Tôi phải nhịn ăn trước khi xét nghiệm.

Phải means have to, must, or need to. It shows obligation or necessity.

So Tôi phải nhịn ăn... means I have to fast...

A useful comparison:

• phải = have to / must
• cần = need to

In medical instructions, phải sounds very natural because it often implies a requirement, not just a suggestion.

Nhịn ăn means to fast or to go without eating for a period of time.

It is made up of:

• nhịn = to refrain from, endure without, hold back from
• ăn = eat

So together, nhịn ăn literally means refrain from eating.

This is the normal phrase for fasting before a medical test.

Related expressions:

• nhịn uống = refrain from drinking
• nhịn ăn uống = refrain from both eating and drinking

No, they are different.

• nhịn ăn = not eat at all for some time; fast
• kiêng ăn = avoid certain foods; be on dietary restrictions / avoid eating something

Examples:

• Tôi phải nhịn ăn trước khi xét nghiệm.
  = I have to fast before the test.

• Tôi phải kiêng ăn đồ ngọt.
  = I have to avoid sweets.

So for a medical test, nhịn ăn is the correct phrase if you mean fasting.

Trước khi means before when it is followed by a verb or clause.

In this sentence:

• trước khi xét nghiệm = before taking/having the test

A helpful rule:

• trước + noun/time
  
  • trước giờ ăn = before mealtime
• trước khi + verb/clause
  
  • trước khi ăn = before eating
  • trước khi xét nghiệm = before taking the test

So trước khi is used here because xét nghiệm is functioning like an action.

Xét nghiệm is a common medical term meaning test, testing, or to test, especially medical/lab testing.

It can behave like either a noun or a verb depending on the sentence.

In context, trước khi xét nghiệm is understood as before having the test / before being tested.

Common uses:

• xét nghiệm máu = blood test
• đi xét nghiệm = go get tested
• làm xét nghiệm = do/take a test
• kết quả xét nghiệm = test results

So yes, it is quite flexible.

Both are possible.

• trước khi xét nghiệm
• trước khi làm xét nghiệm

The version without làm is natural and concise. Vietnamese often leaves out words that are easy to understand from context.

So:

• Tôi phải nhịn ăn trước khi xét nghiệm.
  = natural
• Tôi phải nhịn ăn trước khi làm xét nghiệm.
  = also natural, slightly more explicit

You will hear both.

The structure is:

Tôi + phải + nhịn ăn + trước khi xét nghiệm

Breakdown:

• Tôi = I
• phải = have to / must
• nhịn ăn = fast
• trước khi xét nghiệm = before the test / before being tested

So the pattern is basically:

Subject + obligation marker + main action + time clause

This is a very common Vietnamese sentence pattern.

It is natural and correct, especially in a medical context.

It sounds neutral and standard. A doctor, nurse, or patient could say it.

A slightly fuller version is:

• Tôi phải nhịn ăn trước khi làm xét nghiệm.

A more specific version might be:

• Tôi phải nhịn ăn trước khi xét nghiệm máu.
  = I have to fast before a blood test.

So the original sentence is perfectly natural.

Usually xét nghiệm refers to a medical or laboratory test.

For example:

• xét nghiệm máu = blood test
• xét nghiệm nước tiểu = urine test
• xét nghiệm COVID = COVID test

It is not the usual word for every kind of test in English. For example:

• a school test = usually bài kiểm tra
• a driving test = bài thi lái xe or similar depending on context

So in this sentence, learners should understand xét nghiệm as a medical test.

Yes. Vietnamese pronouns change depending on age, relationship, and social situation.

Examples:

• Em phải nhịn ăn trước khi xét nghiệm.
  = I have to fast... / You have to fast...
  depending on who em refers to in the conversation

• Anh phải nhịn ăn trước khi xét nghiệm.
• Chị phải nhịn ăn trước khi xét nghiệm.

Tôi is a safe, neutral word for I, but in real conversation people often use relationship-based pronouns instead.

These two words can be tricky for English speakers.

nhịn

• nh sounds roughly like ny in canyon, but smoother
• the tone is nặng (the heavy, low, clipped tone)

nghiệm

• ngh begins with the ng sound, like the end of sing, but Vietnamese allows it at the start of a syllable
• it also has the nặng tone on ệm

A rough learner-friendly approximation:

• nhịn ≈ nyin with a low, heavy stop
• nghiệm ≈ nghiem with a low, clipped ending

If pronunciation is your focus, these are definitely words worth listening to from native audio, because English spelling approximations are limited.

Both can mean something like need to fast, but phải is stronger.

• phải nhịn ăn = must / have to fast
• cần nhịn ăn = need to fast / it is necessary to fast

In a medical instruction, phải often sounds more like a firm requirement.

For example:

• Bạn phải nhịn ăn trước khi xét nghiệm.
  = You must fast before the test.

• Bạn cần nhịn ăn trước khi xét nghiệm.
  = You need to fast before the test.

Both are correct, but phải feels more definite.

You can add the type of test after xét nghiệm.

For example:

• Tôi phải nhịn ăn trước khi xét nghiệm máu.
  = I have to fast before the blood test.

Other examples:

• xét nghiệm nước tiểu = urine test
• xét nghiệm đường huyết = blood sugar test
• xét nghiệm cholesterol = cholesterol test

So xét nghiệm is the general medical-testing word, and the thing being tested can be added after it.