sindansuru no de ha naku, hannou wo kakuninsuru.

It means not doing A, but doing B or rather than A, B.

So in this sentence:

• 診断するのではなく = not to diagnose / rather than diagnosing
• 反応を確認する = check the response

Together, the sentence contrasts two actions:

• don’t diagnose
• instead, check the response

This pattern is very common when Japanese wants to replace one action or interpretation with another.

The の turns the verb phrase 診断する into something noun-like, so it can be followed by ではなく.

You can think of it like this:

• 診断する = to diagnose / diagnose
• 診断するの = the act of diagnosing or diagnosing
• 診断するのではなく = not diagnosing / rather than diagnosing

This の is often called a nominalizer.

A rough literal breakdown is:

• 診断する = diagnose
• の = the act of / the thing of
• ではなく = is not / not being, and...

So the structure is not random; the の is what makes the contrast grammar work naturally.

Because the sentence is making a contrast between two options.

Compare:

• 診断しない = do not diagnose
• 診断するのではなく、反応を確認する = it is not a matter of diagnosing; instead, check the response

So 〜のではなく does more than simple negation. It sets up:

• not A
• but B instead

That contrast is the key point.

Also:

• ではなく is more neutral/formal
• じゃなくて is a more casual spoken version

For example:

• 診断するのではなく、反応を確認する。 = more formal/written
• 診断するんじゃなくて、反応を確認する。 = more conversational

Yes.

These are very similar:

• 診断するのではなく
• 診断するのではなくて
• 診断するんじゃなくて

The differences are mostly about tone and style:

• 〜のではなく: concise, formal, often written
• 〜のではなくて: slightly softer, still natural in speech or writing
• 〜んじゃなくて: casual spoken Japanese

So the original sentence sounds like something you might see in:

• instructions
• technical writing
• medical or academic explanation
• a manual or guideline

Because 反応 is the direct object of 確認する.

• 反応 = response / reaction
• を = marks the thing being acted on
• 確認する = confirm / check

So:

• 反応を確認する = check the response

This is a very common verb-object pattern in Japanese.

Other similar examples:

• 結果を確認する = check the results
• 状態を確認する = check the condition
• 内容を確認する = check the contents/details

It can mean both, depending on context.

In many English translations, check sounds most natural:

• check the response

But 確認する literally has the sense of:

• verify
• confirm
• make sure of
• check

So the nuance is a little more deliberate than just casually looking.

In this sentence, 確認する suggests:

• observe the response
• verify what the response is
• make sure how the person/system reacts

Yes, the subject is omitted, which is very normal in Japanese.

The sentence does not explicitly say:

• you
• we
• the doctor
• the examiner
• one

Who is doing the action depends on context.

In English, you might supply different subjects depending on the situation:

• Do not diagnose; check the response instead.
• Rather than diagnosing, we check the response.
• The goal is not to diagnose, but to check the response.

Japanese often leaves this unstated when the context makes it clear.

Yes, it can be.

Japanese often uses short plain-form sentences, especially in:

• instructions
• notes
• manuals
• headings
• technical explanations

The final verb 確認する completes the sentence.

Even though no subject is stated, it is still a perfectly normal Japanese sentence.

It may feel a little like a fragment to an English speaker because English usually wants an explicit subject, but Japanese does not need one here.

The plain form is common in written Japanese and does not automatically mean casual speech.

Here, 確認する can sound like:

• a neutral statement
• an instruction style
• a procedural description
• a formal written note

Depending on context, it could mean something like:

• check the response
• one checks the response
• the purpose is to check the response

Japanese plain form is often used in writing where English might choose different styles.

They describe different kinds of actions.

• 診断する = diagnose
  This means making a judgment or identifying a condition, often in a medical or technical sense.

• 確認する = confirm / check
  This means observing or verifying something without necessarily making a full diagnosis.

So the contrast is something like:

• Don’t try to identify the condition itself
• Instead, check how it responds/reacts

That contrast is the main message of the sentence.

Yes, that is a very natural translation.

Possible natural translations include:

• Instead of diagnosing, check the response.
• Rather than diagnosing, confirm the response.
• The point is not to diagnose, but to check the response.

Which one is best depends on context, but instead of diagnosing, check the reaction/response is a solid translation.

Just note that 反応 can be:

• reaction
• response

and the better choice depends on what kind of response is being discussed.