What’s the Plural of Diagnosis: Understanding Medical Terminology

Diagnosis Becomes Diagnoses — See the Pattern

Look at these words. Can you see what happens?

• one diagnosis → two diagnoses
• one crisis → two crises
• one analysis → two analyses
• one thesis → two theses

Did you see the pattern? The -is at the end changes to -es. This rule works for many English words that come from the Greek language.

Why It Is Diagnoses and Not Diagnosises

In English, many words that end in -sis come from the Greek language. These words do not follow the normal plural rule. Instead of adding -s or -es to the end, the -is changes to -es:

• diagnosis → diagnoses (not “diagnosises”)
• analysis → analyses (not “analysises”)
• parenthesis → parentheses (not “parenthesises”)

A diagnosis is when a doctor finds out what is wrong with someone. For example, if you have a cough and a fever, the doctor checks you and gives you a diagnosis. You say it like “die-ag-NO-sis” for one, and “die-ag-NO-seez” for more than one.

Easy way to remember: Think of crisis → crises. Most people know that one. The word diagnosis works the same way: diagnosis → diagnoses.

How to Use Diagnosis and Diagnoses in Everyday English

• The doctor gave me a clear diagnosis after the blood test. (one diagnosis)
• The hospital made over fifty diagnoses this morning. (more than one)
• I want a second diagnosis before I take the medicine. (one diagnosis)
• The two diagnoses from the doctors were not the same. (more than one)
• An early diagnosis can help you get better faster. (one diagnosis)

You are doing great. Now let us look at the mistake most learners make with this word.

The Most Common Mistake With Diagnosis

Many learners write “diagnosises” because most English plurals just add -s or -es to the end. That is a smart guess — but it does not work here. Even native speakers sometimes pause and think about this one. So if you get confused, you are not alone.

How to remember: Think of it like this — the word ends in -sis. If you see -sis at the end, change it to -ses. Diagnosis → diagnoses. Simple.

Other words like this: analysis → analyses, crisis → crises, thesis → theses, basis → bases, hypothesis → hypotheses.

Test Yourself: Diagnosis or Diagnoses?

Choose the correct answer for each sentence. Click Check to see if you are right.

Keep Going — You Are Building Something

You just learned that diagnosis becomes diagnoses. That is one more plural you will never get wrong again.

But here is something interesting. The doctor who gave you that diagnosis probably did an analysis of your blood first. Analysis ends in -sis too. So what is the plural of analysis? Does it follow the same rule? And how do you use it in a sentence?

Next lesson: What Is the Plural of Analysis?

Sources

Oxford Learner’s Dictionary: diagnosis

Online Etymology Dictionary: diagnosis

Wikipedia: Medical diagnosis

Khamis Maiouf

My name is Khamis Maiouf. I am the creator of the English Teacher Site, dedicated to providing valuable resources and insights for students around the world. With a passion for education and a commitment to helping students enhance their skills, I aim to make English teaching more effective and enjoyable for both educators and students.