Bác sĩ chẩn đoán dị ứng khi tái khám.

It can mean any of those, depending on context.

Vietnamese usually does not mark the/a or singular/plural the way English does. So bác sĩ could be:

• the doctor
• a doctor
• doctors (in a general sense)

In this sentence, English translations might include:

• The doctor diagnosed an allergy during the follow-up visit.
• A doctor diagnosed an allergy during the follow-up visit.
• Doctors diagnose allergies at follow-up visits.

Context tells you which one is intended.

Not necessarily.

Vietnamese verbs do not automatically change form for tense. The verb chẩn đoán stays the same whether the action is past, present, or sometimes even future, unless other time words or context make it clear.

So Bác sĩ chẩn đoán dị ứng khi tái khám could be understood as:

• The doctor diagnosed an allergy at the follow-up visit
• The doctor diagnoses allergies at follow-up visits

If the speaker wanted to make the past clearer, they might add something like đã:

• Bác sĩ đã chẩn đoán dị ứng khi tái khám.

Here, chẩn đoán is a verb, meaning to diagnose.

Structure:

• Bác sĩ = subject
• chẩn đoán = verb
• dị ứng = object
• khi tái khám = time clause

So the sentence is built like:

Doctor + diagnose + allergy + when/at follow-up

Here, dị ứng is a noun phrase, meaning allergy or an allergic condition/reaction.

So chẩn đoán dị ứng means:

• diagnose an allergy
• diagnose an allergic reaction

In other contexts, dị ứng can relate more broadly to being allergic, but in this sentence it functions as the thing being diagnosed.

Because Vietnamese does not usually use articles like a, an, and the.

So where English says:

• diagnose an allergy

Vietnamese simply says:

• chẩn đoán dị ứng

This is completely normal. Vietnamese often leaves definiteness and indefiniteness to context.

Khi means when.

It introduces a time clause:

• khi tái khám = when returning for a follow-up examination / during the follow-up visit

So the sentence is saying that the diagnosis happened when the follow-up visit took place.

Tái khám means to come back for another examination, to have a follow-up exam, or a follow-up visit/checkup.

Breaking it down:

• tái = again / re-
• khám = examine / medical examination

So tái khám is very common in medical contexts and usually refers to a patient returning after an earlier visit.

Usually, it is understood to be the patient who returns for the follow-up visit, not the doctor.

So khi tái khám most naturally means:

• when the patient came back for a follow-up
• during the follow-up visit

Vietnamese often omits subjects in subordinate clauses when they are easy to infer from context. Even though bác sĩ is the main subject of the sentence, the understood subject of tái khám is often the patient.

That is one reason English translations may sound a bit less literal, for example:

• The doctor diagnosed the allergy during the follow-up visit.

Literally, it looks like when re-examining / when having a repeat examination, but in real medical Vietnamese it usually means at the follow-up visit or when the patient returned for follow-up.

So a natural English rendering is usually not hyper-literal. Instead of:

• when re-examining

you would often say:

• during the follow-up visit
• when the patient came back for a checkup

Yes, a little.

Because Vietnamese leaves out articles, tense marking, and sometimes subjects in subordinate clauses, this sentence can be interpreted more than one way without context.

Possible readings include:

• The doctor diagnosed an allergy during the follow-up visit.
• The doctor diagnoses allergies at follow-up visits.

The most likely real-world meaning is the first one, but the sentence by itself is somewhat open.

Because Vietnamese often omits words that are understood from context.

In English, you might say:

• The doctor diagnosed the patient with an allergy when the patient returned for a follow-up.

Vietnamese often avoids repeating information if it is obvious. So instead of explicitly mentioning the patient, it simply says:

• Bác sĩ chẩn đoán dị ứng khi tái khám.

This is normal and natural in Vietnamese.

Yes.

You could also say:

• Khi tái khám, bác sĩ chẩn đoán dị ứng.

This still means roughly:

• During the follow-up visit, the doctor diagnosed an allergy.

Vietnamese often allows this kind of movement for time expressions, especially when the speaker wants to emphasize the timing.

Not in the main grammatical role here.

In this sentence, dị ứng is the object of chẩn đoán, so it is the condition being diagnosed. That makes it function like allergy.

If you wanted to say someone is allergic, Vietnamese often uses a different structure, for example:

• bị dị ứng = to have/suffer an allergic reaction
• dị ứng với... = to be allergic to...

So here:

• chẩn đoán dị ứng = diagnose an allergy not
• diagnose someone is allergic in a direct word-for-word sense

Yes, it is natural and understandable in a medical context.

Vietnamese often uses:

• chẩn đoán + illness/condition

Examples:

• chẩn đoán ung thư = diagnose cancer
• chẩn đoán viêm phổi = diagnose pneumonia
• chẩn đoán dị ứng = diagnose an allergy

So this pattern is very standard.

Yes, it could.

The original sentence is active:

• Bác sĩ chẩn đoán dị ứng khi tái khám.
• The doctor diagnosed an allergy during the follow-up.

If you add passive-style wording, the focus changes. For example:

• Dị ứng được bác sĩ chẩn đoán khi tái khám.
  • The allergy was diagnosed by the doctor during the follow-up.

Or if you rephrase around the patient:

• Bệnh nhân bị dị ứng...
  • The patient had an allergic reaction / was allergic...

So bị and được are not just small extras; they can shift the sentence structure and viewpoint.

Not very natural in English.

A very literal breakdown is something like:

• doctor diagnose allergy when follow-up-examining

That helps you see the structure, but it is not good English.

More natural English versions are:

• The doctor diagnosed an allergy during the follow-up visit.
• The allergy was diagnosed at the follow-up appointment.

This is a good example of how Vietnamese can be compact, with a lot of meaning supplied by context.