Common blood tests and how to prepare
What common blood tests check, why a practitioner might order one, how to prepare for fasting and what results mean. Read the guide, then book a consult.
Bio Chain Clinic Updated 8 min read
A pathology request form can look like a list of codes. This guide covers the common blood tests you’re likely to see on one. It explains what each test looks at and how to prepare.
Key points
- Most common blood tests need no preparation at all.
- A few need a fast, usually 8 to 12 hours with water only.
- Results for most common tests are back within 1 to 2 days.
- A result flagged high or low isn’t a diagnosis on its own.
The facts here come from 2 Australian sources. Pathology Tests Explained is a not-for-profit site run by pathology organisations, and it replaced Lab Tests Online Australia. Healthdirect is the Australian Government’s health information service. Each source is linked at the end, and all were checked on 14 October 2026.
What do common blood tests check?
Common blood tests count your blood cells and check how your kidneys and liver are working. Others measure the fats and sugar in your blood, your iron, your thyroid and your vitamin D. A practitioner usually picks a few of these, not all of them. The table shows what each one looks at and whether it needs any preparation.
| Test | What it looks at | Usual preparation |
|---|---|---|
| Full blood count (FBC) | Your red cells, white cells and platelets | None |
| Kidney function tests | How well your kidneys clear waste, plus salts such as sodium and potassium | Sometimes a fast of 8 to 12 hours |
| Liver function tests (LFT) | Proteins and enzymes made or released by your liver | None |
| Lipid profile | LDL and HDL cholesterol and triglycerides | Sometimes a fast of 8 to 12 hours |
| HbA1c | Your average blood sugar over the past few months | None |
| Fasting blood glucose | Your blood sugar at one point in time | At least 8 hours without food |
| Iron studies | How much iron is in your blood and in storage (ferritin) | No iron supplements first, sometimes a 12 hour fast |
| Thyroid function (TSH) | A first check on how your thyroid is working | None |
| Vitamin D | Your level of 25(OH)D, the form labs measure to check your supply | None |
The preparation column is a general guide. Your request form, or the practitioner who wrote it, has the final word.
Why might a practitioner order a blood test?
A practitioner orders a blood test when the answer could change what happens next. Healthdirect says blood tests can check your general health. They also help diagnose and monitor conditions such as anaemia, infections and low levels of a nutrient. Results can show how your kidneys and liver are coping, too.
Symptoms are often the starting point. Feeling flat and tired, for example, can have many causes. Iron, thyroid and blood count results can help narrow them down.
A test is worth doing when its result would change the plan. If you had the same test recently, your practitioner may use that result instead. If no result would change the advice, a test may not be needed at all. Extra tests cost time and money, and a borderline number can cause worry without changing anything.
How do you prepare for a blood test?
Most common blood tests need no preparation. You can eat, drink and take your usual medicines unless you’re told otherwise. Fasting is the main exception. Your request form should say if you need to fast, and for how long.
If you do need to fast:
- Stop eating 8 to 12 hours before the test. Healthdirect gives this window for a fasting lipid test.
- For a fasting glucose test, go at least 8 hours without food.
- Drink plain water only. Skip tea, coffee, juice and milk until after the test.
- Book an early morning time, so most of the fast happens while you sleep. For an 8 am test with a 12 hour fast, finish dinner by 8 pm the night before.
A few other things are worth knowing. Iron supplements can skew iron studies, so skip them before that test. Some medicines and supplements affect certain results. Check with the practitioner who ordered the test before you change how you take anything.
What happens at the collection centre?
The visit is short. Staff first check that your details on the request form are correct. A trained collector, often called a phlebotomist, then takes blood from a vein in your arm.
They put a band around your upper arm, find a vein and insert a needle. A small amount of blood goes into 1 or more tubes, which are sent to the lab. You press on the spot afterwards until it stops bleeding. Healthdirect suggests going easy on that arm for the rest of the day. Avoiding heavy lifting makes a bruise less likely.
How long do blood test results take?
Most common blood test results are ready within 1 to 2 days, says Healthdirect’s guide to blood testing. Some tests take longer to process.
The lab sends your results to the practitioner who ordered them. Ask when to expect them, and whether they’ll contact you or you should book a follow-up. Some labs will send you a copy if the practitioner asks on the form. Talking them through with your practitioner is still the clearest way to understand them.
What does a high or low result mean?
A result marked H or L sits above or below the lab’s reference range. That range covers the results most healthy people get, so a result outside it isn’t automatically a problem. In the words of Pathology Tests Explained, a flag “does not indicate that you are unwell and may not mean anything in your personal situation.”
Three things are worth keeping in mind:
- Ranges can differ between labs, so only compare your result with the range printed on your own report.
- Age and sex can shift what’s expected, so some tests have separate ranges.
- Blood tests are usually accurate, but they can still give a false positive or a false negative.
That’s why your practitioner reads each number alongside your symptoms, your history and any other tests. Sometimes they’ll suggest a repeat test to confirm a result.
Questions to ask before your blood test
Healthdirect suggests asking a few questions when you’re handed a request form. These ones are a good place to start:
- Why do I need these tests?
- Do I need to fast or change anything beforehand?
- How much will the tests cost?
- How and when will I get the results?
- What happens if a result isn’t in the normal range?
Write the answers on your phone or the back of the form. It saves a second call later.
Common questions about blood tests
Can I drink water before a fasting blood test?
Plain water is the one thing you can have while you fast. Healthdirect’s fasting advice for glucose, lipid and iron tests all allows water. Tea, coffee, juice and milk wait until after the test.
Should I take my usual medicines?
In most cases you keep taking them, unless the practitioner who ordered the test tells you otherwise. Some tests need a change in how or when you take a medicine or supplement. Your request form should say so.
Do I need to fast for every blood test?
Many tests need no fasting at all. Pathology Tests Explained lists no preparation for a full blood count, liver tests, HbA1c, thyroid tests or vitamin D. Fasting applies to some glucose, lipid, kidney and iron tests.
What if I forgot to fast?
Tell the collection staff before they take your blood. Depending on the test, they might still collect it, or ask you to come back another morning. Either way, the lab and your practitioner need to know you’d eaten.
Can I eat straight after the test?
You can eat and drink as normal once your blood is taken. Bringing a snack helps if you’ve fasted since the night before.
Talking it through with a practitioner
Blood tests are one input among many. Sleep, stress, training, food and family history all count too. If you’d like to talk about your health, it starts with a live consultation. If your practitioner thinks blood tests would help, they’ll explain which ones and why.
Read what happens in a telehealth consult or see how a consultation works at Bio Chain Clinic. The frequently asked questions cover fees and booking. When you’re ready, book a consultation.
Sources
- Pathology Tests Explained: Full blood count, Kidney function tests, Liver function tests, Lipid profile, HbA1c, Iron studies, Thyroid function tests, Vitamin D and Reference intervals explained.
- Healthdirect: Guide to blood testing, Cholesterol and lipid blood tests, Blood glucose test and Iron studies.
