotheses.
Another common error is confusing the singular and plural in writing. Some people write diagnosis when they mean several conditions, or they write diagnoses when they mean just one. Paying attention to number (singular vs. plural) is essential in medical contexts because a single diagnosis and multiple diagnoses lead to different treatments.
Comparison Table: Singular vs. Plural
| Singular | Plural | Example Sentence |
|---|---|---|
| diagnosis | diagnoses | The doctor made one diagnosis. |
| analysis | analyses | We reviewed two analyses. |
| crisis | crises | The hospital faced multiple crises. |
| thesis | theses | She wrote three theses. |
| hypothesis | hypotheses | Several hypotheses were tested. |
Natural Examples
Seeing the word in real contexts helps you remember the correct form. Here are examples from different situations.
In a Medical Report (Formal)
The patient received two distinct diagnoses: type 2 diabetes and hypertension.
Notice that the verb received agrees with the plural subject diagnoses. In formal writing, always match the verb to the plural noun.
In an Email to a Colleague (Semi-Formal)
Could you send me the diagnoses for the three new cases we discussed yesterday?
Here, diagnoses is used naturally in a workplace email. It is clear and professional.
In a Conversation (Informal)
I heard the clinic had a few different diagnoses for that outbreak.
Even in casual talk, the plural form diagnoses is correct. Avoid saying diagnosises in any setting.
In a Study or Learning Context
The textbook lists the most common diagnoses for respiratory infections.
When you are studying, you will often see diagnoses in lists or summaries. This is the standard plural.
Common Mistakes
Here are the three most frequent errors learners make, along with the correct versions.
Mistake 1: Adding an Extra Syllable
Incorrect: The lab reported three diagnosises.
Correct: The lab reported three diagnoses.
The word diagnosises does not exist in standard English. It is a common overcorrection. Remember: one diagnosis, two diagnoses.
Mistake 2: Using the Singular Form for Multiple Items
Incorrect: The study looked at several diagnosis.
Correct: The study looked at several diagnoses.
When you have more than one, you must change the ending. Using the singular form for a plural meaning is a grammatical error that can confuse readers.
Mistake 3: Mispronouncing the Plural
Incorrect pronunciation: dye-ag-NOH-sis-iz (sounding like diagnosises)
Correct pronunciation: dye-ag-NOH-seez
The plural diagnoses rhymes with seas, not sisses. Practice saying it aloud: one diagnosis, two diagnoses.
Better Alternatives and When to Use Them
Sometimes you might want to rephrase a sentence to avoid confusion, especially in spoken English. Here are some alternatives.
- Use “cases” – The doctor reviewed three cases. This works well in conversation when the context makes it clear you mean diagnoses.
- Use “findings” – The report listed the findings for each patient. This is common in formal medical writing.
- Use “conditions” – She was treated for two conditions. This is simpler and often used in patient communication.
When you need to be precise, stick with diagnoses. In everyday conversation, you can use cases or conditions to sound more natural. In formal reports or academic writing, always use the correct plural diagnoses.
Formal vs. Informal Tone
The word diagnoses is appropriate in both formal and informal contexts. However, the surrounding language changes.
- Formal (medical journal, report): The differential diagnoses were considered before treatment.
- Informal (talking with a friend): They gave him a couple of diagnoses, but nothing serious.
In emails, you can use either tone depending on your audience. For a supervisor, write: Please find the diagnoses attached. For a coworker you know well, you might say: Here are the diagnoses we talked about.
Nuance: When to Use “Diagnosis” as a Mass Noun
In rare cases, diagnosis can be used as a mass noun (uncountable) when talking about the process or field of diagnosing. For example: Diagnosis is a critical skill for any physician. Here, it refers to the activity, not a specific instance. Do not use the plural diagnoses in this context. The plural is only for countable instances of a diagnosis.
Mini Practice Section
Test yourself with these four questions. Write your answers down, then check them below.
- What is the correct plural of diagnosis?
- Fill in the blank: The hospital reported three new ______ this morning.
- True or false: Diagnosises is an acceptable plural form.
- Which sentence is correct?
A. The doctor made several diagnosis.
B. The doctor made several diagnoses.
Answers
- Diagnoses.
- Diagnoses.
- False. The correct plural is diagnoses.
- B. The doctor made several diagnoses.
Frequently Asked Questions
1. Is “diagnoses” pronounced differently from “diagnosis”?
Yes. Diagnosis (singular) ends with a short -sis sound, like sister without the ter. Diagnoses (plural) ends with a long -seez sound, like seas. Practice: dye-ag-NOH-sis (singular) vs. dye-ag-NOH-seez (plural).
2. Can I use “diagnosis” for multiple conditions if I say “diagnosis of conditions”?
No. Even if you add more words, the noun diagnosis itself must be plural if it refers to more than one. Write diagnoses of conditions or diagnosis of a condition (singular).
3. What about “diagnoses” as a verb?
The verb form is diagnoses (third person singular, present tense), as in The doctor diagnoses patients every day. This is a different word. The plural noun diagnoses is pronounced the same way but used differently. Context tells you which one is meant.
4. Do other medical words follow the same pattern?
Yes. Many medical terms from Greek follow the -sis to -ses pattern. Examples include prognosis → prognoses, metastasis → metastases, and thrombosis → thromboses. Learning this rule helps you with multiple words at once.
Final Tips for Remembering
To avoid mistakes with the plural of diagnosis, keep these three points in mind.
- Think of the pattern: crisis → crises, thesis → theses, diagnosis → diagnoses.
- Never add an extra syllable: If you hear someone say diagnosises, it is incorrect.
- Practice with real sentences: Write a few sentences using diagnoses in different contexts. The more you use it, the more natural it becomes.
For more help with similar confusing plurals, visit our Confusing Plurals section. You can also check our Common Plural Forms for other patterns. If you have questions, see our FAQ or contact us.
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