Table of Content
For a creatinine or BUN test on its own, you usually do not need to fast. What else is in the same blood draw is what changes the answer.
The question of whether a BUN creatinine test needs fasting comes up because these two values almost never travel alone. They are usually bundled into a metabolic panel, and the panel may contain a test that does require an empty stomach. The two instructions that matter most are below.
Is Fasting Required for a Creatinine Blood Test?
Not by itself. MedlinePlus (2023) states that your provider will tell you how to prepare, and that for a creatinine test you may be told not to eat meat for 24 hours beforehand, because meat can temporarily raise creatinine levels.
That is a different instruction from fasting. Skipping meat for a day still lets you eat normally. The fasting requirement appears only when creatinine is measured as part of a basic or comprehensive metabolic panel, and in that case MedlinePlus says you may need to fast for up to 12 hours.
And What About the BUN Test?
For BUN the picture is similar. MedlinePlus (2024) says that usually no special preparation is necessary for a BUN test, but if your provider has ordered other tests on the same blood sample, you may need to fast for several hours.
So the honest answer to "do I need to fast" is that it depends on the order form, not on the creatinine. Ask the clinic what the whole panel contains.
How Many Hours Do You Fast, and Can You Drink Water or Coffee?
When fasting is required, the window is usually 8 to 12 hours depending on which test is driving it. MedlinePlus puts the creatinine-inside-a-panel figure at up to 12 hours.
Water, Coffee and Everything Else
Here is the part people get wrong. MedlinePlus describes fasting as not eating or drinking for the stated window. It does not carve out an exception for coffee, tea, or juice, and coffee with milk or sugar is not a borderline case.
Plain water is the one thing worth asking about directly, since clinics differ on it and dehydration can affect the very numbers being measured. Call the office that ordered the test and ask what their instruction sheet means by nothing to eat or drink. The answer takes thirty seconds and removes the guesswork.
If fasting is genuinely difficult for you, say so when the test is ordered rather than on the morning of the draw. Appointment timing can usually be arranged around it, which is one reason these tests are often booked early in the day.
Will Eating Meat Before the Test Raise Your Creatinine?
It can nudge it up temporarily, which is exactly why the 24-hour meat instruction exists. It is a short-lived effect, not a sign of kidney damage.
Where Creatinine Comes From
The reason sits in what creatinine is. MedlinePlus (2025) explains that creatinine is a waste product of creatine, a chemical the body makes to supply energy mainly to muscles. The kidneys remove creatinine from the blood entirely, so the blood level rises when filtering slows down.
Meat contains creatine, so a large steak the night before adds raw material to the system. The same encyclopedia entry notes that creatinine levels also vary with body size and muscle mass, and that women often run lower than men because they usually carry less muscle.
This is also why one result on its own rarely settles anything. If the number surprises your doctor, repeating the test after a normal day of eating and drinking is a common next step.
What If Glucose or Cholesterol Is in the Same Blood Panel?
Then fasting is likely, and the strictest test in the draw sets the rule. This is the single most useful thing to understand about blood test preparation.
Cholesterol and Glucose Set the Rule
MedlinePlus (2024) says you may need to fast for 9 to 12 hours before a blood cholesterol test, which is why these tests are often done in the morning. For glucose, MedlinePlus (2024) specifies at least eight hours of fasting for a fasting blood glucose test or an oral glucose tolerance test, and notes you may also need to fast if glucose is part of a metabolic panel.
The table below pulls the common combinations together. Treat it as a map of what to ask about, not as a substitute for the instructions your clinic gives you.
|
What Is in the Blood Draw |
Fasting Usually Needed? |
|---|---|
|
Creatinine on its own |
No special preparation, though you may be told to skip meat for 24 hours |
|
BUN on its own |
Usually no special preparation |
|
Creatinine or BUN inside a BMP or CMP |
Possibly, up to 12 hours |
|
A blood cholesterol test in the same draw |
Yes, often 9 to 12 hours |
|
A fasting glucose or glucose tolerance test |
Yes, at least 8 hours |
|
Other blood glucose tests |
No special preparation |
Read down the list and find the strictest line that applies to your draw. If a cholesterol test and a creatinine test are going into the same tube rack, you are fasting for the cholesterol, and the creatinine simply comes along for the ride.
Could Not Fasting Explain a "Bad" Kidney Result?
Sometimes, but dehydration is the more likely culprit than food. Both creatinine and BUN rise when there is too little fluid in the body, and that has nothing to do with whether you ate breakfast.
What Pushes These Numbers Up Temporarily
MedlinePlus lists loss of body fluid among the reasons creatinine can read higher than normal, alongside reduced blood flow to the kidneys, a blocked urinary tract, and breakdown of muscle fibers. For BUN, the same resource lists dehydration, a high-protein diet, burns, certain medicines, and a recent heart attack.
Two of those are everyday situations rather than diseases. Arriving at the lab dehydrated after a long fast, or after a weekend of high-protein meals, can move the numbers without anything being wrong with your kidneys.
Older adults are the group where this matters most, because thirst signals weaken with age and fluid loss is easy to miss. Our article on dehydration in older adults and the signs mistaken for dementia covers how easily that gets missed, and what completely clear urine does and does not tell you looks at the hydration question from the other direction.
What It Does Not Explain
A number that stays high across repeat tests is a different matter. BUN levels also vary by sex and usually increase with age, so your doctor reads the result against your own history rather than against a single cutoff.
The way to settle it is to repeat the test under the conditions your provider asks for, and to let them interpret the pair of results. Guessing from one printout is how people spend a week worrying about a number that was never the problem.
Why Would a Doctor Order a Creatinine or BUN Test?
Mostly to check how well the kidneys are filtering, often before there is any symptom to go on. MedlinePlus notes that a BUN test may be ordered as part of a routine checkup, to monitor treatment of kidney disease, or to screen people at risk, since early kidney disease usually causes no signs at all.
The scale of that silent group is the reason screening exists. The CDC (2024) reports that more than 1 in 7 American adults has chronic kidney disease, and as many as 9 in 10 of them do not know they have it. Specific blood and urine tests are the only way to find it.
What the Kidneys Are Doing
The CDC also puts the workload in perspective: the kidneys, each about the size of a computer mouse, filter all the blood in the body roughly every 30 minutes while also helping control blood pressure and signaling the body to make red blood cells.
High blood pressure and diabetes are the two conditions most often behind a kidney check, which is why these tests appear so often on annual panels. If you want the fuller picture, our articles on chronic kidney disease and what eGFR means and on high blood pressure and its silent risks cover what the follow-up usually looks like.
How Is the Blood Draw Itself Done?
A single blood sample covers both values, usually drawn from a vein inside the elbow or the back of the hand. MedlinePlus describes the sensation as a brief prick or sting, with some throbbing or slight bruising afterward that soon goes away.
There is very little risk to a blood test. MedlinePlus lists the less common issues, all of them ordinary:
Bleeding more than expected at the site
Fainting or feeling lightheaded
Several punctures before a vein is found
Blood collecting under the skin as a bruise
Infection, a slight risk any time the skin is broken
Getting the Sample Without Leaving Home
Getting there is often the harder part, particularly after an overnight fast. Some laboratory services will collect a doctor-ordered sample at home instead, and Sonic Diagnostic Laboratory is one of the Los Angeles County services that draws blood at a patient's address rather than requiring a trip to a collection site.
The order itself still has to come from a clinician. For someone who cannot easily get to an office, a doctor who visits at home can review symptoms, decide which tests make sense, and write the request. Internal medicine physicians such as Mark Barats, MD see patients across Los Angeles and West Hollywood and handle this kind of monitoring as routine work.
Whatever route you take, bring the instruction sheet and the list of what you are taking. Blood tests that get repeated because the preparation was unclear cost more time than the thirty-second phone call would have. The same logic applies to any scheduled panel, including the bloodwork sometimes attached to a sports physical.
FAQ
-
Brushing your teeth is not eating, and most clinics do not treat it as a problem. Gum, mints and lozenges contain sugar or sweeteners, so leave them until after the draw. When in doubt, the instruction sheet from the clinic that ordered the test is the one to follow.
-
Do not change anything on your own. MedlinePlus is clear that certain medicines and supplements can affect creatinine results, and that you should never stop or adjust a medicine without talking to your provider first. Ask when the test is ordered, not the morning of the draw.
-
It is usually more convenient rather than more accurate. Cholesterol tests are often scheduled in the morning precisely because the fasting window fits overnight, so an early slot means you sleep through most of it.
-
It can. Creatinine comes from muscle activity, and intense exertion plus the fluid loss that goes with it can nudge the number up. If you train hard, mention it when you give your sample so the result is read in context.
-
Creatinine is the measured value from your blood. eGFR is a calculated estimate of how well your kidneys are filtering, worked out from that creatinine value along with details such as your age and sex. The two always move in opposite directions.
-
Urine tests have their own instructions, and they are often about timing and collection methods rather than food. Read the sheet for each test separately, since one fasting rule does not cover the whole visit.
Sources Used
Blood Glucose Test – MedlinePlus Medical Tests, U.S. National Library of Medicine (2024)
BUN (Blood Urea Nitrogen) – MedlinePlus Medical Tests, U.S. National Library of Medicine (2024)
Cholesterol Levels – MedlinePlus Medical Tests, U.S. National Library of Medicine (2024)
Chronic Kidney Disease Basics – Centers for Disease Control and Prevention (2024)
Creatinine blood test – MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine (2025)
Creatinine Test – MedlinePlus Medical Tests, U.S. National Library of Medicine (2023)
Fasting for a Blood Test – MedlinePlus (2025)
Written by Sofiia Puhach
Published · Updated
You May Also Like
Most over-the-counter libido supplements have thin evidence behind them, and a urologist's advice is to skip the testosterone boosters.